CA1187967A - Automatic temperature control device for an electric appliance such as an electric blanket - Google Patents
Automatic temperature control device for an electric appliance such as an electric blanketInfo
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- CA1187967A CA1187967A CA000444604A CA444604A CA1187967A CA 1187967 A CA1187967 A CA 1187967A CA 000444604 A CA000444604 A CA 000444604A CA 444604 A CA444604 A CA 444604A CA 1187967 A CA1187967 A CA 1187967A
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Abstract
ABSTRACT OF THE DISCLOSURE
The automatic temperature control device for an electric appliance such as an electric blanket comprises a heater element, a sensor made of a thermosensitive material having an impedance, which changes as the tempera-ture varies, for detecting the temperature of the heater element, a switching element such as an SCR for regulating the supply of electric power to the heater element, and a plurality of electronic circuits comprising specifically a pulse supplying circuit and a pulse discriminating cir-cuit for driving the switching element in response to an output signal from the sensor, thereby assuring a safe operation of the automatic temperature control device against the occurrence of any failure.
The automatic temperature control device for an electric appliance such as an electric blanket comprises a heater element, a sensor made of a thermosensitive material having an impedance, which changes as the tempera-ture varies, for detecting the temperature of the heater element, a switching element such as an SCR for regulating the supply of electric power to the heater element, and a plurality of electronic circuits comprising specifically a pulse supplying circuit and a pulse discriminating cir-cuit for driving the switching element in response to an output signal from the sensor, thereby assuring a safe operation of the automatic temperature control device against the occurrence of any failure.
Description
?~
The present inven-tion ~elates -to an automatic temperatu~e control device ~or use in an electric appliance such ~s an elec-tric hlanke-t or electric carpet Th~s application is a divisional application of copending application No. 378,514, filed May 28, 1981.
The present invention will he described with reference to the aCcomp~n~ing drawings, in which:-lQ
Flg. 1 is a circuit diagram of a conventional automa-tic temperature control device for an elecric blanket;
F~g. 2 shows t~le construction of a wire used ~
-the conventional electric blanket and having double func-tions of heating and temperature sensingi Fig. 3 is a graph showing -the relation between the :impedance and temperature of a sensor used in the wire Fig. 4 is a bloc]c diagram showing an embodiment of an automatic temperature control device for an electric blanket according to the invention;
Fiys. 5, 6 and 7 are circuit diagrams showing details of the respective blocks in Fig. 4; and Figs. 8 and 9 are time~ charts showing voltage and current waveorms in the circuits of Figs. 4 to 7.
~ conventional automatlc temperature control device for an electr~c ~lanket has been constructed as shown in Fig.
1. In Fig. 1, reference numeral 1 denotes an ac source,
The present inven-tion ~elates -to an automatic temperatu~e control device ~or use in an electric appliance such ~s an elec-tric hlanke-t or electric carpet Th~s application is a divisional application of copending application No. 378,514, filed May 28, 1981.
The present invention will he described with reference to the aCcomp~n~ing drawings, in which:-lQ
Flg. 1 is a circuit diagram of a conventional automa-tic temperature control device for an elecric blanket;
F~g. 2 shows t~le construction of a wire used ~
-the conventional electric blanket and having double func-tions of heating and temperature sensingi Fig. 3 is a graph showing -the relation between the :impedance and temperature of a sensor used in the wire Fig. 4 is a bloc]c diagram showing an embodiment of an automatic temperature control device for an electric blanket according to the invention;
Fiys. 5, 6 and 7 are circuit diagrams showing details of the respective blocks in Fig. 4; and Figs. 8 and 9 are time~ charts showing voltage and current waveorms in the circuits of Figs. 4 to 7.
~ conventional automatlc temperature control device for an electr~c ~lanket has been constructed as shown in Fig.
1. In Fig. 1, reference numeral 1 denotes an ac source,
2 a switch, 2 a ~ire having double functions of heating and -temperature sensing and comprising a heater element 4, a sensor 5 a,nd a~ control element 6.
~ .
The wire 3 has ~ construction such ~s shown in Fi~o 2 in ~ich the control ele~ent 6 is wound helic~
~ t~
1 on an insulating core 7. Also, the sensor 5 separates the heater element 4 from the control element 6. Reference numeral 8 denotes an insulating coating~ Practically, the wire 3 is arranged in a serpentine fashion inside an electric blan~et. The sensor 5 has a negative temperature coefficient of resistance and is usually formed by a plastic thermistor made of a thermosensiti~e material.
This sensor 5 has an impedance Z related to the -temperature T as shown in Fig. 3, wherein the impedance Z is a resultant value o a capacitive impedance ZC and a resistive impedance ~R~ and the temperature sensing property of the sensor 5 is greatly influenced by the capacitive impedance ZC at low temperat~lres and by the resistive impedance ZR
at hish temperatures. When a DC voltage is applied to a plastic thermistor, the plastic thermistor is polarized and deteriorated to increase its impedance, so that it is neces-sary to use a plastic thermistor under the application of an ac voltage which is as uniform as possible in its positive and negative polarities.
~hen, the conventional example sho~ in Fig. 1 will be explained in greater detail. In Fig. 1, a swi.ching element 11 (in this example, a semiconductor control device ~enerally known as an SCR is used) is connected in series with the heater element 4 between 25 lines 9 and 10 Similarly, resistors 12 and 13 are con-nec-ted in series therebetween. Between the junction of the resistors 12 and 13 and a gate of the SCR 11, a variable resistor 14, a resistor 15, the control element 6, 1 a diode 17 and a triggering elernent 1~ are connected in series. Arranged between the control element 6 and the heater element 4 is a capacitor 19~ In this automatic temperature control device, during positive half cycles of ~he ac source voltage in which the line 9 is at a posi~
tive po-tential relative to the line 10, the capacitor 19 is charged at a potential which is determined by the resistance values of the resistors 12 and 13, variable resistor 14) resistor 15 and sensor 5, and when the potential on the capacitor 19 reaches a breakdown voltage o:E the triggering element 18, a trigger pulse is applied to the gate of the SCR 11 so that the SCR 11 is turned on to supply electric power to the heater element 4.
The conduction phase angle of the SCR 11 becomes approxi-mately 0 when the heater element 4 is at low -temperature and hence the sensor 5 has a high impedance, thereby supplying maximum electric power to the heater element 4.
While, when the temperature of the heater element 4 is high and the impedance of the sensor 5 is low, the con-duction phase angle becomes approximately 90Q to decreasethe supply of electric power to the heater element 4.
In this way, the automatic temperature control is carried out. Since the phase angle is changed by the variable resistor 1~, the user can obtain a desired blanket temperature by selecting a suitable resistance value for the variable resistor 14.
However, the aforementioned automatic temper-ature control device has the following disadvantages.
1 Namely, since the clrcuit comprisiny the diode 17 is con-nected in parallel with the sensor 5, the ac voltage applied to the sensor 5 at positive half cycles is different from the ac voltage applied thereto at negative half cycles.
Consequently, the sensor 5 is polarized and deteriorated to increase its impedance-, which results in a danger such that the controlled temperature shifts to higher temperatures.
Further, in the event of a failure of the components used such as short-circuiting of the triggering element 18, the supply of electric power to the heater element 4 becomes uncontrollable still maintaining a maximum amount o:E electric power supply, thereby causing a danger such tha-t the heater element 4 is overheated. Additionally, a similar dznger takes place when the SCR 11 is short-circuited, or a self-triggering failure occurs in the SCR
11 in which the SCR 11 is turned on without being triggered by triggering pulses. Thus, it is possible that these dangerous conditions cause a human body injuring accident in the worst case, since an electric blanket is used by babies or aged persons who cannot push aside an overheated blanket by themselves. Further, the automatic temperature control device employing the phase angle control of an SCR has brought about an unfavourable condition to generate noises which cause interference, in particular, ~5 with a radio receiver while it is used by a user lying in his bed.
It is a first main object of this inven-tion to provide an automatic temperature control device which l comprises a heater elernent connected to an ac source, a senso.r for detecting -the temperature of the heate.r element, a teMperature responsive circuit including a grounded-base transistor with its base-emitter circuit connected in series wi-th the sensor and a diode connected across the base-emitter junction of the transistor with the PN junction of the diode and the base-emitter junction o:E the transis-tor connected inversely in parallel with each other, thereby using a collector current of the grounded-base transistor as a temperature signal, an electric potential setting circuit for producing an electric potential thereby to preset a desired temperature, a comparison circuit Eor comparing an output of the temperature responsive circuit with that of the electric potential setting circuit, ancl swltching means d:riven by an output of the comparison circuit to regulate electric power supplied -to the heater element, and which is featured to supply a uniform ac voltage to -the sensor so as to prevent the sensor from being polarized and assure its accurate temperature detection.
An object subsidiary to the first main object is to provide an automatic temperature control device which further comprises an integrating circuit for integrating the output of the temperature responsive circuit (the col-lector current of the grounded-base transistor) to produce an in-tegration output ~o be supplied to the comparison cir-cuit and stopping its integrating oper`ation during positive half cycles of the ac source voltage in which electric power is supplied to the heater element, thereby affording 5~
1 a s-table and accurate temperature signal.
Another object subsidiary to the first main object is to provide a safe automatic temperatuxe control device which further comprises a power supply interrupting circuit including a heating resistor connected in series with the series circuit of the sensor and the base-emit~er circuit of the transistor and a thermal fuse connected to the ac source which can be fused by the heat generated in the heat-lng resistor thereby to interrup-t electric power supply to the heater element, whereby, when the heater element is overhea-ted to cause the sensor to be melted ancl the heater element and the control element to be short--circuited to each other, the heating resistor is supplied with an over~
load which is 17 times as large as its rated load thereby to be heated to lnterrup-t electric power supply to the heater element.
It is a second main object of this invention to provide a highly safe automatic temperature control device which comprises a heater element connected -to an ac source9 a sensor for detecting the temperature oE the heater element, a temperature responsive circuit for detecting the temperature of the heater element by detect-ing an impedance of the sensor by means of the control element which is in contact with the sensor, an electric potential setting circuit for producing an electric - potential thereby to preset a desired temperature, a pulse supplying circuit. for producing a zero-crossing pulse, a pulse discriminating circuit for determining whe-tller ... .
