Improvements with the thermostats and their circuits associated with electric control

FRENCH REPUBLIC

SERVICE

of the On PROBLEMS INTELLECTUAL INDUSTRIELLE PATENT Gr. 15.-Cl. 2. No. 1.018.450

COMPAGNIE FRANÇAISE THOMSON- [...] residing in France (Seine).

Demanded 4 May 1950, to I5^{m h ll,} to Paris.

.-Issued 15 October 1952 Published 7 January 1953.

(2 requests patent deposited in the United States of America 4 May 1949: the 1 mm™ on behalf of. David C. Spooner Jr, andMiltonS. [...] andthe 2^{e on behalf of} m. George C. Crowley. -Declaration the applicant.)

The present invention relates to a thermostat

improved flexible and relatively fai-diameter

wheat, for example of the order of 2.5 mm, which can

be constructed in any desired length, and which can

produce, or step in producing, a actual effect

and predictable control, when a portion which-

concha of its length is varied

predetermined temperature. The aim of the invention is also

the improved apparatus includes control associated with

this thermostat.

The file server maintains its properties in all

a predetermined temperature range, and it is

reversible, i.e. that it returns to its state pri-

pattern after being exposed to a change in

temperature, followed by the return of the temperature

to its initial value, so that the control acts

whenever the thermostat reaches the temperature

requiring control. The thermostat can be

manufactured in large quantities inexpensively.

Since particular application of the invention, is

include, for example, certain heating devices

such as electric blankets, comprising

the thermostat improved, distributed from a ma-

urge substantially uniform throughout.

The control element, in accordance with the inven-

optionally, uses the variations of a feature electric-

cudgel, such as the resistance, the reactance or the im-

[...] presented by a portion of the thermostat as

a variation of its temperature, so as to-

drag a useful and efficient control.

The thermostat according to the invention comprises

two electrical conductors, preferably in

form of threads, separated by a continuous layer of a

solid organic insulating and flexible, physically

stable in the temperature range contemplated,

and having, in this range, variations pre-

visible, and which can be repeated, a characteristic

electrical, resistance, impedance, etc, these variations

for operating a control circuit.

Requesting The company has been found that the cellulose esters, vinyl halogen-containing resins, and polyamides, for example, have a very high room temperature, but, for higher temperatures, pass sufficient current, under the usual domestic voltages, for operating a relay, a signalling device, or the like.

According to a preferred embodiment of the invention, an inner conductor, which may be the cover of a heating wire, is wound around an insulating core, of-like flexibility of the cover; a thin layer of polyamide resin, having a flexibility comparable to that of the core above, is arranged by extrusion or in any other suitable manner, around the assembly above; then a control electrode is wound around the polyamide resin, and the entire unit is enclosed in an insulating jacket like flexibility. When such an assembly is plugged into an appropriate electrical circuit, can be used the current through the polyamide resin, high temperature, to actuate a control circuit, which operates a relay or any other cutting apparatus the current, or producing a signal.

Of course, the invention is not limited to the above application: it may also be used in the windings of the electric machines, to operate an alarm in the event of excessive temperature; the thermostat may be distributed in the volume of a silo, so as to indicate the temperature raising its contents; or, the convolutions can be distribute to control a certain surface.

The present invention has been found particularly advantageous for protecting, against a wear and tear of temperature elevation, the electric blankets, in which heating wires are [...] in a substantial portion of their surface. It is possible to cause undue [...] such a cover, when the normal gene dissipating heat, for example by folding a heating portion under the mattress, or by folding the cover to itself. Have previously been proposed to provide such covers of a number of thermostatic devices scattered over the entire heating surface. The system has the disadvantage of requiring a number of thermostats substantially too high to provide suitable protection, due to the poor thermal conductivity of the material of which the cover is made, and the delay in transmitting heat from a region too hot to a thermostat.

