US3112790A

US3112790A - Control systems

Info

Publication number: US3112790A
Application number: US738874A
Authority: US
Inventors: Russell E Fredrick
Original assignee: Minnesota Mining and Manufacturing Co
Current assignee: 3M Co
Priority date: 1958-05-29
Filing date: 1958-05-29
Publication date: 1963-12-03
Anticipated expiration: 1980-12-03

Description

De@ 3, 1963 R. E. FRI-:BRICK 3,112,790

CONTROL SYSTEMS ly Filed May 29, 1958 2 Sheets-Sheet 1 E 50 55 52 j' 23 z l f6 l 47 lg@ 46 4%( g3 42 I3 I 3@ 55 i 33 6 j@ ,Q0

Ill/Ill.

,ll/Zi )rill/lJlllIlVw-T 73 4 62 INVENTOR. u sell'ed TL'C,

R. E. FREDRICK CONTROL SYSTEMS Dec. 3, 1963 2 Shesets-Sheet 2 Filed May 29, 1958 lo .ltmk

a Q 1N VEN TOR.

autres y CNTRL ySif'tEli/S f Russell E. Fredrick, White Bear Lake, Minn., assigner,

by mesne assignments, to Minnesota Mining and- Manufacturing Company, St. Paul', Minn., a corporation of Delaware v f Filed May 29, 1958, Ser. No. 738,S74

7 Claims. (Cl. 158-431) My present inyention relates to .improvements in control systems, and more particularly to control systems including control means and a plurality of thermoelectric generator means, each subject to a heat source, and -in which at a predeterminedtemperature at any one of the hot -thermojunctions thereof, actuation of the control means is effected. v f

In many instances it is desirable in a control system including a plurality of energized devices to aiiord actuation of control means of the system upon ,ay predetermined reduction in temperature at any one of the energized devices, and accordingly -it is an object of my invention to provide a simple, etticient and economic control system to that end. t

My present invention may, by way of example, have typical utility for a control system includingk a plurality of primary gas burners supplied-with fluid fuel from a common supply manifold, and in Which suitable main control valve means is 'associated with the supply manifold to terminate the supply of iluid y.fuel to. all the primary vburners when the temperature at any one of the plurality of ignition burners drops below a pre-determined temperature. VIn thelapplication in my invention in theinstance noted, I comprehend the provision of ther-moelectric generator means, one associated,'for example,'with a pilot burner for each primary burner, and in which the several y thermoelectric generators are in circuit with lcoi-l means of the'rnainfcontrol valve means to effect actuation of the flatter upon the diminution or failure of any one of the pilot burners so as to shut off fluid fuel to all the primary burners. To this end, each thermoelectric generator means, to achieve the aforementioned control in accordelectric element having aj high negative temperature coeflicient'of resistance at and below a predetermined operating' temperature so that upon `diminution or extinguisht ment of heat vat the hot thermojunctions thereof, :the re.-

Asistance of the control circuit is markedly altered tol actu- 'ate the main control valve means in the circuit to close oil the supply ofpiluid 'fuel to all of the primary burners.

YA further object, of my invention is to provide a control system for a plurality Vof primary heating devices with -which secondary ignitingdevices are associated, control y jatf least vone thermoelectric member having a negative temperature coefficient of resistance so that at a temper- -ature below a predetermined temperature at any 'one of the hot thermojunctions ofthe thermoelectric generator means, the resistance of the circuit is increased so thatthe control means terminates the 'supply' of fluid fuel to the primary burners. f

The aboveland otherl objects'and advantage of my invention will appear from the rfollowing,detailed descripi ,tion of a preferred embodiment of my invention. I

'.Now in order to acquaint .those skilledA in the artV with the manner of constructing and utilizinga control system lin accordance with my invention, I'shall describe in connection with the accompanying drawings Va 'preferredembodim'ent of my invention."

