GB2148633A - Improvements in or relating to electrical circuits - Google Patents

Abstract

An electric blanket comprises an electrical heating coil element R3, and a resistor/capacitor network (R1, R2/C1, C2) controls potentials on the trigger (G) of a thyristor (D). If heating coil R3 or its connections should suffer a break, the thyristor switch D will go into its non- conducting mode to open the circuit. If the broken ends of the heating coil R3 or its connections should touch and remake the break, the thyristor switch D will stay non-conducting, because C1 is still full charged. Hence no arcing can occur at the break. The application of supply power to the heating coil R3 cannot be re-established until isolating switch S is first opened and then re-closed. On the supply being reconnected, the pulse of current recharging capacitor C1 causes a potential to be established across resistor R1, thus charging capacitor C2 to a voltage sufficient to gate switch D to its conducting state once again. <IMAGE>

Description

SPECIFICATION Improvements in or relating to electrical circuits BACKGROUND TO THE INVENTION This invention relates to electrical circuits and is concerned with circuits employing electrical heating coils, and to connections to such coils.

One common form of electrical circuit employing a heating coil is an electric blanket.

It is already appreciated that electric blankets introduce a fire risk which can arise from loose or broken connections. This risk occurs as a result of arc creation. Typically, a broken connection has its ends in close proximity and, being in a flexible medium, these ends can repeatedly reconnect and disconnect power with consequent arcing. The arcing tends to carbonise the insulating medium in which the coils or connections are located and this in turn can initiate an electrical fire. The risk of broken connections is generally accepted as being highest where a flexible cord to a blanket enters the blanket.

The present invention has as its object to remove the risk of an electrical fire arising from a breakage in an electrical heating coil or in a connection thereto.

SUMMARY OF THE INVENTION According to the invention, an electrical circuit comprising at least one electrical heating coil and a control switch, is characterised in that the switch is triggered to close, whereby the switch opens the circuit if the circuit becomes broken, the trigger of the switch having a regulating means operable to close the switch when power is supplied and the circuit is unbroken, and to cause the switch to be uncloseable when power is supplied and the circuit is broken.

The switch is preferably a thyristor and the trigger regulating means preferably comprise a resistor/capacitor network which controls the potentials on the trigger of the thyristor so as to cause either the thyristor to conduct (switch closed) or to prevent the thyristor from conducting (switch open). An indicator lamp may be provided to give visual warning of unsafe failure (e.g.

conduction without triggering) of the thyristor.

BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, wherein Figures 1 to 5 illustrated electric blanket control circuits with monitoring means, all in accordance with the invention.

In the figures, like reference numerals refer to like components. (Typical values of components in the circuits are listed at the end of the specification).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference to Fig. 1, an electrical circuit 10 of an electric blanket comprises an electrical heating coil element R3 and a control switch D. The electric blanket is supplied with current from a power source AC via a full wave rectifier BR so as to provide an unsmoothed DC source, indicated by the conventional positive (+) and negative (-) signs. The circuit 10 is also provided with a two-pole on/off switch S and a fuse F.

The control switch D comprises a thyristor having a gate or trigger electrode G which has its potential controlled by series connected capacitors C1 and C2. Capacitor C1 is shunted by a resistor R2 (relatively high) and capacitor C2 by a resistor RI (relatively low). Control switch D is connected in series to the heating coil R3. The resistors R1 and R2 are connected in parallel with capacitors C2, Cl.

The resistor/capacitor network (R1, R2/C1, C2) comprises regulating means which controls potentials on the trigger (G2) of the thyristor (D) so as to cause either the thyristor to conduct (switch closed) or to prevent the thyristor from conducting (switch open).

On closing isolating switch S, the capacitor C1 commences charging via resistor Ri, and the potential drop across Ri charges up C2. The potential drop across resistor R1 applies a potential to the gate G of the thyristor switch Dto turn it into its conducting mode. Thus the heating coil R3 is connected with the power supply. As capacitor C1 approaches the fully charged state the current in resistor R1 approaches to near zero. Thus the potential applied to gate G will be near to zero. The thyristor switch D however will stay in its fully conducting state until there is an interruption in the current flow.