However, the thermostat of the invention is intimately associated with the heating wire, over the entire heating surface, and it corresponds to any abnormal temperature rise, in any portion of the cover, so as to switch off the current before damage has occurred. A will, open the circuit and can be closed by hand, or may close automatically, when the abnormal temperature has disappeared, according to the type of apparatus used. Since the thermosensitive element returns to its primitive state, the cover can be used again safely, since the element will operate again in another abnormal temperature rise.

On the other hand, the small diameter of the conductor is very advantageous to cartridges of the thermostats used in the prior art, and its high flexibility to withstand frequent bends from a cover, heating or garments, and the like.

The invention also clearly understood by referring to the description that follows and to the accompanying drawing as non-limiting and wherein.

The fig. 1 is an elevation, to large scale, cross-section portion, of one embodiment of a thermostat according to the invention;

The fig. 2 is a variant of the fig. 1;

The fig. 3 is the circuit diagram of associated with the device of Figure 1, said circuit using the inner conductor of the thermostat as charging circuit (for example the heating wire a cover);

The fig. 4 is a diagram of the other control circuit, wherein the load is independent of the thermostat;

Fig. 5 represents The curves show the characteristics/temperature resistance, DC, and the specific inductive powers, according to the temperature, for five polyamide resins used in the invention;

Fig. 6 represents curves The raised on a thermostat according to the invention, of 60 meters long, comprising a polyamide resin, and showing, DC and AC, its resistance, its capacitive reactance and its impedance, for the range of temperatures corresponding to the operating conditions of an electric blanket;

The fig. 7 represents the resistance/temperature curve, DC, and the inducer specific for a halogenated vinyl resin;

Fig. 8 represents The clothoidal curves for a cellulose ester, the materials in question (fig. 7 and 8) that may be used according to the invention;

The fig. 9 is a perspective view of a folded cover;

The fig. 10 schematically shows the distribution of threads in a heated surface, with their associated thermostatic device, such as, by an electric blanket;

The fig. 11 is a diagram of the control circuit wherein the active element is shunted by the electrical conductivity in the control layer, for a certain temperature rise thereof;

The fig. 12 is a diagram of the control circuit wherein the active element is provided in a resonant circuit, comprising the two conductors of the thermostat;

The fig. 13 is a diagram of the control circuit wherein the impedance of the relay is part of the resonant circuit, and comprising cyclic protection against undue heating;

The fig. 14 is a diagram of the control circuit using a glow thermal switch, instead of a ordinary relay;

The fig. 15 is a diagram of the control circuit wherein the filler is formed of one of the conductors of the thermostat, the active element is used in a resonant circuit comprises the second conductor;

The fig. 16 is a diagram of the control circuit wherein the active element comprises means to indicate the temperature rise of any object, or any mass, with which the thermostat may be placed in heat exchange conditions.

In referring fig. 1 and 2, see that the thermostatic device 10 includes a conductor nu 11, preferably ribbon, wound around a flexible core 12, glass fiber, or cellulose acetate, or other suitable flexible material. On the lead 11, and in intimate contact with the food, is arranged, for example, by extrusion, a layer 13 of an organic insulator flexible, having the desired physical characteristics, and the feature that thermoelectric desired, to produce the control that will be described below.

Closed Up tightly on said layer 13, is arranged a conductor nu 14, preferably also strip-shaped. After suitable drying of the layer 13, to reduce the moisture content, is applied a waterproof shield 15 of polyethylene, and an outer insulator 15a. j: The. latter is chosen from materials present both a good isolation, a sufficient wear resistance, and, in the case of heating blankets, and resistant to the cleaning liquor to sec. Polyvinyl chlorides are suitable. The combination of the tape 11 and the flexible core 12 is advantageous, when the object is subjected to repeated bending as frequent in the covers and heating clothes. If the objects are not subjected to repeated bending, it is possible to use a conventional flexible conductor, without providing the core 12.

Depending on Fig. 2, the thermostatic device has two threads, nus 16 and 17, respectively acting as conductors 11 and 14 of Figure 1 wound and tightened, parallel to one another, on the flexible insulating core 18. Sensitive The insulator 19 is applied by extrusion, or otherwise, to embed the wires 16 and 17. Also that the layer 13 of fig. 1, the layer 19 holds the spaced wires from each other. In Figure 1, the layer 13 acts according to its radial thickness and in fig. 2, the layer 19 is active according to the distance between turns of 16 and 17.