' ance with my invention, includes at-least one thermoin the drawings:

' FEGURE 1 is a schematic representation of a controlv system constructed in accordance with my `invention and embodied with a plurality of duid fuel burners;` FIGURE 2 is a longitudinal cross-sectional view vof a thermoelectric generator suitable foruse in the system of FILGURE l; y i

FIGURE 3 is aV graphic illustration ofthe general thermoelectric power characteristics in micro volts/ F. over a selected temperature range of a number of thermoelectric elements having utility for this invention cornposed of manganese and tellurium within the hereinafter disclosed range, and said range of manganese and tellurium including'certain .additions of positive promoters thereum within the hereinafter disclosed range, and said rangeV of manganese and tellurium including additions of positive promoters therein.

Referring now to FIGURE l of the drawings, 3l have shown a controll system indicated generally at 3,' compris- Y .ing control means. 4, which in the embodiment selected fordisclosing my invention comprises asafety device or main valve control means, a plurality of main or primary gas .burners 6 and 7, and a plurality of ignition or secondary burners f8 and y9 are for the main or primary burners 6 and 7, respectively. i i

The control means 4 shown in the drawings is a well known automatic shut-off -device and for ,purposes of the present disclosure it is sicient to note that `it comprises a housing or casing 12 provided with an inlet opening y11' and an outlet opening d4. The inlet opening 13 extends 5 3 to an inlet chamber 15, and an outlet opening 14, con-l, i nects with outlet chamber 16, the two chambers'being Vseparated by a partition 17 aiording upper and lower Valve

seats

18 and 19, respectively. lA valve20 disposed in inlet chamber lhas an armature 22 at `its upperI end,

which is adapted to cooperate with an electromagnetZS including coil means 24 which when energized creates a magnetic field sufficient to hold the armature :22 attracted and valve Z0 in open position with respect to valve seat 1-8 so Ythat fluid fuel admitted through inletopeningil passes through the inlet chamber 15, partition ,17, outlet chamber Ii6, and out-let opening 1.4. A suitable,cage'indicated at 25 provides for the support of valve 20 for reciprocal axialV movement and for support yof the electromagnetic device shown at 23 all in a'knownand conventional manner.

The safety device or control means r4 further comprises reciprocal reset means 30 having 'a manually actuating button 32 and carrying at its inner end a valve 33 adapted to cooperate with valve seat 19 to close cti the iiow of ui-d fuel through ythe valve. The reset means 430 includes spring means indicated at 36 Yand 37Std aiord for normal disposition of valve 33 in open relationlwit'h v respect to valve seat 19. When the coil meansVv 24 {is deenergized or insuiiiciently energized, armature 22 i`s released and valve Ztlvcloses valve 118 tof shut otllflow ofk fluid fuel through the valve. Y kUnder suchconditions,Y the reset stem 3d may be urged upwardly frornkthe positionz t shown in the drawings to a position Whereinntlie prof jecting end 38 ci Vstem 30 engages therbottomofvalve 20 1 to dispose the armature '22 in position-against Vthe electromagnet, yand if the latter is adequately energized by- Ythe coil 2st, the electromagnet reta-ins valve 20 inthe po'sitionjA i i shown in the drawings to afford the ilow ofl fluidffuel through the valve, l

Patented Dec. 3,1963 i As shown, a main manifold 4t) extends from the outlet chamber 16 of the safety device 4, and

branch connection

41 and 42 lead, respectively, to the main or primary burners 6 and 7. Also, in the arrangement shownJ a second pilot or igniter manifold 43 has connection with a port 44 extending through partition 17 opening into the latter intermediate valve seats 1S and 19. The secondary manifold 43 also comprises

branch conduits

46 and 47, which extend to the secondary burners or igniters 3 and 9, respectively.

Also, as illustrated in FIGURE l the coil means 24 of control means 4 is connected in series circuit relation with -a pair of thermoelectric generators indicated generally at 50 and 52, each comprising a pair of

thermoelectric elements

53 and 54. The thermoelectric generators 513 and 52 are of like construction and, as shown, circuit means 55 comprising leads 56 and 57 connect coil means 24 in series circuit relationship with the

thermoelectric elements

53 and 54 of generators 50 and 52. The thermoelectric generators in this instance constitute single thermocouples, but it will be appreciated that thermopiles could be employed if desired. The hot thermojunctions 5S of both generators are adapted to be heated by the flame of the secondary or ignition burners 8 and 9.