If heating coil R3 or its connections should suffer a break, then the current will be interrupted and the thyristor switch D will go into its non-conducting mode to open the circuit.

If the broken ends of the heating coil R3 or its connections should touch and remake the break, the thyristor switch 0 will stay non-connecting. The reason why switch D is caused to be uncloseable is because Cl is still fully charged. Thus there is not sufficient potential on gate G to turn "on" the thyristor switch D. Hence no arcing can occur at the break.

The application of supply power to the heating coil R3 cannot be re-established until isolating switch S is first opened, (allowing capacitor C1 to discharge via resistor R2), and then re-closed.

On supply being reconnected, the pulse of current recharging capacitor Cl causes a potential to be established across resistor R1, thus charging capacitor C2 to a voltage sufficient to gate thyristor switch D to its conducting state once again. (Providing that there are no breaks in heating coil R3 and its connections).

If the power supply is disconnected and then reconnected, for example, as a result of checking the wiring at the supply plug, this will have the same effect as opening and closing the thyristor control switch S.

It is possible to monitor the thyristor switch D in a sense to detect the unsafe failure situation in which it conducts without being first gated. This situation means loss of arc protection and Fig. 2 illustrates an electrical circuit 10a provided with a visual warning system comprising illuminatable means. These means comprise a neon indicator N, and a resistor R4 connected thereto. It will be noted that the position of the switch S has been charged relative to the position shown in Fig. 1.

The operation of the monitor facility included in Fig. 2 will be considered in various possible situations: Situation 1. AC power on; thyristor switch D not conducting; switch S open. The neon N is not illuminated.

Situation 2. AC power on; switch S open and thyristor switch D closed (due to a failure situation in which a short circuit between the cathode and anode of the thyristor exists). The neon N is illuminated in a circuit comprising resistor R4, indicator N and switch D, indicating that there is a fault.

Switch S could be replaced by a thermostat or other form of energy regulating switch.

With reference to Fig. 3, the negative ( - vc) terminal of the converted supply DC of an electrical circuit lOb is connected to the thyristor switch D, and thence to the inner (heating) coil R3 of a dual coil electric blanket heating element having thermoplastic insulation I and an outer coil R30 provided for monitoring purposes. Connection is then made to a heater element El of a thermal switch S1, via the contacts of a second normally closed thermal switch S2, and the contacts of the switch Si. The switch S2 has a heater element E2. That part of the circuit 10b which includes the inner heating coil R3 can be regarded as the main heating portion of the circuit.The cyclic on/off ratio of switch S1 can be adjusted by changing the setting of an adjustable resistor VR 1, by some mechanical device in the switch itself, or both.

The insulation I and outer coil R30 provide sensing or monitoring means associated with substantially all points on the inner heating coil R3 whereby a signal from R3O is derived which is related to the temperature of coil R3. Controller means Q1 (referred to below) responsive to said signal are provided for controlling the flow of current through the thermal switch E1/S1 in a sense to stabilise said temperature.

At intermediate settings various degrees of cyclic on/off control of the blanket heating element can be effected. Typically at a mid-setting, the contacts of switch S1 would desirably be closed for two minutes and open for two minutes.

The circuit provides a variable shunt path for the heater element El of thermal switch S1, both the shunt path and the element El drawing current from the heating coil R3. This path consists of a transistor Olin series with the heater element E2 of switch S2. The conductivity of this shunt path is controlled by a signal at the base of the transistor Q1 by the charging of a capacitor C3 which is responsive to currents leaking through the insulation I into coil R30. The adjustable resistor VR 1 is provided in the charging path of C3 together with current limiting resistors R5 and R7. Current limiting resistors R6 and VR2 are provided in series with Or 1.

If the main heating circuit of R3: E?: S2: S1 were to be used without the other components in the circuit, then El would heat up to open the contacts of switch Si. There would then be a cooling period terminated with switch S1 closing followed by another heating of El. That is, there would be bursts of heating at R3 each followed by a period of no heating. This would give rise to a specific mean blanket temperature. By changing the resistance of El the difference between the on/off periods could be changed and the blanket mean temperature changed.