It is preferable to cover the two wires with a common layer of organic insulating, rather than covering a wire, and winding the other thread on this set, as a result of the mechanical contact more certain of each strand with the thermally sensitive layer. In Figure 2, the thickness of the layer 19 is not critical, except in that it constitutes an insulator at thermally between the atmosphere and the inner surface of this layer, between the two conductors. Therefore, it is possible to make that this layer 19 is sufficiently thick to act as a protective layer of the assembly, in which case can be omitted additional layer 20. The active part of the temperature sensitive material referred to hereinafter as the "control layer".

This material must have, upon a temperature change, a significant change and predictable of one, or more electrical characteristics determining the electrical conductivity of the layer, so that this change may be converted to useful effect control. These features are, for example, the resistance in direct or alternating current, the capacitive reactance, the impedance. One or more of these variations can be used in a control circuit performing the desired goal.

' Such variations should occur abruptly in the operating temperature range, and are to be proportionally much larger than those that occur according to changes in the ambient temperature, andc [...] the surfaces. The amount of variation is machined to a substantial difference between the total value of the electrical characteristic of the entire device to its normal temperature, and the total value of the characteristic when a relatively small portion of the device is heated to a temperature requiring the action of control. Furthermore, the temperature sensitive material must be physically stable in the temperature range considered, so as not to be damaged and to adjust its initial characteristics when the temperature is normal again.

Can be of at least three classes of organic materials, cellulose esters and vinyl halogen-containing resins polyamides, to form the heat sensitive layer, because they remain hard to relatively high temperatures, they are highly resistant to variations of ambient temperature, and sufficiently sudden have a variation of one or more of the aforementioned features, at temperatures below that which would cause the softening of the temperature sensitive material. The cavity can thus be applied as a film of a thickness of the order of magnitude of one tenth of a millimetre.

One of the most favorable materials is a synthetic polymer amide long chain, which has amide groups that repeat in the main chain structure. The material is known as "nylon" (1948, "The modem Plastics Encyclopedia", p. 178). These nylons have certain beneficial physical properties to form the control layer. Hard They remain up to about 260 °C and retain their mechanical strength over a wide range of temperatures; can be applied by extrusion, or by other suitable means, to be an flexible film, continuous and in close contact with the inner conductor. Their hygroscopic properties are overcome by applying a moisture-proof coating, as aforesaid. The curves of fig. 5, 6, 7 and 8 have logarithmic ordered representing respectively resistors in ohms per cm^3, and for a length of 60 metres (10^{12 ohms per} cm³⁾. The abscissa represent the temperatures in degrees [...], except in fig. 6 where temperatures are in degrees Fahrenheit. These curves were raised according to the test methods "c the American Association for Test Materials 'in particular according to" ASTM ' designation "257-46" for the insulation resistance, and "D.150-47 T" 257-46 for measurements alternating current (f = 60 Hertz).

The curves of fig. 5 represent the resistance changes to direct current (curves R " c), and the specific inducer (S. I. C curves. ) as a function of the temperature, of five nylon compositions, made by Du Pont Nemours and of j Cie, under N 8942 J R, R N 9444 J, J R N 9443, FM and FM 3604 3003.

Fig. 7 represents The equivalent curves for a plasticized polyvinyl chloride ; (vinyl resin [...] ) for a range of temperatures ranging from 30° to 110 C.

Fig. 8 represents The clothoidal curves for a cellulose ester, cellulose acetate, between 30° and 110 °C.

Referring that these three types of materials have rapid variations in the strength and specific inducer, with temperature, which are advantageously for operating a control circuit. However, is to be noted that, if the polyamides remain hard up to about 260 °, the polyvinyl chloride and cellulose acetate begin to soften C to 90°.