1 an input signal thereto is a pulse and, as a result of the determination of the input pulse, producing an output pulse in phase with the input pulse, a comparison circuit for comparing an output of the temperature responsive circuit with that of the electric potential setting circuit in synchronism with the zero-crossing pulse and supplying a zero-crossing pulse to the pulse discriminating circuit when the temperature of the heater element is below a preset temperature level t ancl switching means triggered by an output of the pulse discriminating circuit to supply electric power to the heater element, whereby the switching means is triggered by the zero-crossing pulse to enable automatic temperature control with reduced noise genera-tion and the occurrence of any failure in the respective constituent circuits are checked in synchronism with the zero-crossing pulse so as to interrupt the supply of electric power to the heater element when the occurrence of a failure has been detected.
An object subsidiary to the second main object is to provide a highly safe automatic temperature control device wherein the comparison circuit comprises an electric circuit for fixing the output of the electric potential setting circuit to an electric potential level which is lower than a circuit dc power supply voltage by a pre-~5 determined magnitude whe:n a zero-crossing pulse does not occur so that the outputs of the temperature responsive circuit and the electrical potential setting circuit are compared with each other in synchronism wi-th input .. . .. ~ .
~ Q ~
1 zero-crossing pulses thereby to produce output zero-crossing pulses, and wherein, even when any failure occurs in the Olltput of the temperature responsive circuit and a false output signal appears representing that the temperature of the heater element is very low, it is possible to inter-rupt the supply of electric power to the heater element.
Another object subsidiary to the second main object is to provide a highly safe automatic temperature control device wherein an SCR is used as the switching means and there is provided an SCR failure sensing circuit for detecting a non-triggering conduction failure (self--triggering) of the SCR to interrupt the supply oE electric power to the heater element.
A further object subsidiary to the second main object is to provide an inexpensive and hiyhly safe auto-matic temperature control device wherein the pulse discriminating circuit is constituted by resistors, a capacitor and switching elements, and in the absence of input zero-crossing pulses the capacitor stores electric charge supplied from the circuit dc power supply through the resistors, and upon receipt of input zero-crossing pulses, the switching elements are turned on to discharge the stored electric charge to the switching means, thus preventing the application of triggering pulses -to the ~5 switching means when a failure occurs in the electric circuits of the device.
A still further object subsidiary to the second main object is to provide a highly safe automatic temperature 1 con-trol device which further compri.ses a disconnection sensing circuit for preventing zero-crossing pulses from being applied to the pulse discriminating circuit, when breakage of the control element occurs, by causing the output of the temperature responsive circui-t -to be shifted apparently to the side of a lower temperature s-tate and to stay there upon occurrence of such breakac3e of the control element, thereby preventing the temperature under control from becoming high.
A yet further object subsidiary to the second main object is to provide a highly safe automatic temper-ature control. device wherein a plurality of electric circui-ts such as the disconnection sensing circuit, the comparison circuit, etc. are connected so tha-t zero-crossing pulses are successively transmitted through the circuits and output pulses from the last one o:E the circui-ts are applied to the pulse discriminating circuit as inpu-t signals thereto whereby a failure of circuit components in the transmission path of zero-crossing pulses can be checked without requiring additional components for use in the checking and the supply of electric power to the heater component can be in-terrupted when a failure occurs in the circuits.
Reference is firstly made to a block diagram of Fig. 4. For a better understanding of the present inventionl reference should also be ma,de to Figs. 8 and 9. In Fig. 4, a voltage VAc of an ac source 1 is applied to a circuit of a diode 20, a resistor 21 and a capacitor ,. " . ?~ q ~
1 22 by closi.ng a switch 2 to produce a dc vol-tage Vcc to be used as a dc power supply for the circuits A temper-ature responsive circuit as designated by a block 23 detects a current IS resulting from the amplification of an electric current which flows through a ground line of the ac source 1, the temperature responsive circuit 23, a heating resistor 33, the control element 6, the sensor 5, the heater element 4 and the other supply line of the ac source 1 with its magnitude varying with the change of the impedance of the sensor 5, during negative half cycles of -the voltage VAc of the ac source 1, and gives rise to the simultaneous flow of a current IT which is substantially identical with the current IS. Since the sensor 5 is capacitive, this current IS leads in phase by about 90 as shown in Fig. 8 and overlaps, in phase, positive half cycles of the ac source voltage in which an SCR 11 is turned on, so that the waveforrn of the current IS is distorted by the influence of a potential gradient generated by -the heater element 4. Maturally, the current I5 is not distorted when the SCR 11 has been turned offO Therefore, the magnitude of the current IS has great dependency upon the presence or absence of the distortion/ resulting in undesirable variations in the relation between the temper-ature of the heater element 4 and the maynitude of the ~5 current I5. An integrating circuit, such as designated by a block ~4, is adapted to integrate the current IT and produce an output voltage VI in inverse proportion to the temperature of the heater element 4. F'or the reasons set ~ 3~ 3 1 forth above, the integra-ting circuit 24 integrates the current IT which has excluded a region of the current IS
where the distor-tion appears. As the temperature of the heater element 4 rises, the impedance of the sensor 5 decreases, resulting in an increased currerlt IT and a decreased voltage VI. An electric potential setting ci.rcuit as designated by a block 25 is adapted to produce a potential Vs The potential Vs is manually variable and may be adjusted by the user to obtain an optimum con-trolled temperature. A block 26 denotes a pulse supplyingcircuit which produces various pulses V0, Vzl, Vp and VN
.in synchronlsm with the ac source voltage VAc The pulse Vzl stands for zero-crossing pulses which are generated at zero-crossing points through which the waveform of the ac source voltage VAc transfers from negative halE cycles to positive half cycles.
A block 27 denotes a disconnection sensing circuit adapted to detect disconnection or breakage o~ a control element 6 and which produces a voltage V~ to be applied to the control element 6 in synchronism with the zero-crossing pulses Vzl and further produces other zero-crossing pulses Vz2 in synchronism with the zero-crossing pulses Vzl if the control element Ç is not disconnected nor broken, the.reby allowing an electric current to flow there-through. Wit~out the disconnection sensing circuit 27,a decrease in the current IS flowing through the sensor S
upon occurrence of disconnection or breakage of the control elemen-t 6 would give rise to a signal indicative .. --~ 1~ - -P
1 of a decreased temperature of the heater element 4, there-by causing -the temperature under control to rise danger-ously in excess of a preset temperature~
A btock 28 denotes ~ comparison circuit which~
in the absence of the zero-crossing pulses Vz~ app:Lied thereto, fixes the output voltage Vs from the electric potential setting circuit 25 at a level of (V~c ~ VF = VsF, where VF is a constant forward voltage drop of a diode), and which, in the presence of the zero-crossing pulses V~2 from the disconnection sensing circuit 27 r releases the Eixed level in synchronism with the reception of the zero-crossing pulses and compares the output vo]tage V
from the i.ntegrating circuit 24, which is in inverse proportiorl to the temperature of the heater element 4, with a voltaye VST which is the value of the output voltage Vs from the electric potential settiny circuit ~5, at this ti~e, thus producing the other zero-crossing pulses Vz3 in phase with the zero-crossing pulses Vz2 when the detected temperature is below a preset temperature level.
A block 29 denotes a pulse discriminating circuit which, upon receipt of the zero-crossing pulses Vz3 fxom the comparison circuit 28 I produces other zero-crossing pulses Vz4 in phase with the zero-crossing pulses Vz3 which pulses Vz4 are applied to the gate of the SCR 11 to trigger it and supply electric power to the heater element 4. When the detected tempera~ure exceeds a preset temperature level or when a failure occurs in the pulse supplying circuit 26, disconnection sensing circ~1it 27 or . . . . ~, ~,. _ , ... .
cormparison clrcul-t 28, no input pulse Vz3 is app:Lied to the pulse cliscrim:inat:ing circul-t 29, and hence there is no out-put pulse Vz,l from the pulse discrimina-ting c.ircult 29, t~-lereby maintainirlg the SCR 11 untr~ggered. Wherl a failure occurs in Ihe pulse supplying clrcuit 26, disconnection sen-sing circuit 27 or comparison circuit 28, so thdt, a con-tinuous input signal Vz3 is appli,ed to the pulse discriminat-ting circuit 29, the ou-tput signcll Vz4 of t~le pulse ~,scri-mlnating circuit 29 is reduced -to a level (practically not more than 0.2 volts) which is so low as to fail to -trigger the SCR 11. Thus, the SCR 11 is not. triggered also in this case. F'urther, the pulse discriminating circuit 29 itself is so designed that i-ts output signal Vz4 disappears or is reducec1 to a low level which is insu:Eficient to tr.igger the SCR 11 when a failure occurs in the pulse discr,imirlating circuit 29 itself.
With the above-descr:ibed construction, the zero-crossing pulses delivered from the pulse supplylng circuit 26 a.re transmltted successively through the disconnection sensing circuit 27 and comparison circui-t 28, whose sae operation has to be assured, and the final output signal causes the pulse discriminating circuit 29 to tri.gger the SCR :L1. In this manner, the respec-tive circuits confirm the occurrence of no failure -therein along with the accom-plishment of their proper functions while the ze:ro-crossing pulses are generated. Thus, it is possible to accomplish -the two functions simultaneously and instantaneously to ensure that tlle circuits can be kept ~ .... . . . ~n 1 in highly safe conditions, yet without requi.ring any great increase o~ circuits or componen-ts for assuring a safe operation of the device. Even when there are involved o-ther circuits, whose safe operatlon has to be assured, in 5 acldition to the two electric circuits of -the disconnection sensing circuit 27 and comparison circuit 28 as e~emplified in the above construction, the aforernentioned safety system can readily assure a highly safe operation of the device~ Therefore, it can be said that this safety system has a wide field for its application.