However, such a simple arrangement would lack safety protection.

The presence of the shunt E2/Q1 allows control of the amount of current passing through El. The base of Q 1 is connected to R30 by way of resistor R5 and then to the variable 'resistor VRl via limiting resistor R7. The base of Ol is also connected with the capacitor C3 and hence VRl acts to control the potential C3 reaches and the amount which Ol conducts and shunts El.

A typical adjustment of VR 1 is such that, in the cold state of the blanket, 0 1 is nearly fully conducting. Under these conditions, and with switches S, S1 and S2 closed, the blanket represented by R3will heat up but El will not heat rapidly due to the shunting effect of Q1 and E2.

As the blanket heats up, a leakage occurs through the insulation / which results in a lowering of the potential on the "R3Oside" of C3 thus causing the transistor Q1 to conduct less (i.e.

effectively the shunt path of El increases in resistance) and hence more current passes through El which tends to open switch Si to prevent the temperature of the blanket (R3) rising. As the temperature of the blanket rises, so the insulation I increases its conductance, Ol decreases its conductance, and El heats more to open switch S1.

The circuit described above provides a settable self-regulated main heating circuit (R3 and El in series with switch S1) having added circuit elements which sense the heating element (R3) temperature and modifies the regulation of the main heating circuit to lower the power supplied to the heating element as the temperature of the heating element increases and vice versa. The increase in the conductance of the insulation I, which brings this modification of regulation into effect, can arise with a localised temperature increase of the heating element or an "all-over" increase.

The switch S2 and its thermal resistor E2 provides protection in the event that transistor a 1 short circuits. If this happened, then El would appear parallel with E2, so that the selfregulation effect of El would be lost as E2 shunts it. This would cause blanket overheating which now is safeguarded against by heavier current passing through E2 (with the shorting of Q 1) and opening of switch 52, which causes a break in the circuit, resulting in thyristor switch D becoming non-conductive. When S2 closes again, no current will flow in the circuit T1, T2, E2, R3 and D as D remains non-conductive, until such time as switch S is opened and closed.

A break in the circuit which includes R5, R30, R7 and VR1 takes Ol into a state of high impedance. This causes El to take more current whereby switch S1 opens.

The transistor Q1 could be replaced bt thyristor switch means.

The circuit is arranged so that with the supply connected when switch S is closed, (thermal switches S1 and S2 being in their normally closed coid state), capacitor C1 charges up. This causes a potential across R1 which is sufficient to enable the charge on C2 to reach the gate turn on voltage of the thyristor D. Most of the charge on C2 becomes lost via Ri and C1 reaches a steady state. Therefore, if S2 opens or there is a break in the circuit comprising El, R3 and their connections, thyristor switch D will stop conducting, and will remain in the nonconducting state, even if S2 recloses or the break is remade.However, if switch S is closed, and there are no breaks in the circuit El, R3 and their connections, when switch S is closed, the thyristor D will be triggered into its conducting state. This allows current to flow through the heating resistance R3.

When S1 opens, C1 discharges via R2, and on S1 reclosing thyristor switch D is turned on again. (Providing S2 is closed, and there are no breaks in El, R3 and their connections).

If the thyristor switch D suffers a short circuit between its anode and cathode, then the arc protection would be inoperative. The indicator circuit was devised to give some warning of this.

Thus: (1) With switch S open and thyristor D in its correct, i.e. non-conducting state, current flows through resistor R4, neon N and diode D2; the neon N is illuminated.

(2) With switch Sopen and thyristor D suffering a short circuit between its anode and cathode, current normally flowing through the neon N is diverted through diode D1; thus the neon N is not illuminated.

In Figs. 4 and 5 there are shown electric circuits 1 or, 1 Od respectively. Each uses a dual heating coil R3, R3O (as in Fig. 3) and single heating coils R8 and R9. A switch S10 provides three heat positions (1, 2 and 3) as well as an off position (0). The difference between Figures 4 and 5 are the connections made to the heating coils R8 and R9. The arrangement of Fig. 5 provides protection on all three heats. The arrangement of Fig. 4 does not.