The realization according to fig. 2 may be best suited for the latter two materials, because the wires 16 and 17 are closely wound on the central core, and a wire less tendency to move in relation to each other than in the case of Fig. -1, wherein the ribbon 14 is wound under tension. Therefore, although the control layer can somewhat soften, the distance between threads is not almost changed.

A big advantage of the invention to the-art solutions is the fact that in said solutions, the control factor is constituted by the resistance change with temperature of a wire having a certain temperature coefficient, while in the present invention, the abrupt effect control occurs even when only a portion of the thermostat is heated to a temperature requiring the operation of control. This has easily if it is assumed that the heating of a portion of the thermostat, above the temperature of the remainder of this thermostat, reduces the resistance or impedance of the control layer, as if a plurality of individual members of resistance or impedance was placed in parallel between the two conductors. Their effect on the control circuit depends on the sum of their individual impedances resistors or, with respect to the resistance or to the impedance of the total length of the thermostat, to the initial temperature. In prior art devices, providing heat to an area of the active thread, having a known temperature coefficient, simply causes a new value of the series resistance, with a net effect much smaller.

As long as the operating characteristics of the. control circuit are sufficiently known to secure the control factor of the thermally sensitive layer for which the control is to operate, the curves of the fig. 6 indicate the length of the conductors, to be borne at the control temperature, to change sufficiently the control factor of the circuit, for driving the control operation.

Curves " represent variations, with respect to the temperature, of the DC resistance ( R dc), of the AC resistance 60 hertz (sqrt), of the capacitive resistance (Xc) and of the impedance (Z), for a temperature sensitive device of 60 meters long, according to the embodiment of Figure 1, with the exception that the outer isolation were free of impermeable layer 15.

Referring that between 25 and 125° C (75° to 253 F), the DC resistance (R "_c) of the control layer falls to about 6.000® 10 ohms ohms; the AC resistance (R_AK) falls from 310^{e ohms to} ohms about 4.000; the impedance Z falls to 4.000 about 200,000 ohms.

It is found that the drop in the impedance is most abrupt in the range 90-115 °C (° 195-240 F), wherein the control circuitry of the cover are called to operate. We note also that the curves continue to leave beyond 125° C (255 °F), thereby indicating values even lower for higher temperatures, however lower than the softening point of the nylon, which is greater than 260 °C.

The control layer was a polyamide nylon FM 3604. and the outer insulation was polyvinyl chloride.

Assuming the impedance of the control layer is selected as the control factor, see that after the impedance curve of fig. 6, that for relatively low temperatures control, for example in drying chambers or the like, a greater length of the thermostat is to reach the control temperature, that, in cases in which the control temperature is relatively high, because at low temperatures, the impedance variation with temperature are less large as at the higher temperatures.

In other words, after the is viewed from fig. 6 that, since the impedance decreases when the temperature rises, the total impedance of the thermostat decreases upon increase in temperature above the normal in any of its length, and that, therefore, the length of the thermostat to be heated above the normal, to cause the total impedance to be that corresponding to the control point, varies inversely with respect to its temperature rise above normal.

In a cover, for example, the heating circuit is to be cut to a temperature of 96 °C. This represents a temperature of the control layer about 120 °C due to internal heating wire, and, therefore, a relatively short length of the heat-sensitive element may provide the desired response. Examples of response to higher temperatures, include in that the sole of an iron of 18 cm, heated to 175 °C will produce rapidly the control operation, when positioned adjacent the thermostat.

A change in impedance of at least 10/1, at about 38° C, is a value suitable for the construction of this thermostat, in that the length to be heated to the control temperature corresponds to substantially feasible limits.

It is well certain that the dimensions of the different elements of the fig. 1 and 2 are greatly influenced by factors depends on the particular application contemplated. In the case of a heating blanket, the diameter of the core 12 may be of 0.5 mm; the conductor 11 may be one having a width of 0.2 mm and a thickness of 0.05 mm, wound in an amount of 35 turns per 25 mm of length; the conductor 14 may be one having a width of 0,375 mm and a thickness of 0.05 mm, also wound in an amount of 35 revolutions per. 25 mm length. The greater width of the outer conductor is advantageous when the two conductors are wound in the same direction, because it always covers the inner wire. Each of the conductors, but preferably the inner conductor, may be the the heating wire of the cover.