A block 30 denotes an SCR failure sensing circuit~ When it is detected that the SCR 11 is turned on in spite of the absence of the zero-crossing pulses Vz3 from the comparison circuit 28~ the SCR failure sensing ci.rcuit 30 applies a heavy load to a hea-ting resistor 31 whlch load is 17 times as high as a rated load of the heating resistor 31 by making use of outpu-t pulses VN and VD from the pulse supplying circuit 26, so that the heating resistor 31 is heated to cause a thermal. fuse 32 to be fused, thereby interrupting the supply of electric power to the heater element 4. Accordingly, even if the SCR 11 has been short-circuited or has brought about self-triggering thereof, the supply of electric power to -the heater element 4 can be also interruptedO
A wixe 3 has a function -to detect abnormal heating when abnormal heat is generated accidentally in the wire 3. Namely, when the heater elemen-t 4 is abnormally heated until the temperature therearound reaches .. .. . . . . ... ... . . .. . .
t~ jt-`~
1 the rnelting point (167C) of the sensor 5 made oF a plastic thermistor~ the con-trol element 6 is brough-t into short-cirrui-ting contact with the heater elemen-t 4 due to a -tensile s-tress in -the control element 6 remaining from the time o~ lts coiling. Then, the heating resistor 33 is supplied with the ac source voltage 'JAC thereby to be heated by a heavy load current which is 17 times as large as its rated load current. As a result, the thermal fuse 32 is fused to interrupt the supply of electric power to the heater element 4.
In -the foregoing, a description has been made of the overall construc-tion of the safety system of the automatic temperature control devlce according to the invention. The construction of each of the specific circuits employed in the system will then be described in greater detail.
Fig. 5 shows details of the pulse supplying circuit 26 in which the ac source voltage VAc is con-ver-ted into an ac voltage VAc' whose phase is slightly delayed~ as shown in Fig. 8, through a fil-ter constituted by a resistor 34 and a capacitor 35, and the ac voltage VAc' is supplied to a point 36. When the ac voltage VAc' exceeds a reference voltage VF determined by a constant current source 37 and a diode 38 during a rising positive half cycle, a comparator 39 is inverted to produce an output voltage Vp. Thereafter, when the ac vol-tage V~c' continues to increase and reaches a level of the sum of the reference voltage VF and a base-emitter 1 forward drop VB~ o~ a transistor 40, transistors 40 and 41 are tur~ed on to generate an output pulse VD which falls ~o gl-ound potential. According:Ly, the falling point of VD is necessarily delayed from the rising point of Vp, whereas -the rising point of VD :Ls necessarily in advance oE the falling point of Vp. As the ac voltage VAc' enters a negative half cycle, a transistor 43 is turned on, thereby turning off transistors 46 and 47, which have been conductive by being fed from constant current sources 44 and 45, and simultaneously generating an output pulse VN which falls to ground potentialO Reference numeral 49 denotes a base-emitter resistor Eor the tran-sistor 47.
By applying the thus produced pulses Vp and VN
to set and reset terminals of a flip-flop 50, respectively, and then a Q output of the flip-flop 50 and the pulse VN
to a NOR circuit 51, the zero-crossing pulse Vzl is produced from the output terminal of the NOR circuit 51.
A description has hereinabove been made of the ac voltage VAc' whose phase is slightly delayed from that of the ac source voltage VAc ~ the pulse VN which rises during negative half cycles of the ac voltage VAc', the pulse Vp which rises during positive half cycles of the ac voltage VAc', and the pulse VD which falls during 2~ positi.ve hal~ cycles of the ac voltage VAc' and rlses during the positive peri.od of the pulse Vp.
ReEerring to Fig. 6 which shows i.n detail the essential struc-ture of the present invention, details of 3f~D l~
1 the temperature xesponsive circuit ~3, integrating circuit 24, electric potential setting circuit 25, disconnection sensiny circuit 27, comparison circuit 28~ and pulse dis-criminating circuit ~9 will be describedO For giving a better understanding of the present invention, reference should also be made to Fig. 8.
The temperature responsive circuit 23 comprises a grounded base transistor 52 and a diode 53 which are connected in antiparallel with the base-emitter circui-t of the transistor 52 such that the PN junction of the diode 53 and the base-emitter junc-tion of the transistor 52 are connected inversely in parallel with each other. During positive half cycles o~ the ac source voltage V~c ~ the supply of electric power to the heater element 4 is controlled, and during negative half cycles of -the ac source voltage VAc , the impedance of the sensor S is detected as an equivalent of the temperature of -the heater element 4. The current IS flowing through the sensor S
during negative half cycles of the ac source vol-tage VAc is equal to the emitter current of the transistor 52.
In Fig. 8, a period designated by x is one in which electric power is supplied to the heater element 4 and a period designated by y is one in which the supply of electric power to the heater element 4 s-tops. Since ~5 the sensor 5 has a capacitive impedance as shown in Fig. 3/ the phase of the sensor current IS leacls that of the ac source voltage VAc by about 90, so that a portion of the sensor current IS overlaps the period for 17 ~
q~
1 controlling the supply o~ electric power to the heater element 4. As a result, an electric potential. gradient caused in the heater element 4 when electric power is supplied to the heater element 4 affects the sensor S curren-t IS to distor-t the waveform of the overlapping portion of the sensor current IS as shown .in FigO 8.
Naturally, the distortion disappears within the period y in Fig. 8. During positive half cycles of the ac source voltage V~ , the current IS flowing through the sensor 5 passes the diode 53. Since the magnitude of the forward voltage of the diode 53 and the base-emitter ~ol-tage of the transistor 52 are substantially equal -to each other and further either one of the above-mentioned voltages is ~ar smaller than the voltage VAc , the sensor S is fed wi-th a symmetrical ac voltage, whereby the promotion of the deterioration of the sensor 5 by the polarization effect is suppressed. The heating resistor 33 is impressed wi-th -the ac source voltage VAc when the heater element 4 is heated -to reach an abnormally high temperature at which 20 the sensor 5 is melted and the heater element 4 is brought into short-circuiting contact with the control element 6, whereby the hea-ting resistor 33 is heated to fuse the thermal fuse 32~ The resistance (390 Q in this example) of the heating resistor 33 is much smaller than the resistance of the sensor S, so that the insertion oX the resistor 33 does not cause an error in`the sensor current S' The collector current IT, whose magnitude is 1 substantially equal to that of the sensor currert Is) flows through -the collector of the transistor 52.. However, a distortion would appear or disappear in the sensor current IS depending on the supply or the stoppage of supply of electric power to -the heater element 4, thus .resulting in -the corresponding fluctuation of the magnitude of the sensor current I~ or the collector current IT. In order to rernove the above-described disadvantage, the integrat-ing circuit 24 is provided to cut the distortion component of tne collector current IT and to produce a dc signal indicative of the temperature under control by integrating the collector current IT whose distortion component has heen removed.
The integrating circuit 24 comprises a capacitor ~/L5 54 for integrating the col~ctor current IT and a discharge resisto.r 55 for gradually discharging electric charge on the capacitor 54. A transistor.56 is provided to cut the distortion component. This transistor 56 staying in the conductive state under the control of a constant current source 58 is turned off to cut the distortion component when a transistor 57 connected in the base circuit of the transistor 56 is turned on by the pulse Vp which rises during positive half cycles of the ac source voltage VAc.
A diode 59 is provided to assure the turn-ofE oE the transistor 56 when the transistor 57 has been turned on.
A description will be given~of the relation between the ternperature of the heater element 4 and the output voltage VI of the integrating circuit 24 brought ... . - ~ q ~ ' ' ' 1 by the above-mentioned construction. As the temperature of tne heater element 4 increases, the impedance of the sensor 5 decreases and the sensor current IS and the col-lector currenc IT increase. Consequently, the lntegration voltage of the capacitor 54 increases and the output voltage VI of the integrating circuit 24 decreases. On the other hand, as the temperature oE the heater element 4 decreases, the above-mentioned situation is reversed and the outpu-t voltage VI of the integrating circuit 24 increases. Thus, the output voltage VI oE the integrating circuit 24 varies in inverse proportion to the temperature o-f the heater element 4.
Nex~, the electric potential setting circuit 25 has a series circuit of the resistor 60, variable resi.stor 61 and resistor 62 connected across the circuit dc power supply Vcc ~ and its output potential Vs is taken out :Erom a junction of the resistor 60 and varia~le resistor 61. The resistance of the variable resistor 61 may be varied by the user to adjust the outpu-t potential Vs o-E the electric potential setting circui-t 25.
An explanation will be given hereunder of the disconnection sensing circuit 27. The disconnection sensing circuit 27 comprises a transistor 68 which is turned on by being fed with a constant current :Erom a 2S constant current source 63 via diodes 64, 65, 66 and 67 when the zero-crossing pulse Vzl is not supplied to the base o~ a transistor 70. A resistor 69 is a base-emitter resistor .for the transistor 68. When the zero-crossi.ng 1 pulse Vzl is applied to the base of the transistor ~0, the transistor 70 ~s turned on, which causes a transistor 74 t.o be turned cn through a resistor 71 and a current m~rror circuit constituted by a diode 72 and a transistox 73 to produce an output potential Vx at the emi-tter output o:E the transistor 74. When the control element 6 is not disconnected llor broken, a current is supplied from the constant current sOUrce 63 to flow through the control element 6 and simultaneously to cause the transistor 68 to be turned off, -thus producing the zero-crossing pulse Vz2 a-t the co:Llector of the -transistor 68. If the control e~ement 6 is disconnected or brolcen, a current cannot flow through the control element 6 even though the transistor 7~ is turned on, so that -the transistor 68 continues to lS be conductive and hence no zero-crossing pu:Lse Vz2 is genexatedO Reference numeral 75 designates a base~
emitter resistor for the transistor 7~. Diodes 76~ 77 and 78 are provided to protect the transistor 74 from a surge voltage. The diodes 54, 65, 66 and 67 are provided 20 to assure the turn-off of the transistor 68 when the t.ransis-tor 74 is turned on.
As will be seen from the above description, when the control element 6 is not disconnected nor broken, -the zero-crossing pulse ~Z2 is produced in phase with the zero-crossing pulse Vzl when the zero-crossing pul.se V
i5 applied to t:he base of the transistor 7a.
