CA2000954A1 - Shock-proof mains voltage supply outlet and method - Google Patents

Abstract

ABSTRACT

An improved mains outlet and method of operating the same that automatically and safely distinguishes between the conditions of human or animal contact with the outlet terminals and contact with appliances such as light bulbs and consumer products, to prevent any substantial voltage or power from being drawn in the former case and automatically applied substantially full mains voltage when the appliance is connected to the outlet.

Description

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SHOC~-PROOF MAINS ~OLTAGE SUPPLY OUTL~T AND MET~OD

The presènt invention relates to mains voltage supply outlets used to connect movable appliances to the source of power. This includes those outlets of the type used ln residences, offices, businesses, hotels and in public places, to power household and other appliances ranglng from lamps to vacuum cleaners, heaters, toasters, hair dryers and similar devices; and is more particularly directed to insuring the safety of such outlet from electrical shock by the inadvertent or misguided purposeful touching of the outlet terminals by children, animals or adults, as may be encountered through insertion of paper clips or nails or even small fingers into outlet apertures and into contact wlth the metal terminals of the outlet.
Numerous devices have been evolved over the years for mitigating against such dangers including the current use of fi.Yed or rnovable plastic lnserts to cover the outlet apertures and mechanical on-off switches -- both requiring human operation or control. Illustrative of prior approaches or atternpts at improved plugs and the like are 2~ s~

U.S. Letters Patent Nos. 3,169,239; 3,368,110; 3,441,799;

3,706,008; 3,864,~81; 3,909,566; 4,o80,640; 4,175,255;

4,306,374; 4,484,1~5; 4,584,430; and 4,722,021. Most of these devices show methods of disconnecting power from the appliance ln case of overload or describe mechanical devices to prevent inadvertent contact with the source of power.
Underlying the present invention, however, is the concept of employing appropriate electronic circuits cormected between the mains supply lines and the outlet terminals that~ in effect, respond automatically to the lmpedance presented between the outlet terminals unambiguously to distinguish between a condition where the human body is connected thereto and the condition where an electrical appliance that is to be powered ls so connected -- insuring automatically that no or only a trivial amount of voltage or power is available in the former case, and ~ubstantlally full power is connected to the appliance in the latter instance.
An obJect of the inventlon, accordingly, is to provide a new and improved mains outlet and method of operating the same that obviate the above-described problems and automatically and ~afely distinguish between the condition~

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of human or animal contact with the outlet and appliance contact therewlth to prevent any substantial voltage or po~er being drawrl in the former case and to apply substantially full mains voltage ln the latter condition.
Other and further obJects will be explained hereinaEter and are more fully dellneated ln the appended claims.
In summary, from one of its broader aspects, the invention embraces a method of rendering an outlet connected to a mains power supply safe from shock upon human touching of the outlet terminals, that comprises, sensing the impedance presented between said outlet terminals;
responding to said sensing to apply no or very small and "6afe" amounts of voltage and power from said supply for sensed impedance values corresponding to the relatively high impedance presented by the human body, wet or dry; and responding to said sensing to apply substantially full supply voltage for sensed impedance values corresponding to the relatively low impedance presented by appliances.
Preferred and best mode appara~us and details are hereinafter presented.
The inventlon will now be described with reference to the accompanying dra~ings~ Fig. 1 of whlch is a simpllfied s~

circuit diagram of a part of a circuit useful for the purposes of the lnvention and which is exemplary to explain the principles underlying the invention;
Figs. 2 and 4 are similar circuit diagrams of modifications; and Flg. 3 ls a preferred implementation Or the inven-tlon.
Referrlng to Fi6. 1, some Or the principles underlying the invention will be e~plalned in simplifled fashion, even though the circuit of ~ig. 1 is not commercially adapted for operation without additional refinements as later explained.
In thls Fig. 1, there is shown a transistor 10 which has its base biased to, for example, +5 volts by resistor 4, which may, for example, be of the order of 250,000 ohms and a resistor 2 whlch may have a much lesser value of the order of 12,500 ohms, thus reducing the voltage at the base of the transistor to about the said ~5 volts. The emitter of this transistor is biased by resist,or 6, which may also have a value of the order of 250,000 ohms, and resistor 8, which may be of the order of 250,000 ohms~ resulting in an emitter voltage of about +10 volts. Thus the transistor 10 cannot conduct and the voltage at the terminals 7 and 5, which represent the outlet, will remaln at a triv~al value of s~