In the construction of fig. 2, the wires 16 and 17 can be strands 36 AWG types, and the core 18 has a diameter of 0.5 mm. In Figure 1 the control layer 13 may be as thin as the need of forming a film normal, flexible, tough and free of tears and holes. In the example of above application, with an alternating voltage of 115 V, and with use of a polyamide of the fig. 5, a fast response with temperature, and appropriate features electrically and mechanical, are obtained with a radial thickness of insulating 0.25 mm thickness and an outer layer 15c of 0,375 mm; diameter of the total of the thermostats fig. 1 or 2 can reach commercially less than 2.5 mm.

The fig. 3 and 4 represent the application of the thermostat of the present invention in a safety circuit, or switching after excess Heating, to protect the electrical heating blankets against burns.

In Figure 3, the inner conductor 11 serves as a heating wire, and the outer thread 14 serves as a connection with the control circuit. The temperature of the cover is normally controlled relative to the ambient temperature, by a control device 21 ; it is known, on the other hand, that the thermostat 10 form many convolutions in the cover, in passages reserved for this purpose.

As seen fig. 3, the electrical system contained in the cover 22 can be connected by a plug and a socket multi-pin 23, to the control system, housed in the control box 24 (fig. 9'). A plug P. establishes the connection with the usual sector 115 V, 60 hertz. In this fig. 3, the load circuit includes the inner conductor 11, into two sections which are connected in parallel to the current source; the resistance of the heating wire 11 is of the order of 65 ohms. The outer conductor 14 is the wire for operating the control, and the control layer 13 is a layer of 0.15 mm of one of the materials specified above, the nylon is preferred. Its impedance change is used to finally act a cutoff relay 25. The control device 21, being sensitive to the room temperature, comprises a bimetal 26, with an adjustment knob 27 outside. The relay coil 25 is connected to the terminals of an impedance bridge, comprising a capacitor of 0.1 μ 28 f, in a resonant circuit having an inductor 29 of 75 Henrys, the other two legs being constituted by two resistors 30,31, of about 6,800 ohms. The impedance of the coil of the relay 25 is to be of the order of 90,000 ohms. The voltage from the resonant circuit is about 130 volts to the terminals of the relay 25. The relay is closed a little below this value and falls for 75 V. 21 If the contacts are closed, the current can be attached to the heating wire 11, by closing momentarily normally open switch 32, which closes a circuit by the driver 33, the two parts of the wire 11, the conductor 34, the resistance 35 of 12,000 ohms, the switch 32 and the conductor 36. The wire 14, which preferably has a resistance of ohms 400 in this example, is in series with the resonant circuit 28-29, to which it is connected by the conductors 37-38, result in the socket 23.

25 The relay closes its contacts 39 and closes the circuit of the heating wire 11. 30-31 The resistors constitute voltage divider, such that if a short circuit has occurred at the ends of the cable 11, while the relay 25 is closed, the voltage at the terminals of the coil-of the relay fall to about half of that of the sector; the relay is wrongly opens the circuit and would intersect.

In normal operation, the resonant circuit and the load circuit are supplemented by the driver 40, the contacts of the relay 39 and the conductors 41 and 36. The resistance 35 voltage limiting is then in the resonant circuit, to create the bonding voltage of the relay. A neon lamp 42 is powered by a circuit including a resistor 43 of 200,000 ohms, the conductors 44,40, the contacts of the relay 39 and the conductors 41 and 36, to indicate that the liner is under tension.

The cover continues to be supplied only subjected to the bimetallic element 26, as long as the temperature of the control layer 13 is less than that which requires the cut.

Dn Resonance control circuit is not affected by the operation of the device 21 and the relay 25 is kept closed.