The comparison circuit 28 will now be explainedO
The comparison circuit 28 is adapted to compare -the output .. .. ~ . , . ... . , .. , . . .... . . ~, ~ .
1 poten-tial VI from the integrating circuit 24 with the output poten-tlal Vs from the electric potential setting circuit 250 In the absence of the zero-crossing pu ! se Vz2 from the disconnection sensing circuit ~7, the transi.stor b8 remains conductive, and a txansistor 80 also stays in the con-ductivP state with its base current drawn through a resistor 79, thereby ensuring that the output potential Vs from the electric potential setting circuit 25 is fixed to a level VsF which is lower than the circuit dc power supply voltage Vcc by the forward voltage drop VF of a diode 81. Accordingly, the relation of Vs > Vl holds between the output potential V~ of the electric potential se-tting circuit 25 and the output potential VI o~ the integrating cirGuit 24, which renders tne output level of the comparator 82 low. When the zero-crossing pulse Vz2 is produced by the turn-off of the transistor 68, the ~ransis~or 80 is turned off to release the ou-tput potential VS of the electric potential setting circuit 25 from its fixed potential level to compare the output potential V
of the integrating circuit 24 with the preset value VsT
of Vs provided by the electric potential setting circuit 250 In this comparison, during the period x sho~l in Fig. 8 where the temperature of the heater element 4 remains lower than the preset temperature, every time when -the relation ~ Vs ~ VI becomes satisfied, the output of the comparator 82 turns to a high level to thereby produce the ~ero crossing pulse Vz3~ On the other hand, during the period y during which the temperature of the heater element 4 .... ~ . . .. ., .. .. . ....... ., . .. , .. ~ . . . . ... . .. . . . .
P ~
1 remains higher than the preset temperature, the relation o Vs ~ VI holds to maintain the ou-tput of the comparator 82 at a low level, thereby preventing the generation of the zero-crossing pulse Vz3~ Rererence numeral 83 denotes a base-emitter resistor for the transistor 80~ The diode 81 fulfils a useful purpose as described helowO When a failure occurs in the tempera-ture responsive circuit 23 or -the integrating circuit 24, namely, when an open-circuit failure occurs in the transistor 52, for example, the ou-tput potential VI of the integrating circuit 24 becomes equal to the circuit dc power supply voltage V
Assuming that the diode 81 is not provided, the output po-tential Vs of the potential setti.ng circuit 25, which has been .Eixed to a level VsF lower than the circuit dc lS power supply voltase Vcc by the emitter-coll.ector satura-tion voltage VcE5a~ of the transistor 80 when the transistor 80 is conductive, would be released to its non-fixed potential level VsT in synchronism with the generation of the zero~crossing pulse Vz2. Therefore, normally the relation of VI > Vs is maintainecl rendering the output le~rel of the comparator 82 high, thus preventing the output zero~crossing pulse Vz3 from being generated by the comparison cixcuit 28. It should be noted that the comparator 82 has an input offset voltage VI0. Accordi~gly, when the relat:ion of VIo > (VI - Vs) holds, the output level of -the comparator 82 becomes low when the transistor 80 is turned on~ As a result the zero-crossing pulse Vz3 is continuously produced from the comparison circuit 28, ... .. ~ 1~
1 resulting in the dangerous overheating of the heater el.ement ~. However, owing to the provision of the diode 81, it is possible to have the potential Vs lowexed to the level VsT further by the magnitude of the forward voltage drop V~ acros.s the diode 81 even when the transistor 80 is conducti~e, thereby eliminating the adverse affect by the input offset voltage of the comparator 82.
Next, an explanation will be given o the pulse discriminating circu~t 29. When the pulse dlscriminating circui~ 29 does not receive the zero-crossiny pulse Vz3 from the comparison circuit 28 (i.e., when the output level of the comparator 82 is low), transistors ~4 and 85 are nonconductive and a capacitor 86 is gradually charged from -the circuit dc power supply voltage Vcc through a resistor 87. Upon receipt of the zero-crossing pulse Vz3 from the comparison circuit 28 by the base of the trans i.stor 84, the transistor 84 is rendered conduc-tive -through a resistor 88 and the transistor 85 is also rendered conduc-tive through a resistor 89. As a result, electric charge stored in the capacitor 86 discharges forming the zero-crossing pulse Vz4 through the translstor 85 and resistor 90 to the gate of the SCR 11. Reference numeral 91 denotes a base-emitter resistor for the transistor 85, and reference numeral 92 a gate resistor ~5 for the SCR 11~
The pulse discriminating circuit 29 makes use of a specific property of the instantaneously rising pulse waveform of the zero~crOssing pulses at the 1 zero-crossing points thereby to effect pulse discrimination.
- ~ In this example, since the ratio of an occurrence period to a non-occurrence perlod of the zero-crossing pulses is taken to b~ 2 or more, even when the resistor 87 has a considerably large resistance value, it is possible to produce at æero-crossing points the pulse Vz4 of a magnitude sufficient to trigger the SCR 11. As an example in this casPj when th~ circuit dc power supply voltage Vcc is 5 volts and the resistance of the gate resis-tor 92 of the SCR 11 is 1 K~, the resistox 87 may have a resistance value of 33 KQ. Accordingly, even if there occurs a failure to render the transistor 85 continuously conductive (for exanlpler in the cases o~F an open-circuitiny failure of each of the transistor 52, the diode 64 and the resistor 79~ and a short-circuiting failure of the transistor 85), the gate of the SCR 11 is supplied only with a voltage not more than a division of the circuit dc power supply vol-tage Vcc by the resistors 87, 90 and 92, for example~
0.15 volt in this case. Since the appli~d voltaye does not exceed a non-triggering voltage VGD of the SCR 11, the SCR 11 is not triggered, and the supply oF electric power to the heater element 4 stops. This state is indicated by a period z in Fig. 8. The period z in FigO 8 is illustrated under the assumption that an open-circuiting failure has occurred in the transistor 52.
Fig. 7 shows a concrete construction of theSCR failure sensing ciruci~ 30. This circuit will be described making reference also to Fig. 9.
=aS~ -1 In Fig. 9, the period "a" deno-tes a state where the SCR 11 is normai but nonconductive, the per.iod "b"
denotes a state where the SCR 11 is normal and conductive, the period "c" denotes a state where a self~triggering failure has occurred in the SCR 11, and the period "d"
denotes a state ~here a short-circuiting failu.re has occur-- red in the SCR 11. In Fig. 7~ resistors 93 and 94, a transistor 95, diodes 96 and 97, a transistor 98, a constant current source 99, a resistor 100 and a transistor 101 constitute an electric circuit for confirming the turn-off of the SCR 11. During negative half cycles o the ac source voltage VAc , the transistors 95 and 98 are turned off and the txansistor 101 is turned on, so -that the collector voltage waveform VO~ of the transi.s~or 100 is at a low level. In the period "a" in Fig. 9 where the SCR
11 is nonconductive, during positive half cycles of the ac power source voltage VAc an input ac voltage VO rises as the ac power source voltage VAc rises, so -that the transistors 95 and 98 are turned on and the transistor 101 is turned off to raise its collector voltage VOO to a high level. Numeral 102 designates a flip-flop circuit, which is provided for storing the~ presence or absence o-f the zero~crossing pulse Vz3 delivered from the comparison circuit 28.
In the period "a" where the temperature of the heater element 4 is higher than the pr.eset temperature, the ~.ero~cross.iny pulse Vz3 is not produced and the flip-flop circuit 102 is kept reset by the inpu-t reset q,;~jv~
1 pu:Lse VN to maintain its output voltage VO at a low level.
Numeral 103 designates a NOR circuit, which produces an ou-tput voltage Vy of a high level when all of its input voltages are of a low level, but which does not deliver any output voltage of a high level during the period ~a~7 t since the pulse5 VD and/or VOo of a ~igh level are supplied to the ~OR circuit 103 during the pericd l'a"~
In the period "D" tr.e pulse Vz3 is produced to turn on the SC~ 11 when the temperature of the heater element 4 Ealls below the preset temperatureO The input voltage VO does not rise during positive halE cycles of the ac power source voltaye VAc , so that it follows that the trans:istors 95 and 98 remain nonconductive and the transistor 101 remain conductive to maintain its collector vol-tage VOO at a low level. On the other hand, the flip~
flop circuit ]02 is sel by the input set pulse Vz3 and maintains its output voltage VQ at a high level during positive half cycles of the ac voltage VAc. As a result high level inputs to the ~OR circuit 103 are maintair.ed by the pulses VD and/or VO to prevent a high level output voltage from being generated by the NOR circuit 103.
In this manner, in the normal operation, the generation of a high level output voltage by the NOR circuit 103 is prevented.
~5 In the period "c" where the SCR 11 conducts due -to lts self-triggering in spite of the absence of the pulse Vz3 , the Elip-flop circuit 102 main-tains its output voltage VQ at a low level because of the absence of an .... .. ... , a~l~ .
~ o l input set pulse thereto. As a result~ there occurs a period in positive half cycles of the ac voltage VA~ where all the inputs to the NOR circuit 103 take a low level to render the output voltage Vy o tle NOR circult 103 hiyh. Consequently, an SCR 104 is triggered to supply the heating resistor 31 with a heavy load which is 17 times as large as its rated load. Thus, the heating resistor 31 is heated to fuse the thermal fuse 32, thereby interrupt~
ing the supply of electric power to the heater element 4.
lQ In the period "d" where the SCR 11 conducts due to a short-circuiting failure in spite of the absence of the pulse Vz3 , the SCR 104 is triggered in -the same manner as ~he period "c", and the h~ating resistor 31 is heated to fuse the thermal fuse 32 so that the supply o~ electric power to the heater element 4 may be interrup-ted.
In Fig. 7, a diode 10S is provided to protect the transistor 95 and the diodes 96 and 97 against a high backward voltage which appears during negative half cycles of the ac power source voltage VAc. Numeral 106 designates a gate resistor for t'sle SCR 104.
~ .
