US2624825A - Temperature control circuit - Google Patents

Description

5 2 1 I l l l IIJ 8 n vw m m V., 2, f L r w e oo n n tr r C T r mmm I ||.|I Ig w P .s R1 n m. ijf O H LLl @new o. wmp. y R01 .D con um@ .Td GMM mm m T Jan. e, 1953 Patented Jan. 6, 1953 TEMPERATURE CONTROL CIRCUIT George C. Crowley, Stratford, Conn., assignor to General Electric Company, a corporation o'f New York Application April 11, 1951, Serial No.220,440

(Cl. E19-24)) 3 Claims. l

vMy invention relates to temperature responsive Vcontrol circuits and more particularly to such control circuits for use with electric blankets and the like.

Heretofore, various control circuits have been developed for providing over-temperature protection for electric blankets and like heating devices, several ci these circuits being disclosed in 'my copending application, Serial No. 91,402, now Patent 2,565,478, patented August 28, 1951, entitled Temperature Responsive Control Circuit, iiled May 4, 1949, and owned by the General Electric Company, assignee of Vthe present invention. In the circuits 'shown therein, the operation of .the automatic circuit breaking means is controlled by a temperature sensitive structure which contains a pair of conductors having between them a thin layer of material which is substantially an insulator at one temperature and at a predetermined high temperature Vis a conductor of electricity of conventional domestic voltage and frequency. Upon overheating of the blanket, the thin layer of material passes current from one conductor to the other, and thereby causes operation of thecircuit opening means. The temperature sensitive structure itself is disclosed and claimed in the copending application or" Spooner et al., Serial No. 91,396, entitled Thermosensitive Devices and Apparatus Incorporating the Same, iiled May 4, 1949, now Patent 2,581,212, patented January 1, 1952, and also assigned to the General Electric Company.

Although the circuits disclosed in my aforementioned application, 'Serial No. 91,402, have, on the whole, given excellent results when used with electric blankets, they do not make quite enough allowance for a mistake made by certain vinexperienced persons in operating the blankets.

More specifically, although the control circuits must, of cou-rse, operate when only a portion of the blanket is overheated as well as when the entire blanket is too hot, these inexperienced persons often do not realize that danger exists when only a portion of the blanket is overheated, and they attempt to re-energize the heating circuit by jamming closed the on switch before the overheated portion of the blanket is allowed to cool to the temperature at which it may operate safely.

In a number of the aforementioned control circuits, the automatic 'circuit opening means consist of the contacts of an electromagnetic .relay.and Ait has been found that the jamming Vclosed .of kthe on switchl yupon an Aoverheat conr:dition results rapid .opening and clcsing, c.,

chattering, of this relay. Naturally, the chattering is objectionable both because kit causes-'severe wear of the relay vand its contacts 'and also b'ecause'the blanket is energized during the periods that the relay is closed. Moreover, the vchattering is extremely discomforting to the owne'rfof the blanket, often convincing him that there is something wrong with the control 4circuit or with the blanket itself. This conclusion is ra particularly undesirable one 'because ordinarily overheating of a portion of the blanket does not indicate a circuit failure, 'since any portion of the blanket may overheat merely because there is a tight fold or tuck formed vin it which prevents the normal dissipation of heat.

1t is a general object of my invention, therefore, to provide a new and improved control circuit ior use with electric blankets and the like, and it is a more specific object of my invention to provide such a control circuit in which the jamming closed of the on switch upon an overheat condition does not .result in chattering lof the control relay.

llihe features of my invention which vI believe to be novel are pointed out with particularity in the appended claims. The invention, itself, however, both as to organization and mode of operation, together with further objects )and advantages thereof, may be bestunderstood by reference to the following description to be taken in conjunction with the accompanying V drawing which is a schematic diagram of Va control 'circuit embodying my invention. l

Referring to the diagram, .one 4conductor or heater resistor I of a combined heat generating and thermosensitive element 2 is separatedfrom the second conductor or signal wire 3 of the velefment by a thin layer (not shown) of one of 'the organic materials disclosed in the aforementioned Spooner et al. application, Serial No. 91,396, a synthetic polyamide resin, commonly called nylon, being preferable. The thin layer of organic material is lessentiallyan insulator at normal operation temperatures of elementi and is a conductor of electricity at predetermined higher temperatures. The combined heating and therm-osensitive element 2 may be arranged inside a blanket 4 in any one of the suitable-patterns well known to the art and may be joined through a multi-terminal plug and socket 5 to a control circuit which is housed in the 'control box 6. A plug 1 provides the supply terminals for connecting the control circuit to a source of power, for example, a 'conventional'fl-l lvolt, IBi) cycle, A. C. domestic circuit.