A circuit according to the invention can be viewed as being divided into two parts, namely a protective part and a control part. Any breakage in a heating coil (or connection thereto) in the protective part will result in the control switch opening the circuit. On the other hand, any breakage in the control part of the circuit will not cause the control switch to open.

The control part of the circuit allows the use of such components as multi-position switches, (in order to obtain different heat outputs), and thermostats. To provide a single-heat electric blanket with adequate protection is relatively straightforward. To provide a multi-heat blanket with similar protection is not.

The protective part of the circuit is particularly advantageous where electric blankets are provided with simple in-line exterior plug and socket connectors. Often components of such connectors become loose whereby arcing can result, leading to an electrical fire.

In the case of the present invention, should loosening of the connector components occur, the control switch will open the circuit. The fault in the connector can then be rectified, with the circuit remaining open until the electrical supply is first switched "off" and then "on" once more. The circuit will then operate normally.

Approximate values of the more important components shown in the various figures are as follows: Ri (Figs. 1 to 5) -10 to 200 ohms C1 (Figs. 1 to 5 ---0.1 uF R2(Figs. 1 to 5) -10 M ohms C2(Figs. 1 to 5) -0.1 uF R3(Figs. 1, 2 s 3) - 480 ohms C3(Fig. 3) -2.2 uF El (Fig. 3) -30 ohms R3 (Figs. 4 s 5) -300 ohms E2 (Fig. 3) -10 ohms R4 (Figs. 2 8 3) -27 K ohms VR1 (Fig. 3) -10 K ohms R5 (Fig. 3) -1.0 M ohms VR2 (Fig. 3) -1.0 M ohms R6(Fig. 3) -10 Kohms Ol (Fig. 3) -Type BFY-51 R7(Fig. 3) -700 Kohms D (Figs. 1 to 5) -Type 106.D R8 (Figs. 4 s 5) 400 ohms BR (Figs. 1 to 5) -1.0 Amp R9 (Figs. 4 s 5) 100 ohms bridge rectifier R30(Fig. 3) -500 ohms F(Figs. 1, 2 8 3)500 M.A.

R30(Figs. 4 s 5) -300 ohms F(Figs. 4 8 5) 400 M.A.

Claims (11)

1. An electrical circuit comprising at least one electrical heating coil and a control switch, characterised in that the switch is triggered to open the circuit if the circuit becomes broken, the trigger of the switch having regulating means operable to close the switch when power is supplied and the circuit is unbroken, and to cause the switch to be uncloseable when power is supplied and the circuit is broken.

2. A circuit as claimed in Claim 1, wherein the switch is a thyristor and the regulating means is a resistor/capacitor network which controls potentials on the trigger of the thyristor so as to cause either the thyristor to conduct or to prevent the thyristor from conducting.

3. A circuit as claimed in Claim 2, wherein the network comprises series connected capacitors with respective parallel resistors, one of which is of relatively higher resistance than the other, disposed so that the capacitor in parallel with the higher resistance is capable of retaining its charge to prevent the thyristor from conducting when the circuit is broken and is capable of discharging through said higher resistance when power is no longer supplied to the circuit.

4. A circuit as claimed in any preceding claim, wherein it is provided with visual warning means of unsafe failure of the switch.

5. An electrical circuit as claimed in any one of Claims 1 to 4, wherein the heating coils are operable to provide a multi-heat output, and triggering of the control switch only functions on the higher heat settings of the coils where arcing at a break in the circuit can constitute a danger.

6. An electric blanket comprising an electrical circuit as claimed in any one of Claims 1 to 5.

7. An electrical circuit substantially as hereinbefore described with reference to Fig. 1 of the accompanying drawings.

8. An electrical circuit substantially as hereinbefore described with reference to Fig. 2 of the accompanying drawings.

9. An electrical circuit substantially as hereinbefore described with reference to Fig. 3 of the accompanying drawings.

10. An electrical circuit substantially as hereinbefore described with reference to Fig. 4 of the accompanying drawings.

11. An electrical circuit substantially as hereinbefore described with reference to Fig. 5 of the accompanying drawings.

1 2. Every novel feature and every novel combination of features disclosed herein.