In the operation of the control apparatus shown in FIGURE l, it will be clear that when both igniters or secondary burners 8 and 9 are energized, the flames thereof heat the hot junctions 5S of both thermoelectric generators to afford an electric current suiiicient to effect energization of electromagnet 23 to hold armature 22 in attracted position. In this position gas is free to ow through inlet 13 through the valve housing to outlet 14 to manifold 46 for the primary burners 4l and 42. Also in the position of the parts related gas ows through port 44, manifold 43, and

branch conduits

46 and 47 to the secondary or ignition burners 8 and 9. The ignition burners 8 and 9, of course, afford for the ignition of the uid fuel emanating from burners 6 and 7. Thus with the burners 6 and 7 energized and both pilots or igniters S and 9 in operation, the safety device 4 is maintained in open position and the system is in operation.

The thermoelectric generators '50 and 52. are characterized as above noted by a plurality of thermoelectric elements `53 and 54, and such thermoelectric elements are constructed of materials having different thermoelectric properties and for purposes of my present invention at least one of the thermoelectric elements has a high negative temperature coeicient of resistance. Let it be assumed that each of the thermoelectric elements 54 has a negative temperature coefficient of resistance at and below the normal predetermined operating temperature. It will then be apparent that upon extinguishment or predetermined diminution of the name at either of the igniters or secondary burners 8 and 9, reducing the temperature of the associated hot thermojunction 5S below a predetermined temperature, the negative temperature coefcient of resistance of the thermoelectric element 54 comes into play to afford an increase in the electrical resistance in circuit means 55 so that insu'icient current ows in coil 24 for the electromagnet 23 to retain the armature 22. When such temperature condition occurs the armature 22 is released, and valve 26 seats on valve seat 13 closing off the flow of uid fuel to all the primary and secondary burners. The system cannot then be placed into operation without actuating reset means 3@ to dispose armature 22 in juxtaposition of electromagnet 23 and reenergization of the secondary burners or igniters 8 and 9 must be effected. It will be seen that in utilizing the reset means 36, the valve 33 seats against seat 19 so that during the resetting operation fluid fuel is not free to iiow through the main manifold dit, but uid fuel does flow through the manifold 43 for the secondary'hurners or igniters 8 and 9. Upon reignition of the secondary burners or igniters 8 and 9 and suliicient heating of the hot thermojunctions 58 has occurred to generate current in coil 24 4 of electromagnet 23 to hold the armature 22 in attracted position, release of the reset means places the system again in operation. It will thus be seen that my invention aiords control of the safety device 4 upon the Occurrence of an undesirable or unsatisfactory condition at either of the secondary burners or igniters S or 9.

The generators 56 and 52 of my invention may take any number of forms and one suitable construction in that regard is shown in FIGURE 2 of the drawings. As shown in FIGURE 2 of the drawings, the thermoelectric element 54 may comprise certain manganese-tellurium compositions, hereinafter referred to in greater detail, and the second thermoelectric element may comprise a stainless steel enclosure 53. The outer or second thermoelectric element 53 comprises a heat probe portion 6i)I within which a contact electrode 62 is disposed. 'I'he contact 62 is suitably electrically joined to the closed outer end of heat probe 6o at hot thermojunction 5S and at its inner end has connection with the inner thermoelectric element 54. rThe outer thermoelcctric element 53 as shown is secured to a sleeve 63 which may afford a heat sink for effecting cooling of the outer cold thermojunction means of the generator. The inner end of the thermoelectric element 54 at the inner cold thermojunction has a contact electrode 66 secured thereto and it is provided with a shank portion 67 extending coaxially of sleeve 63 and through openings provided in transverse bushing members 68 contained within the sleeve 63. A pair of disc compression springs 69 are arranged between bushings 68 with the arrangement providing for the maintaining of the inner thermoelectric element 54 under compression to strengthen the latter against breakage. In this connection it 1will be seen that one of the washers 68 seats against a shoulder within sleeve 63, and the other washer 68 is in abutment with the enlarged flanged end of the contact electrode 66 so that with the spring means 69 disposed therebetween a compressive biasing force is applied to the inner thermoelectr-ic element 54. A conductor 70 extends from the contact electrode 66 and through an insulating sleeve 72 mounted adjacent the end of sleeve 63 opposite the hot thermojunction of the generator. The sleeve 63 has electrical connection -with outer thermoelectric element 53 and at its outer end includes a metallic sleeve 73 which affords for the electr-ical connection of a lead thereto. As shown, the insulating sleeve 72 electrically insulatesV leads 70 and sleeve 73 from each other to afford for circuit connection of the generator in an electric circuit as for example with conductors 56 and 57 in circuit means 55 of FIGURE 1. The inner lead 7? is preferably provided with a loop or slack p0rtion 75 which serves to prevent the .transmission of shocks applied to the lead from reaching the contact 66 and inner thermoelectric element 54.