The wire 3 has ~ construction such ~s shown in Fi~o 2 in ~ich the control ele~ent 6 is wound helic~
~ t~
1 on an insulating core 7. Also, the sensor 5 separates the heater element 4 from the control element 6. Reference numeral 8 denotes an insulating coating~ Practically, the wire 3 is arranged in a serpentine fashion inside an electric blan~et. The sensor 5 has a negative temperature coefficient of resistance and is usually formed by a plastic thermistor made of a thermosensiti~e material.
This sensor 5 has an impedance Z related to the -temperature T as shown in Fig. 3, wherein the impedance Z is a resultant value o a capacitive impedance ZC and a resistive impedance ~R~ and the temperature sensing property of the sensor 5 is greatly influenced by the capacitive impedance ZC at low temperat~lres and by the resistive impedance ZR
at hish temperatures. When a DC voltage is applied to a plastic thermistor, the plastic thermistor is polarized and deteriorated to increase its impedance, so that it is neces-sary to use a plastic thermistor under the application of an ac voltage which is as uniform as possible in its positive and negative polarities.
~hen, the conventional example sho~ in Fig. 1 will be explained in greater detail. In Fig. 1, a swi.ching element 11 (in this example, a semiconductor control device ~enerally known as an SCR is used) is connected in series with the heater element 4 between 25 lines 9 and 10 Similarly, resistors 12 and 13 are con-nec-ted in series therebetween. Between the junction of the resistors 12 and 13 and a gate of the SCR 11, a variable resistor 14, a resistor 15, the control element 6, 1 a diode 17 and a triggering elernent 1~ are connected in series. Arranged between the control element 6 and the heater element 4 is a capacitor 19~ In this automatic temperature control device, during positive half cycles of ~he ac source voltage in which the line 9 is at a posi~
tive po-tential relative to the line 10, the capacitor 19 is charged at a potential which is determined by the resistance values of the resistors 12 and 13, variable resistor 14) resistor 15 and sensor 5, and when the potential on the capacitor 19 reaches a breakdown voltage o:E the triggering element 18, a trigger pulse is applied to the gate of the SCR 11 so that the SCR 11 is turned on to supply electric power to the heater element 4.
The conduction phase angle of the SCR 11 becomes approxi-mately 0 when the heater element 4 is at low -temperature and hence the sensor 5 has a high impedance, thereby supplying maximum electric power to the heater element 4.
While, when the temperature of the heater element 4 is high and the impedance of the sensor 5 is low, the con-duction phase angle becomes approximately 90Q to decreasethe supply of electric power to the heater element 4.
In this way, the automatic temperature control is carried out. Since the phase angle is changed by the variable resistor 1~, the user can obtain a desired blanket temperature by selecting a suitable resistance value for the variable resistor 14.
However, the aforementioned automatic temper-ature control device has the following disadvantages.
1 Namely, since the clrcuit comprisiny the diode 17 is con-nected in parallel with the sensor 5, the ac voltage applied to the sensor 5 at positive half cycles is different from the ac voltage applied thereto at negative half cycles.
Consequently, the sensor 5 is polarized and deteriorated to increase its impedance-, which results in a danger such that the controlled temperature shifts to higher temperatures.
Further, in the event of a failure of the components used such as short-circuiting of the triggering element 18, the supply of electric power to the heater element 4 becomes uncontrollable still maintaining a maximum amount o:E electric power supply, thereby causing a danger such tha-t the heater element 4 is overheated. Additionally, a similar dznger takes place when the SCR 11 is short-circuited, or a self-triggering failure occurs in the SCR
11 in which the SCR 11 is turned on without being triggered by triggering pulses. Thus, it is possible that these dangerous conditions cause a human body injuring accident in the worst case, since an electric blanket is used by babies or aged persons who cannot push aside an overheated blanket by themselves. Further, the automatic temperature control device employing the phase angle control of an SCR has brought about an unfavourable condition to generate noises which cause interference, in particular, ~5 with a radio receiver while it is used by a user lying in his bed.
It is a first main object of this inven-tion to provide an automatic temperature control device which l comprises a heater elernent connected to an ac source, a senso.r for detecting -the temperature of the heate.r element, a teMperature responsive circuit including a grounded-base transistor with its base-emitter circuit connected in series wi-th the sensor and a diode connected across the base-emitter junction of the transistor with the PN junction of the diode and the base-emitter junction o:E the transis-tor connected inversely in parallel with each other, thereby using a collector current of the grounded-base transistor as a temperature signal, an electric potential setting circuit for producing an electric potential thereby to preset a desired temperature, a comparison circuit Eor comparing an output of the temperature responsive circuit with that of the electric potential setting circuit, ancl swltching means d:riven by an output of the comparison circuit to regulate electric power supplied -to the heater element, and which is featured to supply a uniform ac voltage to -the sensor so as to prevent the sensor from being polarized and assure its accurate temperature detection.
An object subsidiary to the first main object is to provide an automatic temperature control device which further comprises an integrating circuit for integrating the output of the temperature responsive circuit (the col-lector current of the grounded-base transistor) to produce an in-tegration output ~o be supplied to the comparison cir-cuit and stopping its integrating oper`ation during positive half cycles of the ac source voltage in which electric power is supplied to the heater element, thereby affording 5~
1 a s-table and accurate temperature signal.
Another object subsidiary to the first main object is to provide a safe automatic temperatuxe control device which further comprises a power supply interrupting circuit including a heating resistor connected in series with the series circuit of the sensor and the base-emit~er circuit of the transistor and a thermal fuse connected to the ac source which can be fused by the heat generated in the heat-lng resistor thereby to interrup-t electric power supply to the heater element, whereby, when the heater element is overhea-ted to cause the sensor to be melted ancl the heater element and the control element to be short--circuited to each other, the heating resistor is supplied with an over~
load which is 17 times as large as its rated load thereby to be heated to lnterrup-t electric power supply to the heater element.
It is a second main object of this invention to provide a highly safe automatic temperature control device which comprises a heater element connected -to an ac source9 a sensor for detecting the temperature oE the heater element, a temperature responsive circuit for detecting the temperature of the heater element by detect-ing an impedance of the sensor by means of the control element which is in contact with the sensor, an electric potential setting circuit for producing an electric - potential thereby to preset a desired temperature, a pulse supplying circuit. for producing a zero-crossing pulse, a pulse discriminating circuit for determining whe-tller ... .
1 an input signal thereto is a pulse and, as a result of the determination of the input pulse, producing an output pulse in phase with the input pulse, a comparison circuit for comparing an output of the temperature responsive circuit with that of the electric potential setting circuit in synchronism with the zero-crossing pulse and supplying a zero-crossing pulse to the pulse discriminating circuit when the temperature of the heater element is below a preset temperature level t ancl switching means triggered by an output of the pulse discriminating circuit to supply electric power to the heater element, whereby the switching means is triggered by the zero-crossing pulse to enable automatic temperature control with reduced noise genera-tion and the occurrence of any failure in the respective constituent circuits are checked in synchronism with the zero-crossing pulse so as to interrupt the supply of electric power to the heater element when the occurrence of a failure has been detected.
An object subsidiary to the second main object is to provide a highly safe automatic temperature control device wherein the comparison circuit comprises an electric circuit for fixing the output of the electric potential setting circuit to an electric potential level which is lower than a circuit dc power supply voltage by a pre-~5 determined magnitude whe:n a zero-crossing pulse does not occur so that the outputs of the temperature responsive circuit and the electrical potential setting circuit are compared with each other in synchronism wi-th input .. . .. ~ .
~ Q ~
1 zero-crossing pulses thereby to produce output zero-crossing pulses, and wherein, even when any failure occurs in the Olltput of the temperature responsive circuit and a false output signal appears representing that the temperature of the heater element is very low, it is possible to inter-rupt the supply of electric power to the heater element.
Another object subsidiary to the second main object is to provide a highly safe automatic temperature control device wherein an SCR is used as the switching means and there is provided an SCR failure sensing circuit for detecting a non-triggering conduction failure (self--triggering) of the SCR to interrupt the supply oE electric power to the heater element.
A further object subsidiary to the second main object is to provide an inexpensive and hiyhly safe auto-matic temperature control device wherein the pulse discriminating circuit is constituted by resistors, a capacitor and switching elements, and in the absence of input zero-crossing pulses the capacitor stores electric charge supplied from the circuit dc power supply through the resistors, and upon receipt of input zero-crossing pulses, the switching elements are turned on to discharge the stored electric charge to the switching means, thus preventing the application of triggering pulses -to the ~5 switching means when a failure occurs in the electric circuits of the device.
A still further object subsidiary to the second main object is to provide a highly safe automatic temperature 1 con-trol device which further compri.ses a disconnection sensing circuit for preventing zero-crossing pulses from being applied to the pulse discriminating circuit, when breakage of the control element occurs, by causing the output of the temperature responsive circui-t -to be shifted apparently to the side of a lower temperature s-tate and to stay there upon occurrence of such breakac3e of the control element, thereby preventing the temperature under control from becoming high.
A yet further object subsidiary to the second main object is to provide a highly safe automatic temper-ature control. device wherein a plurality of electric circui-ts such as the disconnection sensing circuit, the comparison circuit, etc. are connected so tha-t zero-crossing pulses are successively transmitted through the circuits and output pulses from the last one o:E the circui-ts are applied to the pulse discriminating circuit as inpu-t signals thereto whereby a failure of circuit components in the transmission path of zero-crossing pulses can be checked without requiring additional components for use in the checking and the supply of electric power to the heater component can be in-terrupted when a failure occurs in the circuits.