The control circuit performs a dual function. Firstly, it serves to regulate the current supply to the blanket load or heating circuit, which is comprised of resistor I, and secondly, it serves to completely cie-energize the load circuit upon the overheating of any portion of the blanket. Thus, resistor l, which, for example, may be divided into two parallel-connected sections having an over-all resistance of approximately 60 ohms, is connected to one side of the power source through a cycling control 8 and to the other side of the source through the contacts 9 of a control device, such as electromagnetic relay I0.

Cycling control 8 regulates the blanket ternperature under normal operating conditions and preferably is of the room temperature-responsive type of control described and claimed in the U. S. patent to William K. Kearsley, 2,195,958, April 2, 1940. It accomplishes this temperature regulation through a cycling bimetallic switch member l Irwhich alternately opens and closes the heating circuit in response to the heat supplied to it by a series resistance I2 and the surrounding atmosphere. In order that adjustment of the control may be made to obtain various blanket temperatures, an external regulating knob I3 is provided to act on switch member i I However, as mentioned above, in addition to the normal temperature regulation of the blanket, the control circuit also provides over-temperature protection to completely de-energize the blanket circuit upon any dangerous overheating in the blanket. It is by means of relay I that such protection is obtained. The operating coil Id or relay I0 is connected to be energized across oppo- -the power source or line but capacitor I'I and resistor I5 are connected to the load side of a switch I0 which is biased to the open position and manually operable to the closed position. The other side of switch I9 is joined directly to the opposite side of the line.

Connected in parallel with or across switch I3, as far as the resonant circuit is concerned, is a series circuit consisting of contacts 9 and a 12,000

ohm resistor 20. However, with respect to the heating circuit, it may be said that contacts 9 are in parallel with the series combination of switch i9 and resistor 20. In any case, the closing of contacts 9 serves to energize the heating circuit directly and to energize the impedance bridge through resistor 20. Also connected to be energized directly by the closing of contacts 5l is an operation indicating circuit consisting of a 200,000 resistor 2| and a neon glow-lamp 22 serially connected. As its name intimates, this circuit is added merely to provide a means for indicating to the user whether the blanket is in operation or not.

Means for de-energizing the blanket circuits at the option of the user is provided by a switch 23 which, like switch I9, is biased to the open position and manually operable to the closed position. Although, as will become apparent hereinafter, there are various ways inwhiob. swll. 23, may be connected into the circuit, preferably it is connected between the load side of switch I0 and some point in the resonant circuit so that when closed, it forms a parallel path around a portion of the circuit eiective to disturb the circuit resonance. Thus, as shown in the diagram, one side of switch 23 is joined to switch I9 and the other side is joined to the junction of choke I8 and signal wire 3.

Also connected to this junction point, which actually is one of the aforementioned opposite terminals of the impedance bridge, is the relay operating coil Id. The other point to which coil I is connected, i. e., the other bridge terminal, is the junction of resistors I5 and I6. With the other circuit components being of the magnitudes given, the impedance presented between the bridge terminals by coil Is should be approximately 90,000 ohms with the relay armature closed, and the construction of relay I0 should be such that it operates to close contacts 9 with approximately 115 volts applied to the operating coil lil and drops out to open the contacts when the coil voltage falls to a value someplace between -75 volts depending on the relay calibration. It must be understood, however, that these numerical values, as Well as those given for the various resistances and impedances, are for a circuit to be energized from the standard volt, 60 cycle, A.-C. domestic voltage source. Different voltage sources, of course, would require different values for the circuit components.

To energize the circuit, assuming that the thermostatic cycling control 8 is calling for heat,- switch i9 is closed to connect the impedance bridge directly across the line. Resistors I5 and I0 act as voltage dividers to cause the voltage appearing at the one terminal of the bridge to be approximately half the line voltage. The voltage at the other terminal of the bridge is, however,

considerably higher due to the effect of the resonant circuit; in other words, the voltage drop across choke i8 is much greater than the voltage drop across resistor I6. In fact, the voltage across choke I8 rises considerably above line voltage so that with a line voltage of 115 volts, a voltage of approximately volts appears across coil I4. This causes relay I0 to operate and close contacts 9 placing the heating circuit directly across the line.

As soon as relay I0 operates, switch I9 may be released since the impedance bridge is energized through contacts 9 and resistor 20 to keep relay I0 locked in the closed position. The addition or resistor 20 to the bridge circuit limits the current in the resonant circuit and the voltage divider circuit, but under normal conditions does not diminish the voltage across coil I4 enough for the relay to drop out. This is true since the voltage falls on at both the bridge terminals, in the case of the terminal between resistors I5 and I6 falling to about 1/3 of the line voltage. Thus, assuming that the entire circuit is in good working order, once relay I0 operates, the heating circuit will remain in operation dependent upon the cycling of control 8.