The entire generator assembly is preferably hermetically sealed to afford protection primarily for the inner thermoelectric member 54 which, when it is constituted of certain manganese-tellurium compositions hereinafter disclosed in detail, is subject to deleterious attack by oxygen and hence should be protected from oxygen. The seal structure atthe end of sleeve 63 opposite the hot

junction including conductors

70 and 73, and insulator '72 affords with the sleeve 63, and thermoelectric member 53 a hermetic enclosure for the thermoelectric member 54.

As above indicated the thermoelectric generator of the present invention comprises

thermoelectric elements

53 and 54 of different thermoelectric characteristics, and at least one of the thermoelectric elements should possess a high negative temperature coefficient of resistance. By way of example the aforementioned compositions of manganese and tellurium disclosed in the copending application of myself and Clarence R. Manser, Serial No. 715,454, tiled February 14, 1958, and now Patent No. 2,890,260, are well suited for this purpose. It is there disclosed that electrical conductor elements of good ance substantially all manganese.

t the components.

' (MnsC).

crucible.

thermoelectric and thermistor characteristics may cornprise intermetallic compositions consisting essentially of manganese and tellurium within the range of from 69.9 percent to 72.0 percent by weight tellurium and the bal- The aforementioned compositions may contain other matter in trace amounts without deleteriously affecting the utility of the compositions for the purposes above related, but such impurity should not exceed an order of magnitude of 111%, by weight, except for addition of positive promoters, of which, as examples, sodium and lithium may be present in amounts up to maximum concentrations of one atomic percent. The addition of positive promoter chemical elements, other than the elements manganese and tellurium of which the compositions mainly consist, are effective to enhance certain of thel electrical properties of the thermoelectric elements.

The aforesaid thermoelectric elements 54 may be yformed from the compositions aforementioned, and such compositions may be suitably prepared in thefollowing manner:

The thermoelectric elements may be prepared by reacting pure unoxidized manganese directly tellurium in .appropriateA proportions to afford the aforementioned range in a refractory container .which does not react with However, owing to inconveniences in preparing the elements with starting constituents of manganese and tellurium, it is preferable to first reduce commercial electrolytic manganese which normally contains a small amount of manganese oxide in a carbon Crucible and an inert atmosphere, for a period of several minutes at a temperature of 2600 F. which converts all of the manganese, oxide and metal to manganese carbide The manganese carbide and tellurium in suitable amounts to afford the aforementioned range of ltellurium of from 69.9% tto 72.0% by weight, balance manganese, in the end composition are then reacted in a carbon Crucible under a suitable atmosphere such as hydrogen or Co2. The aforementioned reaction between the manganese carbide and tellurium is violent and it is desirable that the Crucible be fitted loosely with a carbon cap to prevent material from splattering out of the It is advantageous in carrying out the aforementioned reaction to use a tellurium excess over the amount of tellurium required to form the Vintermetallic fcompounds of manganese and tellurium of the aforementioned range of the order of0.5 weight percent, or more,.

to insure the complete reaction withthe manganese carbide. The rate of reaction of MngCand Te or metallic Mn land Te is quite low, and the reaction rate can be yraised substantially by increasing the proportion of tellurium in the initial .melt over that desiredinV the final composition'. The additional tellurium over and above the aforesaid range, present inthe melt' as undesirable .excesses in the form of MnTe2, may be removed to bring the composition within the aforementioned rangeof manganese and tellurium by vacuum melting of thematerial.