ab~5 l~/ v~ts. 5~ long as the trans1stor lO does not conduct, the ma~imum current from the malns supply lA and lB, such as the 110-115 volt or 220 volt supplies used ~7 throughout the world, which can flow to terminals is of the !, order of a totally safe 0.40 milliamperes. When the external resistance applled to termlnals 5 and 7 o~ the outlet ls reduced far below the resistance values of the human body, becomes less than about, say, 500 ohms, the emltter voltage falls below the base voltage and the transistor 10 wlll conduct. There ls then the classlcal case of an emitter follower operation, wherein the voltage across the outlet terminals 5 and 7 will be equal to the 5-volt base voltage appearing in transistor lO.
The above explains the basic operation of the electronic swltching system of the circuit interposed between the mains supply llnes lA and lB and the outlet terminals 7 and 5, though in actual practice, non-llnear devices must be incorporated to produce full power at the termlnals 5 and 7 when the impedance presented across the outlet terminals 5 and 7 is less than a relatively low value, of at most a few hundred ohms as prescribed by an appliance.

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In accordance with the invention, the outlet 5-7 ls rendered safe from shock upon human touchlng of the outlet terminals 5 and 7 through the interposition of the electronlc impedance or resistance sensing and switching circuit such a~ that in Fig. 1 lnterposed bètween the outlet terminals 5 and 7 and the power supply llnes lA and lB. The electronic switching device 10 of the circuit, a~ above indicated, is biased to permit no or a trivial and safe amount of voltage and thu~ power to be passed through the circuit from the power supply lines lA and lB to the outlet terminals 5 and 7 when the impedance between the terminals 5 and 7 is relatively high, say of the order of the impedance of the human body when fingers, wet or dry, are touched to ths terminals 5 and 7, thereby to prevent any shock. This is somewhat analogous to the trivial voltage applied to toy electric railroad tracks and trains which has long been recognized as a totally safe condition, even for children touching the terminals. As prevlously indlcated, these relatively hlgh impedances have been measured to be of the order of from hundreds of thousands of ohms to several megohm~, depending upon the wet or dry condition of the fingers or the portion of the body that becomes connected s~

between the outlet plug terminals 5 and 7. When, however~ a load impedance is sensed across the terminals 5 and 7 that is relatlvely low compared to the above, say, in practice, for lamps and similar electrlcal devices o~ the order of a few hundred ohms and less, the above-mentioned condition of conduction of the transistor 10 takes place and the mains voltage is then applied with substantially full power available to energize the appliance that has been plugged into the outlet as previously described.
The clrcult of Fig. 1 has been described in simplifled form ln connection with the positive cycles of the supply voltage applied at the lines lA and lB. In order not to lose the energy of the negative half-cycles, thls circuit may be combined with a complementary clrcuit~as shown in Fig. 2 to accommodate for the negatlve half-cycles as well.
If a circuit which is symmetrical to ground is desired, the circuit of Fig. 4 containing transistors 30A, 30B, 30C and 30D and corresponding resistor networks 32A through D, 34A
through D, 36A and 36B and 38A and 38B may be employed.
In preferred ~orm, however, the ~witching devices can a~sume the form o~ trlacs and diac~ whlch are not polarity sensitive and therefore there is no need to provide multlple 9~