An increase in temperature of the control layer is accompanied by a reduction of impedance (fig. 6), thereby causing the resonant circuit out of resonance. At this time, the voltage at the terminals of the relay 25 falls below the bonding voltage and the circuit is interrupted.

The normal operation is restored by momentarily closing the button 32 if the temperature of the thermostat has fallen sufficiently below the control temperature.

To open intentionally the circuit, there is provided a switch 45, normally open, dn across relay 25. When this switch is closed, the relay coil is de-energized and the relay opens its contacts 39.

Referring that the malfunction of any part of the control circuit shuts off power to the coil 25 of the relay, by destroying the resonance. The resistance 35, in series with the conductor 11, when closing the switch 32, serves to reduce the current in the conductor, such that if the switch 32 is remained constantly closed, to short the control circuit, there is then no appreciable heating of the cover.

The circuit of fig. 4 operates in a manner to shunt the relay of monitoring by the conduction is established between the two wires of the thermostat, when the control layer reaches the temperature requiring the control.

To dn circuit of the fig. 3, the circuit of the fig. 4 provides a cycle, i. e., by feedback to the normal temperature, the relay is again energized and restores the cover under voltage.

The thermostat is independent of the load and, although the use thereof in a cover 22 [...]. 4 and 10) has been described, it is clear that it can be used in many other kinds of applications.

The cover which have been used in the example above has a heating wire 50. The thermostat 10 of fig. 1 or 10a of fig. 2, is distributed in conduits, so as to be sensitive across the cover. The load 50 is connected to the source of current when the relay 51 closes its contacts 52. The conductors 11 and 14 (fig. 1), or 15 and 17 (fig. 2) are in series with each other and with the coil of the relay 53, a resistor 54 and limiting. The assembly is operated by the momentary closure of the switch 55, and is stopped by the switch 56 which short-circuits the relay coil dn 53.

At operating temperatures which are common, i.e. at about 38° C, the resistance of the control layer is very high; the circuit comprises the coil of the relay 53, in series with the conductors of the thermostat. When one of the parts thereof reaches the control temperature; for example in a fold of the cover of the fig.. 9 ( if is forgotten cut off the power), the decrease in resistance or impedance of the control layer of this part creates a shunt between the conductors and deenergizes the relay 51. The temperature increase can be graduated, because the circuit will be deenergized before the cover has been burning.

In the non-limiting example of fig. 4, the resistance of the load 50 was 65 to 70 ohms; dn each of the conductors thermostat had a resistance of less than 500 ohms; the resistance 54 was 12,000 ohms and the coil 53 had an impedance of ohms 90,000.

In Figures 11 to 14, is represented the load 81 in any desired manner, as it can be a relay, a starter switch for an engine, or in general by any electrical appliance current consuming, raising the load indirectly or directly the temperature of the control layer.

The control circuit of Figure 11 can be used in direct or alternating current. The thermostat 80 is independent of the amount of load 81, and the conductors 11 and 14 are connected in series with the coil 62 of a relay 83, , which is a known voltages or closing the opening. When the coil is energized, its core 84 attracts the armature 85 which, despite the spring 86,87 closes the contacts in series with the load. A limiting resistor 88 creates a normal operational voltage a little below the closing of the relay, said resistance being in series in the circuit of the relay bonding, as known.

Since the contact 87 are normally open, is provided a hand switch 99 to close the circuit of the coil of the relay, for initiating operation of the circuit. Similarly, to deenergize the relay coil oh when desired, is provided a hand switch 91, which short-circuits the coil of the relay 83.

As will be understood that the thermostat is in a condition of thermal exchange with the load 81, which can be any apparatus electrically powered. For [...] conditions the temperature of the feedstock, the temperature of the control layer 72 is such that the layer acts as an insulator. But if the charge produces an undue [...], even on a short length of the control layer, the temperature of the chamber reaches a value such, for example 120° C, that the isolation of the layer decreases sufficiently to that current flows between the conductors 11 and 14, is then passed through resistance 88 and the mass results in terminal; shunts the load and the coil of the relay, so that its attraction falls below the value that can hold the armature 85 against the action of the spring 86. The circuit of the load is thus cut and it will remain until it is re-activates the switch 90. If any of the conductors of the circuit the thermostat is cut, the coil 82 is de-energized. Also, as long as the control layer maintains its high temperature control, it is impractical to operate the relay 83, even if the switch 90 is kept closed, given the established connection between the conductors 11 and 14 and closed by the switch 90 to ground.