To de-energize the circuit, i. e., turn ofi the blanket, switch 23 is closed. The closing of switch 23 effectively shorts out capacitor I'l and disturbs the resonance of the normally resonant circuit. This causes the voltage across'choke I3 to drop appreciably, while the voltage across coil I6 remains the same, approximately 1/3 the line voltage. The voltage between the bridge terminals therefore decreases to a value insuiiicient to keep relay fIJ locked in, and the relay drops out opening contacts 9 `and*de-energizing the entire circuit. Obviously, switch '23 maybe placed at any point in the circuit where, by its closing, it will reduce the voltage between the bridge terminals enough to allow relay It) to drop-out.

Means are also provided for an automatic circuit opening if the blanket overheats to a predetermined cut-oif temperature, for example, when it heats over an external temperature of 180 F. if the temperature of the blanket rises t0 or beyond this value, the impedance of the nylon layer between conductors I and 3 decreases suiiiciently in magnitude that it Vpasses a-ppreciable-current from heater resistor I tosignal wire 3. The current paths formed -between resistor I and signal rwire .3 effectively complete vparallel cirouitsaround capacitor I'I and choke I 8 which disturb the resonance of the normally vresonant cir- Cuit. As above, the disturbance ofthe series resonant eff-ect causes the voltage across choke I8 to drop .off and since the voltage across resistor I6 does not change, results lin the voltage across .coil I4 falling to a Value insulicient to keep relay I from dropping .out to `clear the circuit. Thus, overheating of any portion of the blanket brings about an automatic shut-oir" of the blanket.

It is when the blanket shuts on" automatically in this manner that an inexperienced owner may jam the on switch I9 closed trying to turn the blanket on again. Under normal operating conditions of the original circuit disclosed in the copending application, Serial No. 91,402, i. e. in the original circuit with its relay contacts closed and the on switch open, the current limiting resistor 2! `(using the numbers of the present case) isdn-series with condenser Il and choke |13 across the supply potential, and the voltage across each of the voltage divider resistors I5 and I5 is approximately 1/2 the line voltage. However, when the relay contacts open due to an overheat and the on switch is jammed, the voltage across cach of the voltage divider resistors drops to approximately 1/3 of line voltage, due to the fact that these resistors are then connected in series with the current limiting resistor 2i). The Voltages across choke I8 and condenser Il, however, increase as their circuit is then connected directly across the power source. vSince the Voltage across the choke increases while the voltages across the voltage divider resistors decrease, the result is an increase in the voltage across the relay coil. This increased voltage across the coil causes the relay to operate and close the relay contacts, which, in turn, increases the voltage on the voltage divider resistors. The rise in voltage across the resistors decreases the voltage on the relay coil and allows the relay to drop out. The cycle then repeats itself, causing rap-id chattering of the relay.

However, in this new circuit, current limiting resistor 2) is in series with impedance bridge as a whole when the relay contacts are closed. When the contacts open, due to overheat, and the on switch is jammed closed, resistor 2G is removed from the bridge circuit entirely, both the voltage divider circuit and the resonant circuit being connected directly across the power source. Thus, opening of the relay contacts and jamming of the on switch changes the potential to all the bridge elements and thereby does not cause any unbalance of the voltage appearing at the relay coil. The relay then stays out and no chattering occurs.

When the nylon control layer between resistor I and signal wire 3, and thus, the blanket itself,

has cooled sufficiently that v.there are no longer any parallel circuits effective to disturb theresonant circuit, the closing of switch .I9 will :again restore the blanket to normal operation. However, besides the shut-off due to overheating, the blanket control will also automatically de-ener- .gize the blanket due to the permanent failur-'eof any circuit component, vsince such a failure Adisturbs the resonance of the resonant circuit. In that case, though, reclosing of the on 'switch after a length of time suiiicient to allow cooling of the blanket will not return the blanket to operation, and thereby the user may determine whether his blanket has shut voi Adue toaminor causes, such Yas a tight fold .in ,some portion of the blanket, as is the usual case, or Vwhether it has Vshut -or due to failure of one ci the :circuit components.

The subject matter of this ,application is also related to that of G. C. Crowley, R. G. Holmes, and P. A. Check, Serial No. 236,208, filed July ll, 1951, and to that of vJ. W. McNairy., Serial No. 134,002, iiled December 20, 1949, all having ythe same assignee as theinstant application.

Although the circuit produces satisfactory 'results using the values stated for the various circuit components, these values are meant to be merely illustrative. Obviously, Ynumerous modications and alterations may be made by those skilled in the art without actually departing from the invention, and I therefore aim in the vappended claims to cover all suchequivalent variations as fall within the true Aspirit and scope or the invention.