Reference is firstly made to a block diagram of Fig. 4. For a better understanding of the present inventionl reference should also be ma,de to Figs. 8 and 9. In Fig. 4, a voltage VAc of an ac source 1 is applied to a circuit of a diode 20, a resistor 21 and a capacitor ,. " . ?~ q ~
1 22 by closi.ng a switch 2 to produce a dc vol-tage Vcc to be used as a dc power supply for the circuits A temper-ature responsive circuit as designated by a block 23 detects a current IS resulting from the amplification of an electric current which flows through a ground line of the ac source 1, the temperature responsive circuit 23, a heating resistor 33, the control element 6, the sensor 5, the heater element 4 and the other supply line of the ac source 1 with its magnitude varying with the change of the impedance of the sensor 5, during negative half cycles of -the voltage VAc of the ac source 1, and gives rise to the simultaneous flow of a current IT which is substantially identical with the current IS. Since the sensor 5 is capacitive, this current IS leads in phase by about 90 as shown in Fig. 8 and overlaps, in phase, positive half cycles of the ac source voltage in which an SCR 11 is turned on, so that the waveforrn of the current IS is distorted by the influence of a potential gradient generated by -the heater element 4. Maturally, the current I5 is not distorted when the SCR 11 has been turned offO Therefore, the magnitude of the current IS has great dependency upon the presence or absence of the distortion/ resulting in undesirable variations in the relation between the temper-ature of the heater element 4 and the maynitude of the ~5 current I5. An integrating circuit, such as designated by a block ~4, is adapted to integrate the current IT and produce an output voltage VI in inverse proportion to the temperature of the heater element 4. F'or the reasons set ~ 3~ 3 1 forth above, the integra-ting circuit 24 integrates the current IT which has excluded a region of the current IS
where the distor-tion appears. As the temperature of the heater element 4 rises, the impedance of the sensor 5 decreases, resulting in an increased currerlt IT and a decreased voltage VI. An electric potential setting ci.rcuit as designated by a block 25 is adapted to produce a potential Vs The potential Vs is manually variable and may be adjusted by the user to obtain an optimum con-trolled temperature. A block 26 denotes a pulse supplyingcircuit which produces various pulses V0, Vzl, Vp and VN
.in synchronlsm with the ac source voltage VAc The pulse Vzl stands for zero-crossing pulses which are generated at zero-crossing points through which the waveform of the ac source voltage VAc transfers from negative halE cycles to positive half cycles.
A block 27 denotes a disconnection sensing circuit adapted to detect disconnection or breakage o~ a control element 6 and which produces a voltage V~ to be applied to the control element 6 in synchronism with the zero-crossing pulses Vzl and further produces other zero-crossing pulses Vz2 in synchronism with the zero-crossing pulses Vzl if the control element Ç is not disconnected nor broken, the.reby allowing an electric current to flow there-through. Wit~out the disconnection sensing circuit 27,a decrease in the current IS flowing through the sensor S
upon occurrence of disconnection or breakage of the control elemen-t 6 would give rise to a signal indicative .. --~ 1~ - -P
1 of a decreased temperature of the heater element 4, there-by causing -the temperature under control to rise danger-ously in excess of a preset temperature~
A btock 28 denotes ~ comparison circuit which~
in the absence of the zero-crossing pulses Vz~ app:Lied thereto, fixes the output voltage Vs from the electric potential setting circuit 25 at a level of (V~c ~ VF = VsF, where VF is a constant forward voltage drop of a diode), and which, in the presence of the zero-crossing pulses V~2 from the disconnection sensing circuit 27 r releases the Eixed level in synchronism with the reception of the zero-crossing pulses and compares the output vo]tage V
from the i.ntegrating circuit 24, which is in inverse proportiorl to the temperature of the heater element 4, with a voltaye VST which is the value of the output voltage Vs from the electric potential settiny circuit ~5, at this ti~e, thus producing the other zero-crossing pulses Vz3 in phase with the zero-crossing pulses Vz2 when the detected temperature is below a preset temperature level.
A block 29 denotes a pulse discriminating circuit which, upon receipt of the zero-crossing pulses Vz3 fxom the comparison circuit 28 I produces other zero-crossing pulses Vz4 in phase with the zero-crossing pulses Vz3 which pulses Vz4 are applied to the gate of the SCR 11 to trigger it and supply electric power to the heater element 4. When the detected tempera~ure exceeds a preset temperature level or when a failure occurs in the pulse supplying circuit 26, disconnection sensing circ~1it 27 or . . . . ~, ~,. _ , ... .
cormparison clrcul-t 28, no input pulse Vz3 is app:Lied to the pulse cliscrim:inat:ing circul-t 29, and hence there is no out-put pulse Vz,l from the pulse discrimina-ting c.ircult 29, t~-lereby maintainirlg the SCR 11 untr~ggered. Wherl a failure occurs in Ihe pulse supplying clrcuit 26, disconnection sen-sing circuit 27 or comparison circuit 28, so thdt, a con-tinuous input signal Vz3 is appli,ed to the pulse discriminat-ting circuit 29, the ou-tput signcll Vz4 of t~le pulse ~,scri-mlnating circuit 29 is reduced -to a level (practically not more than 0.2 volts) which is so low as to fail to -trigger the SCR 11. Thus, the SCR 11 is not. triggered also in this case. F'urther, the pulse discriminating circuit 29 itself is so designed that i-ts output signal Vz4 disappears or is reducec1 to a low level which is insu:Eficient to tr.igger the SCR 11 when a failure occurs in the pulse discr,imirlating circuit 29 itself.
With the above-descr:ibed construction, the zero-crossing pulses delivered from the pulse supplylng circuit 26 a.re transmltted successively through the disconnection sensing circuit 27 and comparison circui-t 28, whose sae operation has to be assured, and the final output signal causes the pulse discriminating circuit 29 to tri.gger the SCR :L1. In this manner, the respec-tive circuits confirm the occurrence of no failure -therein along with the accom-plishment of their proper functions while the ze:ro-crossing pulses are generated. Thus, it is possible to accomplish -the two functions simultaneously and instantaneously to ensure that tlle circuits can be kept ~ .... . . . ~n 1 in highly safe conditions, yet without requi.ring any great increase o~ circuits or componen-ts for assuring a safe operation of the device. Even when there are involved o-ther circuits, whose safe operatlon has to be assured, in 5 acldition to the two electric circuits of -the disconnection sensing circuit 27 and comparison circuit 28 as e~emplified in the above construction, the aforernentioned safety system can readily assure a highly safe operation of the device~ Therefore, it can be said that this safety system has a wide field for its application.
A block 30 denotes an SCR failure sensing circuit~ When it is detected that the SCR 11 is turned on in spite of the absence of the zero-crossing pulses Vz3 from the comparison circuit 28~ the SCR failure sensing ci.rcuit 30 applies a heavy load to a hea-ting resistor 31 whlch load is 17 times as high as a rated load of the heating resistor 31 by making use of outpu-t pulses VN and VD from the pulse supplying circuit 26, so that the heating resistor 31 is heated to cause a thermal. fuse 32 to be fused, thereby interrupting the supply of electric power to the heater element 4. Accordingly, even if the SCR 11 has been short-circuited or has brought about self-triggering thereof, the supply of electric power to -the heater element 4 can be also interruptedO
A wixe 3 has a function -to detect abnormal heating when abnormal heat is generated accidentally in the wire 3. Namely, when the heater elemen-t 4 is abnormally heated until the temperature therearound reaches .. .. . . . . ... ... . . .. . .
t~ jt-`~
1 the rnelting point (167C) of the sensor 5 made oF a plastic thermistor~ the con-trol element 6 is brough-t into short-cirrui-ting contact with the heater elemen-t 4 due to a -tensile s-tress in -the control element 6 remaining from the time o~ lts coiling. Then, the heating resistor 33 is supplied with the ac source voltage 'JAC thereby to be heated by a heavy load current which is 17 times as large as its rated load current. As a result, the thermal fuse 32 is fused to interrupt the supply of electric power to the heater element 4.
In -the foregoing, a description has been made of the overall construc-tion of the safety system of the automatic temperature control devlce according to the invention. The construction of each of the specific circuits employed in the system will then be described in greater detail.
Fig. 5 shows details of the pulse supplying circuit 26 in which the ac source voltage VAc is con-ver-ted into an ac voltage VAc' whose phase is slightly delayed~ as shown in Fig. 8, through a fil-ter constituted by a resistor 34 and a capacitor 35, and the ac voltage VAc' is supplied to a point 36. When the ac voltage VAc' exceeds a reference voltage VF determined by a constant current source 37 and a diode 38 during a rising positive half cycle, a comparator 39 is inverted to produce an output voltage Vp. Thereafter, when the ac vol-tage V~c' continues to increase and reaches a level of the sum of the reference voltage VF and a base-emitter 1 forward drop VB~ o~ a transistor 40, transistors 40 and 41 are tur~ed on to generate an output pulse VD which falls ~o gl-ound potential. According:Ly, the falling point of VD is necessarily delayed from the rising point of Vp, whereas -the rising point of VD :Ls necessarily in advance oE the falling point of Vp. As the ac voltage VAc' enters a negative half cycle, a transistor 43 is turned on, thereby turning off transistors 46 and 47, which have been conductive by being fed from constant current sources 44 and 45, and simultaneously generating an output pulse VN which falls to ground potentialO Reference numeral 49 denotes a base-emitter resistor Eor the tran-sistor 47.
By applying the thus produced pulses Vp and VN
to set and reset terminals of a flip-flop 50, respectively, and then a Q output of the flip-flop 50 and the pulse VN
to a NOR circuit 51, the zero-crossing pulse Vzl is produced from the output terminal of the NOR circuit 51.
A description has hereinabove been made of the ac voltage VAc' whose phase is slightly delayed from that of the ac source voltage VAc ~ the pulse VN which rises during negative half cycles of the ac voltage VAc', the pulse Vp which rises during positive half cycles of the ac voltage VAc', and the pulse VD which falls during 2~ positi.ve hal~ cycles of the ac voltage VAc' and rlses during the positive peri.od of the pulse Vp.
ReEerring to Fig. 6 which shows i.n detail the essential struc-ture of the present invention, details of 3f~D l~
1 the temperature xesponsive circuit ~3, integrating circuit 24, electric potential setting circuit 25, disconnection sensiny circuit 27, comparison circuit 28~ and pulse dis-criminating circuit ~9 will be describedO For giving a better understanding of the present invention, reference should also be made to Fig. 8.