In y such vacuum` melting the equilibrium MnTezMnTe-i-Te 'is shifted to the right as the temperature is raised and 'to condense on a cooler 'section ofthe system. By holdi ing the'temperature of the melt above the melting point of MnTe (2150 F.) while reducing the pressure to the Vorder of .1" Hg will, in several minutes, the time depending on how much tellurium is to `be removed, bring the .composition within the aforesaid range, and an almost pure metallographic phase will be produced, for Which thejvapor pressure of Vtellurium'over the solid is negili-` Q ble -at temperatures up Ato its melting point.

When the thermoelectric elements V54 are of manganese and tellurium the aforementioned range is important.

Should, lfor. example, the manganese concentration ex- Further, as to tellurium excesses over and above 69.9%

by weight of the total composition of manganeseand tellurium combine with the single phase MnTe to form MnTe2 which can be accommodated up to 72.0% tellurium by weight of the total composition of manganesev and tellurium. Thus, unlike manganese excesses of the aforementioned range, the thermoelectric generator elements 54 can accommodate 8% by weight excess tellurium in the aforementioned form of MnTe2 without appreciable significant reduction in thermoelectric power.

Turning again to the aforementioned' method, it is expedient to incorporate all of the above operations of reducing, reacting, and removing excess tellurium to pro-y vide thermoelectric elements 54 within the above range of manganese and tellurium in one heating cycle. This can be done by putting appropriate proportions of manganese, carbide and tellurium in a closed carbon mold, and then heating the same to the order of 2600" F. for several minutes underan atmosphere of inert gas.V The temperature is then reduced slight-ly andthe mold is evacuated to remove the excess tellurium from the melt by vaporization. The released carbon from yMn3C will form an insignificant scum on the surface of the Asolidified ingot.

The thermoelectric elements produced, as above discussed, may have their electrical conductivities increased by the addition, in a conventional way,.of P-type additions or promoters, as for example, sodium or 'lithium up to concentrations of one atomic percent. v l Y In the graph of FIGURE, 3 of the drawings, the` curve bearing reference numeral 89 generally .indicates lthe thermo-electric power in microvolts per 1 F. as Vagainst temperature in compositions of tellurium and manganese, in which tellurium ranges from l69.9% toV 72.0% by Weight, and the balance substantially all manganese. The curves indicated at 812, 83, S14 andl '85 give the re-V utilization of positive promoter elements, as Vsodium orY lithium, is accompanied by reduction in electrical resistivity as shown, for example, in FIGURE 4 by curves In YFIGURE, 4,v theY 816, `87, d3 and 89' respectively. I general graphic representation of unpromoted tellurium and manganese in kthe range of `from 69.9% to 72.0% tellurium .by weight and balance substantially all manganese is exhibited by curve 9i). In regard to the' several curves lof FIGURE 4, it Will be readily apparent that they are characterized by desirable negative Vtem-v perature co-eiiicients, it being observed that VFIGURE 4 represents the log vof the electrical resistivity expressed n in ohm-inches plotted, as previously noted, for conveni' ence against the reciprocal of absolute (Kelvin) temperature, symbolically expressed l103TK- `It will bef observed from an examination of FIGURES` 3 and 4 that nominal loss of thermoelectric power 'is more than offset` by reduced electrical resistivity .of the promoted man@ ganese-tellurium thermoelectric elements'. Y f

As previously indicated, any residual impurities occur# ing in lelectrolytic manganese is not `effective to alter the conductivity of the thermoelectric elementsof this in-j vention as 4do the positive promoter elements' sodium Yand lithium by reason of which elementary purematerials are not required in order to provide reproducible' results, providing unknown or trace impurities do not, exceedan v' order of magnitude .Oll %1 by Vweight ofthe .manganese and tellurium. I

Thermoelectrical'elements produced in .accordance V;,

The reduction of with the aforementioned methods are always P-type at all temperatures below the melting points of the compositions. The aforedescribed thermoelectric elements may be operated at high temperatures, approaching 1600o F., Without `sublimation of the elements.

The contacts or

electrodes

62 and 66 for thermoelectric elements 54 of the aforedescribed compositions may be of steel or steel alloys having pressure engagement with the ends of elements 54.