Figure 12 is a control circuit against the [...], similar to that of Figure 11, but in which has been added a capacitor 92 in series with the coil of the relay 93 94. The inductance and capacitance brought into series form a resonant circuit which produces a voltage substantially higher than the supply voltage. With the usual sector alternating 115 volts, 60 Hertz, the resonant voltage can be of the order of 150 volts. In such a circuit, the relay is provided to operate at a voltage slightly lower, for example 130 volts, and to open when the impedance variation of the control layer load the control circuit and ' flattens the tip of the resonance voltage. At that time, the voltage available at the terminals of the coil of the relay is of the order of 75 volts, this voltage being insufficient to hold the armature on its contacts.

In this Figure " 12 95 resistance creates a voltage for applying glue to the relay, for example 130 volts. The load circuit 81 is connected to the mains when the armature 96 97 closes the contacts;

it remains so connected until a portion of the control layer 72 reaches the level of the control temperature, thereby reducing its impedance; the current then flows between the conductors 11 and 14, which loads the resonant circuit and, away from the resonant conditions, the resonant voltage lowers' below the value of the bonding voltage of the relay, which therefore opens. The switches 90 and 91 play the same role as those of same reference of Figure 11.

In the circuit of fig. 12, the coil 93 has a reactance of 90,000 ohms, the capacitor 92 has a capacity of 0.1 microfarads, the conductors 11 and 14 each having a resistance of less than 500 ohms. The resistance 95 may be of the order of 12,000 ohms.

The circuit of the fig. 12 operates in a manner similar to that of Figure. It; is noted however that by short-circuiting the capacitor 92, the circuit resonates more, thereby open the relay 94. Most also that if the switch 90 is constantly kept closed, the 95 resistance is in series with the load, which greatly lowers the current. The resistance 95 may also be provided, with respect to the load, to prevent operation of the firearm.

Figure 13 is a variant of Figure 12, for obtaining a cyclic control wherein the relay opens at an undue [...], and closes when the thermostat returns to its normal temperature. A resonant circuit comprises the conductors 11 and 14 in series with the relay 94 and the capacitor 92, and gives rise to a sufficiently high voltage to operate the relay and thereby bring the armature 96 to close the contacts 97. When the temperature increases, the impedance of the control layer 72 decreases, the resonant circuit is charged and the resonant voltage drops to a value incapable of maintaining the relay closed. The armature 96, biased by its return spring opens the contacts 97.

The ground connection for the control circuit is such that when the resonance is restored, following a lowering of the temperature, the control layer allows the coil of the relay being again excited, thereby reestablishing the circuit. A hand switch 98 may be provided for disconnecting the load from the current source.

The control circuit of Figure 14 uses a glow switch 100, instead of the relay above. It requires special reactance, such that the induction coil 101 of 75 [...], in series with conductors 11 and 14 and the capacitor 102 of 0.1 mfd.