clrcuits such as shown in Fig. 2 to take care of the condition of both the positive and negative polarity half-cycles. Furthermore, mains plug outlets are normally wired with a ground and a hot conductor, at least in the United States~ and there is no need to provide a balanced output in such instances.
The precedlng has descrlbed the translstor clrcuits in which the resistor 6 of Fig. 1, for example, wlll dellver very small current lnto the hlgh impedance load of the human _ _ _ , _ body touching the outlet and in which the transistor 10 in switching to conduction will deliver ~dditional current of the type necessary to enable powering of a low output impedance device, ~uch as appliance, when it ls applied -to the outlet terminals 5 and 7. The addltlonal current thus applied to the low impedance device is limited because, in the emitter-follower circuit of Fig. 1, the current in the transistor cannot rise beyond the point where the voltage acros3 the emitter circuit equals the voltage at the base.
Thus, ~n a commercially ùseful system, to develop effective power in the output clrcult for po~ering the deslred appliance when plugged into the outlet termlnals 5 and 7, the transistors are supplemented with or preferably replaced 35~

by non-linear devices such as the triac and diac type electronic switching illustrated in Fig. 3. Referring to that figure, the triac 110 replaces the transistor 10 of Fig. 1. The triac has terminals 112, 114 and 116. In the data sheets these are generally referred to respectively as "Main Terminal-l" (112)~ "~ain Terminal-2" (114) and "Gate"
(116). Critical voltages are the voltages occurring between terminals 112 and 116. The triac, of course, is a device which in its idle condition has a high lmpedance between lts main terminals 112 and 114, and which can be switched to a low impedance device by applying a voltage pulse of a magnitude which exceeds the voltage at the terminal 112 and causes the switching of the triac. This conversion will take place with supply voltage of either polarlty.
A voltage divider consisting of resistors 102 and 104 is installed at the gate side and another voltage dlvider consisting of resistors 106 and 108 is installed in the "mains terminal" side of the circuit. The Junc-tion of resistors 106 and 108 is connected to the "Maln Terminal-l"
(112), and to the output terminal 12~ of the outlet shown at 122. The ~unction of resistors 102 and 104 is connected to the gate 116 of the triac serially through a diac 120, having the property that lt presents a very hlgh lmpedance to currents of either polarity until the voltage across the same has reached a certain threshhold; and then, when that voltage is exceeded, the dlac becomes ~ a very low impedance device. When the voltage across the diac is reduced to elther zero or to a very low value, it reconverts into a high impedance device As,a result of these properties, the diac prevents any significant amount of current fro~ entering the triac gate 116 untll the time that the voltage appearing at the ~unction o~ resistors 102 and 104 is sensed to be sufficiently high to trigger the triac to its switched conducting mode. A condenser 118 is connected across resistor 104 and the energy stored in the condenser at the time of triggering is applied through diac 120 to the gate 116 of the triac and helps to insure positive triggering o~ the triac. Because of the external capacities, and specifically because of the capaclty between the terminal~ and the gate, a very ~hort duration spike on the power supply can, in some instances, cause a spike to appear on the gate terminal 116 and such could cause a spurious triggering o~ the triac. Insertlon of resistor 130, whlch is a low re~istance value~ prevents such spikes 2~ S~

from a~fectlng triac operation. If desired~ a protectlve fuse 128, as shown, may be inserted in the power supply.
The operatlon of the clrcult shown in Flg~ 3 is as follows: so long as a hlgh impedance which may conslst o~
parts of the human body is sensed at the output terminals 124 and 126 of the outlet, or when there ls inflnite impeda~ce or the human body or some portlon thereof is .~ . .. . ~
connected across those terminals, the voltages appearing at the gate 116 are equal to or lower than the voltage which appears on the "Termlnal-l" (112) and the triac cannot conduct. Suitable numerical examples of voltages are shown in the flgure. The resistance between the two hands of an adult person measure to be, say, about 500,000 ohms when test leads of a conventional analyzer are squeezed between salt water the moistened thumb and index finger of each hand, resulting in an equivalent resistance between the terminals 124 and 126 of about 43,000 ohms. As a result of the presence of resistance 130 there occurs a voltage of about llc4 volts at "Terminal-l" (112) and at the gate. In thi~ case the triac will not conduct. In contrast with thi~, when a 25-watt ligh-t bulb is plugged into the outlet termlnal~, a resistance to ground at terminal 112 and the 5a~