Further, electrical contact may be made with thermoelectric elements E4 by fusing together the thermoelectric elements with an Mo-Fe electrode previously wetted with a small amount of a lead rich lead telluriurn composition doped, for example, with sodium as disclosed in the Patent No. 2,811,440, dated October 29, 1957, and assigned to the assignee of this application. ln this instance, the lead excess, lead-tellurium sodium doped composition acts as a solder joining the

iron electrodes

62 and 66 and the thermoelectric element 54.

As previously noted in the utilization of thermoelectric elements of the aforementioned compositions at high temperatures, the thermoelectric elements should be hermetically sealed in an oxygen free environment.

If desired, annealing of the thermoelectric elements above described may be accomplished to the end of improving the same. This may be accomplished by annealing the thermoelcctric elements for several hour-s at 1400io F. and allowing the thermoelectric elements to furnace cool with the operation being accomplished in a reducing atmosphere. The annealing step results in stabilizing the properties of the thermoelectric elements at high temperature operations.

In connection with thermoelectric elements in the range of from 69.9% to 72.0% tellurium, balance essentially all manganese, it has been observed that at the higher tellurium concentrations, operating temperatures may be somewhat limited by the vaporization of tellurium. However, where high temperature operation is not important, simplification of the production of the thermoelectric elements may be effected by the use of the greater amount of tellurium and eliminating the vacuum melting procedure without seriously affecting the electrical properties of the thermoelectric elements.

Thermoelectric elements of the aforementioned compositions are superior to other presently known thermoelectric materials at temperatures above 800 F And, it is believed that such thermoelectric elements are capable for a hot junction operating temperature approaching 1600 F.

When the aforedescribed manganese-tellurium compositions are utilized for thermoelectric elements 54 it is preferred that thermoelectric elements 53 be of negative polarity to aiford etlicient production of thermoelectric current by the thermoelectric generator. For this purpose thermoelectric elements 53 may be of steel or steel alloys.

It will be understood that various other thermoelectric materials may be utilized in my invention and the aforementioned manganese-tellurium compositions and steel allord a preferred embodiment of my invention. Also contact electrodes 62 and y66 may comprise any contact material suitable for use with the selected thermoelectric materials and the materials for

contacts

62 and 66 hereinabove described have utility for the disclo-sed manganese-tellurium compositions.

In the construction of a control system in accordance with the above a control circuit was afforded having a cold resistance of 1.8 ohms and an operating resistance with the hot thermojunctions of the generator at a predetermined satisfactory operating temperature of .2 ohm which it will be readily apparent to those skilled in the art is amply effective to achieve the operation of the control `system as above described, l claim:

`1. An electrical circuit comprising a load device, electrical energy source means including a thermoelectric generator in circuit with said load device for energization thereof, said generator having hot junction means and said source means ailording a predetermined value of electrical energization to said load device when said hot junction means is subjected to a predetermined operating temperature level7 said generator including at least one thermoelement of material having a high negative temperature coefficient of resistance at and below said operating temperature level and being connected in said circuit in such a manner that on a decrease in the tempera-ture of said thermojunction means from said predetermined operating level, rthe resultant increase in the resistance of said one thermoelement effects a substantial decrease in the electrical energy supplied to said load device, irrespective of any concomitant variation in the electrical output of said generator.

2. An electrical circuit including at least one thermoelectric generator as a power source therefor, said one generator having hot junction means and said circuit having a predetermined value of operating resistance when said hot junction means is subjected to a predetermined operating temperature level, said one generator including iat least one thermoelement of material having a high ynegative temperature coeilicient of resistance at and below said operating temperature level causing, on a decrease in the temperature of said thermojunction means from said predetermined operating level, a substantial increase in the resistance of said circuit.

3. A control system comprising, control means having coil means of predetermined operating current value, a plurality of thermoelectric generators in circuit with said coil means each having thermoelectric elements, thermoelectric properties and at least one of which generator has a thermoelement having a negative temperature coetlicient of resistance at and below a predetermined operating temperature, said thermoeletcric elements of each generator -being joined to alord hot thermojunction means :for each of the respective generators for heating to operating temperature by different heat sources, whereby when the hot thermojunction of said at 'least one generator is at a temperature below said predetermined operating temperature, said thermoelectric element thereof which has the negative temperature coeicient of resistance eilects an increase in the resistance of said circuit to afford a current therein of a value lower than said predetermined operating current value for said coil means.'