The temperature responsive circuit 23 comprises a grounded base transistor 52 and a diode 53 which are connected in antiparallel with the base-emitter circui-t of the transistor 52 such that the PN junction of the diode 53 and the base-emitter junc-tion of the transistor 52 are connected inversely in parallel with each other. During positive half cycles o~ the ac source voltage V~c ~ the supply of electric power to the heater element 4 is controlled, and during negative half cycles of -the ac source voltage VAc , the impedance of the sensor S is detected as an equivalent of the temperature of -the heater element 4. The current IS flowing through the sensor S
during negative half cycles of the ac source vol-tage VAc is equal to the emitter current of the transistor 52.
In Fig. 8, a period designated by x is one in which electric power is supplied to the heater element 4 and a period designated by y is one in which the supply of electric power to the heater element 4 s-tops. Since ~5 the sensor 5 has a capacitive impedance as shown in Fig. 3/ the phase of the sensor current IS leacls that of the ac source voltage VAc by about 90, so that a portion of the sensor current IS overlaps the period for 17 ~
q~
1 controlling the supply o~ electric power to the heater element 4. As a result, an electric potential. gradient caused in the heater element 4 when electric power is supplied to the heater element 4 affects the sensor S curren-t IS to distor-t the waveform of the overlapping portion of the sensor current IS as shown .in FigO 8.
Naturally, the distortion disappears within the period y in Fig. 8. During positive half cycles of the ac source voltage V~ , the current IS flowing through the sensor 5 passes the diode 53. Since the magnitude of the forward voltage of the diode 53 and the base-emitter ~ol-tage of the transistor 52 are substantially equal -to each other and further either one of the above-mentioned voltages is ~ar smaller than the voltage VAc , the sensor S is fed wi-th a symmetrical ac voltage, whereby the promotion of the deterioration of the sensor 5 by the polarization effect is suppressed. The heating resistor 33 is impressed wi-th -the ac source voltage VAc when the heater element 4 is heated -to reach an abnormally high temperature at which 20 the sensor 5 is melted and the heater element 4 is brought into short-circuiting contact with the control element 6, whereby the hea-ting resistor 33 is heated to fuse the thermal fuse 32~ The resistance (390 Q in this example) of the heating resistor 33 is much smaller than the resistance of the sensor S, so that the insertion oX the resistor 33 does not cause an error in`the sensor current S' The collector current IT, whose magnitude is 1 substantially equal to that of the sensor currert Is) flows through -the collector of the transistor 52.. However, a distortion would appear or disappear in the sensor current IS depending on the supply or the stoppage of supply of electric power to -the heater element 4, thus .resulting in -the corresponding fluctuation of the magnitude of the sensor current I~ or the collector current IT. In order to rernove the above-described disadvantage, the integrat-ing circuit 24 is provided to cut the distortion component of tne collector current IT and to produce a dc signal indicative of the temperature under control by integrating the collector current IT whose distortion component has heen removed.
The integrating circuit 24 comprises a capacitor ~/L5 54 for integrating the col~ctor current IT and a discharge resisto.r 55 for gradually discharging electric charge on the capacitor 54. A transistor.56 is provided to cut the distortion component. This transistor 56 staying in the conductive state under the control of a constant current source 58 is turned off to cut the distortion component when a transistor 57 connected in the base circuit of the transistor 56 is turned on by the pulse Vp which rises during positive half cycles of the ac source voltage VAc.
A diode 59 is provided to assure the turn-ofE oE the transistor 56 when the transistor 57 has been turned on.
A description will be given~of the relation between the ternperature of the heater element 4 and the output voltage VI of the integrating circuit 24 brought ... . - ~ q ~ ' ' ' 1 by the above-mentioned construction. As the temperature of tne heater element 4 increases, the impedance of the sensor 5 decreases and the sensor current IS and the col-lector currenc IT increase. Consequently, the lntegration voltage of the capacitor 54 increases and the output voltage VI of the integrating circuit 24 decreases. On the other hand, as the temperature oE the heater element 4 decreases, the above-mentioned situation is reversed and the outpu-t voltage VI of the integrating circuit 24 increases. Thus, the output voltage VI oE the integrating circuit 24 varies in inverse proportion to the temperature o-f the heater element 4.
Nex~, the electric potential setting circuit 25 has a series circuit of the resistor 60, variable resi.stor 61 and resistor 62 connected across the circuit dc power supply Vcc ~ and its output potential Vs is taken out :Erom a junction of the resistor 60 and varia~le resistor 61. The resistance of the variable resistor 61 may be varied by the user to adjust the outpu-t potential Vs o-E the electric potential setting circui-t 25.
An explanation will be given hereunder of the disconnection sensing circuit 27. The disconnection sensing circuit 27 comprises a transistor 68 which is turned on by being fed with a constant current :Erom a 2S constant current source 63 via diodes 64, 65, 66 and 67 when the zero-crossing pulse Vzl is not supplied to the base o~ a transistor 70. A resistor 69 is a base-emitter resistor .for the transistor 68. When the zero-crossi.ng 1 pulse Vzl is applied to the base of the transistor ~0, the transistor 70 ~s turned on, which causes a transistor 74 t.o be turned cn through a resistor 71 and a current m~rror circuit constituted by a diode 72 and a transistox 73 to produce an output potential Vx at the emi-tter output o:E the transistor 74. When the control element 6 is not disconnected llor broken, a current is supplied from the constant current sOUrce 63 to flow through the control element 6 and simultaneously to cause the transistor 68 to be turned off, -thus producing the zero-crossing pulse Vz2 a-t the co:Llector of the -transistor 68. If the control e~ement 6 is disconnected or brolcen, a current cannot flow through the control element 6 even though the transistor 7~ is turned on, so that -the transistor 68 continues to lS be conductive and hence no zero-crossing pu:Lse Vz2 is genexatedO Reference numeral 75 designates a base~
emitter resistor for the transistor 7~. Diodes 76~ 77 and 78 are provided to protect the transistor 74 from a surge voltage. The diodes 54, 65, 66 and 67 are provided 20 to assure the turn-off of the transistor 68 when the t.ransis-tor 74 is turned on.
As will be seen from the above description, when the control element 6 is not disconnected nor broken, -the zero-crossing pulse ~Z2 is produced in phase with the zero-crossing pulse Vzl when the zero-crossing pul.se V
i5 applied to t:he base of the transistor 7a.
The comparison circuit 28 will now be explainedO
The comparison circuit 28 is adapted to compare -the output .. .. ~ . , . ... . , .. , . . .... . . ~, ~ .
1 poten-tial VI from the integrating circuit 24 with the output poten-tlal Vs from the electric potential setting circuit 250 In the absence of the zero-crossing pu ! se Vz2 from the disconnection sensing circuit ~7, the transi.stor b8 remains conductive, and a txansistor 80 also stays in the con-ductivP state with its base current drawn through a resistor 79, thereby ensuring that the output potential Vs from the electric potential setting circuit 25 is fixed to a level VsF which is lower than the circuit dc power supply voltage Vcc by the forward voltage drop VF of a diode 81. Accordingly, the relation of Vs > Vl holds between the output potential V~ of the electric potential se-tting circuit 25 and the output potential VI o~ the integrating cirGuit 24, which renders tne output level of the comparator 82 low. When the zero-crossing pulse Vz2 is produced by the turn-off of the transistor 68, the ~ransis~or 80 is turned off to release the ou-tput potential VS of the electric potential setting circuit 25 from its fixed potential level to compare the output potential V
of the integrating circuit 24 with the preset value VsT
of Vs provided by the electric potential setting circuit 250 In this comparison, during the period x sho~l in Fig. 8 where the temperature of the heater element 4 remains lower than the preset temperature, every time when -the relation ~ Vs ~ VI becomes satisfied, the output of the comparator 82 turns to a high level to thereby produce the ~ero crossing pulse Vz3~ On the other hand, during the period y during which the temperature of the heater element 4 .... ~ . . .. ., .. .. . ....... ., . .. , .. ~ . . . . ... . .. . . . .
P ~
1 remains higher than the preset temperature, the relation o Vs ~ VI holds to maintain the ou-tput of the comparator 82 at a low level, thereby preventing the generation of the zero-crossing pulse Vz3~ Rererence numeral 83 denotes a base-emitter resistor for the transistor 80~ The diode 81 fulfils a useful purpose as described helowO When a failure occurs in the tempera-ture responsive circuit 23 or -the integrating circuit 24, namely, when an open-circuit failure occurs in the transistor 52, for example, the ou-tput potential VI of the integrating circuit 24 becomes equal to the circuit dc power supply voltage V
Assuming that the diode 81 is not provided, the output po-tential Vs of the potential setti.ng circuit 25, which has been .Eixed to a level VsF lower than the circuit dc lS power supply voltase Vcc by the emitter-coll.ector satura-tion voltage VcE5a~ of the transistor 80 when the transistor 80 is conductive, would be released to its non-fixed potential level VsT in synchronism with the generation of the zero~crossing pulse Vz2. Therefore, normally the relation of VI > Vs is maintainecl rendering the output le~rel of the comparator 82 high, thus preventing the output zero~crossing pulse Vz3 from being generated by the comparison cixcuit 28. It should be noted that the comparator 82 has an input offset voltage VI0. Accordi~gly, when the relat:ion of VIo > (VI - Vs) holds, the output level of -the comparator 82 becomes low when the transistor 80 is turned on~ As a result the zero-crossing pulse Vz3 is continuously produced from the comparison circuit 28, ... .. ~ 1~
1 resulting in the dangerous overheating of the heater el.ement ~. However, owing to the provision of the diode 81, it is possible to have the potential Vs lowexed to the level VsT further by the magnitude of the forward voltage drop V~ acros.s the diode 81 even when the transistor 80 is conducti~e, thereby eliminating the adverse affect by the input offset voltage of the comparator 82.