The relay is constituted by a gas-filled envelope 103, in which there is sealed a discharge electrode 104, a bimetallic electrode 105 with a contact 106, a fixed contact 107 being connected to the grounded conductor. When the bimetallic electrode is cold, its contact 106 is removed from the fixed contact 107, thereby opening the circuit of the load. Therefore, it is necessary to raise the temperature of the bimetallic element to cause the contacts 106 and 107 are closed, said element being maintained at that temperature, so that the load circuit remains closed. This temperature can be obtained by a glow discharge between the electrodes 104 and 105, and see that the control of the load 81 is carried out by controlling the voltage on the electrode 104. This is achieved by connecting the lower electrode in a resonant circuit, wherein the resonant voltage eta. dget and maintains-the discharge. | The control circuit is operated by the closure of the switch 108 normally open; a circuit is established between the source, the wire 11, the capacitor 102, the electrode 14, the coil 101, the switch 108 and ground. The circuit constants are such. that a voltage of 150 volts is resonance rise, when the circuit is powered from the mains alternating 115 volts, 60 Hertz. The electrodes 104 and 105, connected to the terminals of the inductor 101, are subjected to the resonance voltage and discharge occurs between them. After a period of time, the bimetallic electrode 104 is heated to such an extent that its contact 106 contacts the fixed contact 107, thus completing the circuit of the load. The closure may be indicated by a neon lamp 110, connected between the source and the mass, through the bimetallic element 105, and the contacts 106 and 107. When the lamp 110 is turned on, the switch 108 can be released. As long as the lamp 110 is illuminated, the load 81 is under tension.

Resonance is maintained as long as the temperature of the control layer has a temperature substantially less than that of the temperature requiring the operation of control. When the temperature rises unacceptably, the impedance of the control layer decreases to such an extent that a contract flows between the conductor 11 and 14, the capacitor 102 is thus shunted and the resonant voltage leaves its peak value to a value insufficient to maintain the glow discharge.

After a period of time, the cooling of the bimetallic element 105 separating the contacts 106 and 107 and the load circuit is cut. Notably, the glow relay protects against undue [...], but the resonant conditions does not recover, when the control layer is returned to its normal temperature. The charging circuit cannot be reconnected that with a new closing of the switch 108 and connected that it will remain if the temperature of the control layer is sufficiently decreased below the switch-off temperature. If to be cut by hand the load 81, it is sufficient to close the switch 111 normally open, the coil 101 thereby [...] and interrupts the operation of the resonant circuit.

Persistent As long as the conditions that caused a temperature too high, the control circuit will cut the circuit of the load, each time the control layer reaches the temperature for which the switching is desired. To protect the system, in the event that the user would otherwise hold the switch 108 closed, thus short-circuiting the control circuit, a resistor 112 is provided, which is connected in series with the load when the switch 108 is closed. This resistance 112 is to be much higher than that of the load, in order to reduce the current therein to a value that no longer occurs no rise in temperature. In the case of a electric heating blanket, wherein the filler is represented by the heating wire (whose resistance is about 60 ohms), the resistance 112 is to be of the order of 3,000 ohms.

Figure 15 is a control circuit, cyclic-running, wherein the filler is formed by the wire heating inner 11 of the cover, in particular as in the case of fig. 3.

Since therein, the conductor 11 is connected to the supply mains by the control device bimetallic blade 120, and a relay 140 has a frame 141, opening normally contact 142, by the return spring 143, or any other equivalent device.

The relay coil 140 is in series with the conductor 14 and the capacitor 144 ; the reactance of the coil of the relay and the impedance of the capacitor are calculated to form resonant circuit, for a normal temperature of the control layer 72. A hand switch 145 can be closed, at the start of operation, the resonant circuit, the voltage is applied an relay 140, and to attract his armature that closes the contact 142 and thus completes the circuit of the load. A neon lamp 146 is connected to the mains, when the relay has operated, indicating that the warmer is under tension. If a point of the cover reaches a undue temperature, which causes the conductivity of the control layer 72, a current flows between the wires 11 and 14 which loads the resonant circuit and reduces the voltage applied to the relay 140 to a value lower than the bonding voltage of this relay; the armature intersects the contact 142, thereby breaking the circuit of the heating wire it and turning off the lamp 146.

Fig. 16 is with the use of a signalling circuit, and it exhibits more particularly a plant in which the load is made by an apparatus signaller S, which is to operate when the control layer 72 of the thermostat 80 reaches a predetermined temperature. The thermostat 80 may be disposed along the walls of a room is of a drying oven, for example, or it may be arranged in or coal pile in silos, to respond to a undue temperature rise which could occur therein.

The a resonant circuit, although the shunt circuit Fig. 11 can be used, especially in situations where a normal operating temperature is high, for example 175 to