4. ln combina-tion, a plurality of primary burners, an ignition burner for each of said primary burners, main control valve means including `coil means for controlling the ow of fluid fuel to said man burners, a thermoelectric generator at each of said ignition burners having hot thermojunctions ad-apted to be heated to a predetermined normal operating temperature by the latter, means connecting said thermoelectric generators in circuit with vsaid coil means of main control valve means, and said thermoelectric generators each including at least one thermoelectric element having a negative temperature coefficient of resistance at and below said predetermined normal operating temperature and adapted upon a predetermined -reduction in said hot thermojunction temperature to aiford said circuit means increased electrica-l resistance characteristics causing substantial de-energization of said coil means of said main control valve means to terminate the iiow of fluid fuel to said primary burners.

5. In combination, a plurality of heat sources, a plurality of thermoelectric `generators one for each heat source, said 4thermoelectric generators each comprising a plurality of :thermoelectric elements of dilerent thermoelectric properties connected to afford hot thermojunction means for heating to a predetermined normal operating temperature by said heat sources, each of said generators having at least one thermoelectric element which has a negative temperature coeiiicient of resistance at and below said normal operating temperature, a safety device, circuit means `connecting said thermoelectric generators .fuel to said burner, a thermoelectric generator at said burner having a hot thermojunction adapted -to be heated to a predetermined normal operating temperature by the burner, means connecting said thermoelectric generator in circuit with `said coil means, and said thermoelectric gen' erator including a thermoelectric element ha'ving a negative temperature coefficient of resistance at and below said predetermined normal operating temperature and adapted fupon a predetermined reduction Vin said ho-t thermojunction temperature to afford said circuit means increased electrical resistance characteristics causing substantial de-energization yof said coil means of said main control valve means -to terminate the iiow of fluid fuel to said burner.

7. In combination, a heat source, a thermoelectric generator comprising a plurality of thermoelectric elements connected to afford hot thermojunction means to be heated to a predetermined normal operating temperature by said heat source, said generator having at least ture coeicient of resistance at and below said normal one thermoe-lectric element which has a negative tempera- 30 operating temperature, a safety device, circuit means connecting said thermoelectric generator and said safety device, said safety' device being adapted to be energized by Y the thermoelectric current produced by said thermoe-lectric generator, and said one element of said'therrno electric generator affording increased resistance to said circuit means for de-energizing said safety device when the temperature at its associated hot thermoj'unction drops below said predetermined operating temperature.

References Cited in the le o f this patent UNITED STATES PATENTS 2,351,277: Mantz June 13, 1944 2,676,196 Marsden Apr. 20, 1954 2,811,571 Fritts et al. Oct. 29, 1957 2,823,741 Thornbery Feb. 18, 195'8 2,858,350 Fritts et al. Oct. 28,1958

FOREIGN PATENTS 708,491 France July 24, 1931 OTHER REFERENCES Properties and Uses of Thermistors-Thermally Sensitive Resistors, by Becker, Green, `and Pearson, vol. 65 of Transactions, November 1946, 15 sheets, page 713 only referred to.

semi-Conductors, D. A. wright, John wny & sons,

New York, 1950 (page 361).

Solid State Physics, vol. 9, 1959, pages 121 and 123, Academic Press, New York.

Claims

1. AN ELECTRICAL CIRCUIT COMPRISING A LOAD DEVICE, ELECTRICAL ENERGY SOURCE MEANS INCLUDING A THERMOELECTRIC GENERATOR IN CIRCUIT WITH SAID LOAD DEVICE FOR ENERGIZATION THEREOF, SAID GENERATOR HAVING HOT JUNCTION MEANS AND SAID SOURCE MEANS AFFORDING A PREDETERMINED VALUE OF ELECTRICAL ENERGIZATION TO SAID LOAD DEVICE WHEN SAID HOT JUNCTION MEANS IS SUBJECTED TO A PREDETERMINED OPERATING TEMPERATURE LEVEL, SAID GENERATOR INCLUDING AT LEAST ONE THERMOELEMENT OF MATERIAL HAVING A HIGH NEGATIVE TEMPERATURE COEFFICIENT OF RESISTANCE AT AND BELOW