What I claim .as new .and desire to secure by Lett-ers Patent of `the United States is:

l. In a control .circuit including a control element comprising a heater resistor and a signal wire having between ithem a layer of material which is substantially an insulator at one temperature and at a predetermined higher temperature a conductor of electricity, a pair of supply terminals, a manually operable switch biased to the open position, an impedance bridge connected in series with said switch across said terminals, 'two legs of said bridge collectivelycomprising in serial relationship a capacitor, said signal wire and reactance means, and the other two legs co1- lectively comprising a pair of resistors serially connected, a control relay having an operating coil and contacts biased to the open position, a current limiting resistor, a circuit connecting said contacts and said current limiting resistor in serial relationship across said switch, means connecting said operaing coil across said impedance bridge to derive voltage therefrom to operate said contacts to the closed position, and a circuit connecting said contacts and said heater resistor serially across said supply terminals to enable the current paths formed between said signal wire and said heater resistor upon conducting status of said layer to complete parallel circuits about two legs of said impedance bridge to effect a reduction in the voltage on said operating coil to a value such that said contacts return to said open position and said switch during said conducting status of said layer is ineiective upon being closed to energize said operating coil to again close said contacts.

2. In a control circuit including a control element comprising a heater resistor and a signal wire having between them a layer of material which is substantially an insulator at one temperature and at a predetermined higher temperature a conductor of electricity at conventional domestic voltage and frequency, a pair of supply terminals, a manually operable switch biased to the open position, an impedance bridge connected in series with said switch across said terminals, two legs of said bridge collectively comprising a series resonant circuit including a capacitor, said signal wire and a reactance means, and the other two legs collectively comprising a pair of resistors serially connected, a control relay having an operating coil and contacts biased to the open position, a current limiting resistor, a circuit connecting said contacts and said current limitingr resistor in serial relationship across said switch, means connecting said operatingcoil across said impedance bridge toderive voltage therefrom to operate said contacts to the closed position, a thermosensitive cycling control having a cycling Iswitch member, and a circuit connecting said contacts, said cycling switch member and said heater resistor serially across said supply terminals to enable the current path formed between said signal wire and said heater resistor upon conducting status of said layer to complete parallel circuits about two legs of said impedance bridge eiective to reduce the magnitude of energization of said operating coil to allow said contacts to return to said open position and effective to prevent operation of said contacts to the closed .position upon the operation of said switch during said conducting status of said layer.

3. In a control circuit including a control element comprising a heater resistor and a signal wire having between them a thin layer of material which is substantially an insulator at one temperature and at a predetermined higher temperature a conductor of electricity at conventional domestic voltage and frequency, a pair of supply terminals, a pair of switches biased to the open position and manually operable to the closed position, an impedance bridge connected directly to one of said supply terminals and connected to the other of said supply terminals by one of said switches, two legs of said bridge collectively comprising in serial relationship a capacitor, said signal wire and reactance means,

and the other two legs collectively comprising a pair of matched resistors serially connected, a control relay having an operating coil and contacts biased to the open position, a current limiting resistor, a circuit connecting said current limiting resistor and said contacts in serial relation across said one switch, means connecting said operating coil across said impedance bridge to derive Voltage therefrom to operate said contacts to the closed position, means connecting the other of said switches in parallel relationship with a portion of said resonant circuit, a thermosensitive cycling control having a cycling switch member, and a circuit connecting said contacts, said cycling switch member and said heater resistor in serial relationship across said supply terminals to enable the current paths formed between said signal wire and said heater resistor upon the attainment of conducting status of said layer to complete parallel circuits about two legs of said impedance bridge eiective to reduce the magnitude of energization of said operating coil to allow said contacts to return to said open position and effective to prevent operation of said contacts to the closed position upon the operation of said switch during said conducting status of said layer.

GEORGE C. CROWLEY.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 603,594 Cutler May 3, 1898 862,740 Kallman Aug. 6, 1907 1,325,048 Simon Dec. 16, 1919 1,934,531 Guettler Nov. 17, 1933 2,123,063 Peters July 5, 1938 2,334,530 Andrews Nov. 16, 1943 2,413,125 Walbridge Dec. 24, 1946 2,448,470 Rypinski Aug. 31, 1948 2,470,633 Mershon May 17, 1949 2,565,478 Crowley Aug. 28, 1951 2,581,212 Spooner et al Jan. l, 1952 2,588,926 Holmes Mar. 11, 1952 FOREIGN PATENTS Number Country Date 338,880 Great Britain Nov. 18, 1930

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