Next, an explanation will be given o the pulse discriminating circu~t 29. When the pulse dlscriminating circui~ 29 does not receive the zero-crossiny pulse Vz3 from the comparison circuit 28 (i.e., when the output level of the comparator 82 is low), transistors ~4 and 85 are nonconductive and a capacitor 86 is gradually charged from -the circuit dc power supply voltage Vcc through a resistor 87. Upon receipt of the zero-crossing pulse Vz3 from the comparison circuit 28 by the base of the trans i.stor 84, the transistor 84 is rendered conduc-tive -through a resistor 88 and the transistor 85 is also rendered conduc-tive through a resistor 89. As a result, electric charge stored in the capacitor 86 discharges forming the zero-crossing pulse Vz4 through the translstor 85 and resistor 90 to the gate of the SCR 11. Reference numeral 91 denotes a base-emitter resistor for the transistor 85, and reference numeral 92 a gate resistor ~5 for the SCR 11~
The pulse discriminating circuit 29 makes use of a specific property of the instantaneously rising pulse waveform of the zero~crOssing pulses at the 1 zero-crossing points thereby to effect pulse discrimination.
- ~ In this example, since the ratio of an occurrence period to a non-occurrence perlod of the zero-crossing pulses is taken to b~ 2 or more, even when the resistor 87 has a considerably large resistance value, it is possible to produce at æero-crossing points the pulse Vz4 of a magnitude sufficient to trigger the SCR 11. As an example in this casPj when th~ circuit dc power supply voltage Vcc is 5 volts and the resistance of the gate resis-tor 92 of the SCR 11 is 1 K~, the resistox 87 may have a resistance value of 33 KQ. Accordingly, even if there occurs a failure to render the transistor 85 continuously conductive (for exanlpler in the cases o~F an open-circuitiny failure of each of the transistor 52, the diode 64 and the resistor 79~ and a short-circuiting failure of the transistor 85), the gate of the SCR 11 is supplied only with a voltage not more than a division of the circuit dc power supply vol-tage Vcc by the resistors 87, 90 and 92, for example~
0.15 volt in this case. Since the appli~d voltaye does not exceed a non-triggering voltage VGD of the SCR 11, the SCR 11 is not triggered, and the supply oF electric power to the heater element 4 stops. This state is indicated by a period z in Fig. 8. The period z in FigO 8 is illustrated under the assumption that an open-circuiting failure has occurred in the transistor 52.
Fig. 7 shows a concrete construction of theSCR failure sensing ciruci~ 30. This circuit will be described making reference also to Fig. 9.
=aS~ -1 In Fig. 9, the period "a" deno-tes a state where the SCR 11 is normai but nonconductive, the per.iod "b"
denotes a state where the SCR 11 is normal and conductive, the period "c" denotes a state where a self~triggering failure has occurred in the SCR 11, and the period "d"
denotes a state ~here a short-circuiting failu.re has occur-- red in the SCR 11. In Fig. 7~ resistors 93 and 94, a transistor 95, diodes 96 and 97, a transistor 98, a constant current source 99, a resistor 100 and a transistor 101 constitute an electric circuit for confirming the turn-off of the SCR 11. During negative half cycles o the ac source voltage VAc , the transistors 95 and 98 are turned off and the txansistor 101 is turned on, so -that the collector voltage waveform VO~ of the transi.s~or 100 is at a low level. In the period "a" in Fig. 9 where the SCR
11 is nonconductive, during positive half cycles of the ac power source voltage VAc an input ac voltage VO rises as the ac power source voltage VAc rises, so -that the transistors 95 and 98 are turned on and the transistor 101 is turned off to raise its collector voltage VOO to a high level. Numeral 102 designates a flip-flop circuit, which is provided for storing the~ presence or absence o-f the zero~crossing pulse Vz3 delivered from the comparison circuit 28.
In the period "a" where the temperature of the heater element 4 is higher than the pr.eset temperature, the ~.ero~cross.iny pulse Vz3 is not produced and the flip-flop circuit 102 is kept reset by the inpu-t reset q,;~jv~
1 pu:Lse VN to maintain its output voltage VO at a low level.
Numeral 103 designates a NOR circuit, which produces an ou-tput voltage Vy of a high level when all of its input voltages are of a low level, but which does not deliver any output voltage of a high level during the period ~a~7 t since the pulse5 VD and/or VOo of a ~igh level are supplied to the ~OR circuit 103 during the pericd l'a"~
In the period "D" tr.e pulse Vz3 is produced to turn on the SC~ 11 when the temperature of the heater element 4 Ealls below the preset temperatureO The input voltage VO does not rise during positive halE cycles of the ac power source voltaye VAc , so that it follows that the trans:istors 95 and 98 remain nonconductive and the transistor 101 remain conductive to maintain its collector vol-tage VOO at a low level. On the other hand, the flip~
flop circuit ]02 is sel by the input set pulse Vz3 and maintains its output voltage VQ at a high level during positive half cycles of the ac voltage VAc. As a result high level inputs to the ~OR circuit 103 are maintair.ed by the pulses VD and/or VO to prevent a high level output voltage from being generated by the NOR circuit 103.
In this manner, in the normal operation, the generation of a high level output voltage by the NOR circuit 103 is prevented.
~5 In the period "c" where the SCR 11 conducts due -to lts self-triggering in spite of the absence of the pulse Vz3 , the Elip-flop circuit 102 main-tains its output voltage VQ at a low level because of the absence of an .... .. ... , a~l~ .
~ o l input set pulse thereto. As a result~ there occurs a period in positive half cycles of the ac voltage VA~ where all the inputs to the NOR circuit 103 take a low level to render the output voltage Vy o tle NOR circult 103 hiyh. Consequently, an SCR 104 is triggered to supply the heating resistor 31 with a heavy load which is 17 times as large as its rated load. Thus, the heating resistor 31 is heated to fuse the thermal fuse 32, thereby interrupt~
ing the supply of electric power to the heater element 4.
lQ In the period "d" where the SCR 11 conducts due to a short-circuiting failure in spite of the absence of the pulse Vz3 , the SCR 104 is triggered in -the same manner as ~he period "c", and the h~ating resistor 31 is heated to fuse the thermal fuse 32 so that the supply o~ electric power to the heater element 4 may be interrup-ted.
In Fig. 7, a diode 10S is provided to protect the transistor 95 and the diodes 96 and 97 against a high backward voltage which appears during negative half cycles of the ac power source voltage VAc. Numeral 106 designates a gate resistor for t'sle SCR 104.
Claims (7)
1. An automatic temperature control device for an electric appliance comprising: a heater element connected to an alternating current source to be fed therefrom; a sensor made of a thermosensitive material having an impedance, which changes as the temperature varies, for detecting the temp-erature of said heater element; a temperature responsive circuit or detecting the temperature of said heater element by detecting the impedance of said sensor; an electric potential setting circuit for producing an electric potential for setting a desired temperature; a pulse supplying circuit for producing a zero-crossing pulse; a pulse discriminating circuit for determining whether an input signal thereto is a pulse and, as a result of the determination that an input signal pulse, is for producing an output pulse in phase with the input pulse; a comparison circuit for comparing an output signal of said temperature responsive circuit with a an output signal of said electric potential setting circuit in synchronism with said zero-crossing pulse and producing another zero-crossing pulse when the temperature of said heater element is below a preset temperature, the output signal of said comparison circuit being supplied as an input signal to said pulse discriminating circuit; and switching means triggered by the output pulse from said pulse discrimin-ating circuit for regulating the supply of electric power to said heater element.
2. An automatic temperature control device ac-cording to claim 1, further comprising at least one control element cooperating with said sensor for supplying a temp-erature sensing electric current which varies depending on the impedance of said sensor, wherein the thermosensitive material of said sensor is interposed between said heater element and said control element, said heater element, said sensor and said control element composing a wire.
3. An automatic temperature control device according to claim 1, wherein said comparison circuit com-prises a circuit for fixing the output level of said elec-tric potential setting circuit to an electric potential which is lower than a circuit dc power supply voltage by a pre-determined magnitude during a period in which the zero-crossing pulse does not occur.
4. An automatic temperature control device ac-cording to claim 1, wherein said switching means is an SCR, and an SCR failure sensing circuit is provided to detect the conduction of the SCR, when the SCR is not triggered, and thereby to stop the supply of electric power to said heater element.
5. An automatic temperature control device ac-cording to claim 1, wherein said pulse discriminating circuit comprises resistors, a capacitor and switching elements and in the absence of the input zero-crossing pulse the capacitor stores electric charge supplied from a circuit dc power supply through a resistor, and upon a receipt of the zero-crossing pulse, the switching elements are turned on to discharge the electric charge stored in the capacitor to said switching means.
6. An automatic temperature control device ac-cording to claim 2, further comprising a disconnection sens-ing circuit for preventing the zero-crossing pulse from being applied to said pulse discriminating circuit when a breakage failure occurs in said control element,
7. An automatic temperature control device ac-cording to claim 6, wherein a plurality of electronic cir-cuits including said disconnection sensing circuit, and said comparison circuit, are connected so that the zero-crossing pulse is successively transmitted through the electric circuits, and a final output pulse from the electric circuit is applied to said pulse discriminating circuit.
Applications Claiming Priority (7)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
JP73064/80 | 1980-05-30 | ||
JP73063/80 | 1980-05-30 | ||
JP7306380A JPS56168231A (en) | 1980-05-30 | 1980-05-30 | Safety circuit |
JP7306480A JPS56168232A (en) | 1980-05-30 | 1980-05-30 | Temperature controlling circuit |
JP10610280A JPS5731013A (en) | 1980-07-31 | 1980-07-31 | Temperature control device |
JP106102/80 | 1980-07-31 | ||
CA000378514A CA1183925A (en) | 1980-05-30 | 1981-05-28 | Automatic temperature control device for an electric appliance such as an electric blanket |
Related Parent Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
CA000378514A Division CA1183925A (en) | 1980-05-30 | 1981-05-28 | Automatic temperature control device for an electric appliance such as an electric blanket |
Publications (1)
Publication Number | Publication Date |
---|---|
CA1187967A true CA1187967A (en) | 1985-05-28 |
Family
ID=27426306
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
CA000444604A Expired CA1187967A (en) | 1980-05-30 | 1984-01-03 | Automatic temperature control device for an electric appliance such as an electric blanket |
Country Status (1)
Country | Link |
---|---|
CA (1) | CA1187967A (en) |
-
1984
- 1984-01-03 CA CA000444604A patent/CA1187967A/en not_active Expired
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