gate 116 of about 400 ohms is sensed. ~t this point the voltage across resistor 108 is momentarily reduced to a fraction of a volt. This triggers the diac which in turn triggers the triac which thereafter shows a voltage drop of a fraction of a volt.
The resisLance-capacitance values and voltage appear-ing in the figure are approximate vaLues to those used and observed in experimental apparatus.
Referring to the before-described Fig. 4, a furt11er transistor version embodying a "symmetrical to ground"
output is shown. The transistors in this circuit can be replaced by non-linear devices such as those described in connection ~ith Fig. 3.
In practice, it is to be understood that these cir-cuits may be formed on chips or in very small packages and made integral with the outlet plug itself or can be made in the form of kits to be added to existing mains outlets terminals, extension cords, etc.
Current sensing may also be used to determine the impedance presented at the outlet terminals instead oE
voltge sensing (dividing) as herein lllustrated and which is par~icularly useful for the use oE motors and other inductive loads introducing phase shif~s.

12a The saee outLet circuit Oe the invention may also be interposed between conventional ground fault inLerruptor-ground neutral detector systems (for example, the type in "Industrial Blocks," p. 9-100 of the current National Semi-Conductor Application ~landbook) and the appliance or load ouLlet, usLng the outlet Oe the invention for the appliance, to render such sysLems safe from leakage current and o~her unsaFe conditions that may occ-lr therein.
~ urLher moclifications will occur to Lhose skilled in this art and such are considered to fall within the spiriL
and scope of Lhe lnvent:lon as defined in the appended claims.

Claims (11)

1. A method of rendering an outlet connected to a mains power supply safe from shock upon human touching of the outlet terminals, that comprises, sensing the impedance presented between said terminals;
responding to said sensing to apply substantially no or trivial totally safe voltage and power from said supply for sensed impedance values corresponding to the relatively high impedance presented by the human body, wet or dry, or by animals; and responding to said sensing to apply substantially full supply voltage for sensed impedance values corresponding to the relatively low impedance presented by appliances.

2. A method of rendering an outlet connected to a mains power supply safe from inflicting shock upon human touching of the plug terminals, that comprises, interposing an impedance-sensitive electronic circuit between the terminals and the power supply lines, said circuit having electronic switching means; biasing said electronic switching means to permit no or a trivial safe amount of voltage and power to be passed through the circuit from the power supply lines to the outlet terminals when impedances well above several hundred ohms are connected thereacross, as when human fingers, wet or dry, touch the terminals; and adjusting said electronic switching means to become substantially fully conductive to apply substantially full voltage from the supply line to the said terminals when a load impedance less than several hundred ohms, as from electrical appliances, is connected across said terminals.

3. An outlet for applying voltage from mains power supply lines to the outlet terminals having, in combination, an electronic circuit interposed between the supply lines and the outlet terminals, and comprising electronic switching means, means for biasing said electronic switching means to prevent any substantial voltage or power from being applied to said outlet terminals when the impedance presented thereacross is of the relatively high impedance values presented by the human body, and means automatically operable upon the presenting between said outlet terminals of the relatively low impedance values of appliances for causing the switching means to apply thereto substantially full voltage from said supply lines.

4. An outlet as claimed in claim 3 and in which said relatively high impedance values include values of the order of hundreds to thousands of kilohms and said relatively low impedance values include values of the order of hundreds of ohms and less.

5. An outlet as claimed in claim 4 and in which said electronic switching means comprise serially connected triac and diac devices.

6. An outlet as claimed in claim 4 and in which said electronic switching means comprise complementary SCR or transistor devices.

7. An outlet as claimed in claim 1 and in which the impedance at the outlet is determined by voltage sensing.

8. An outlet as claimed in claim 1 and in which the impedance at the outlet is determined by current sensing.

9. An outlet as claimed in claim 1 and in which the trivial safe voltage is of the order of 10-20 volts and the resulting current drawn therefrom by the relative high impedance presented to the outlet terminals is of a milliampere or a few milliamperes.

10. An outlet as claimed in claim 9 connected to a ground fault detector system.

11. An outlet as claimed in claim 10 connected to the output of said ground fault detector system, with the output terminals of said outlet connected to the load such as an applicance.