US3919679A

US3919679A - Time-delay switch

Info

Publication number: US3919679A
Application number: US499382A
Authority: US
Inventors: Geoffrey W Kingma
Original assignee: Texas Instruments Inc
Current assignee: Texas Instruments Inc
Priority date: 1974-08-21
Filing date: 1974-08-21
Publication date: 1975-11-11
Anticipated expiration: 1992-11-11

Abstract

A time-delay switch comprising a base, a first flexible switch arm cantilevered from the base and carrying a first contact on the free end thereof and a second flexible switch arm cantilevered from the base and carrying a second contact on the free end thereof engageable and disengageable with the first contact for making and breaking an electrical circuit. The switch also includes a thermal actuator comprising a bimetallic arm cantilevered from the base for flexing toward and away from the switch arms in response to changes in temperature, the first and second switch arms and the bimetallic arm being substantially parallel to and spaced from one another for flexing movement in a common plane and about spaced transverse axes for each of the arms lying in a plane substantially perpendicular to the common plane contiguous to the base. A coupling mechanically links the bimetallic arm and at least one of the switch arms to effect movement of the free end thereof relative to the base and to the other switch arm in response to flexing of the bimetallic arm between a closed position in which the first and second contacts are in engagement with one another for making the above-stated electrical circuit and an open position in which the contacts are clear of one another for breaking this circuit. The switch further includes a self-regulating heater interposed between the fixed ends of the bimetallic arm and the second switch arm and having opposed current-carrying faces respectively in electrical and thermal contact with the fixed ends of the second switch arm and with the bimetallic arm for heating the latter thereby to cause the bimetallic arm to flex and to thus effect movement of the contacts between their closed and open positions in delayed response to energization of said heater.

Description

United States Patent [1 1 Kingma 1 1 TIME-DELAY SWITCH Geoffrey W. Kingma, Downsview, Canada {73] Assignee: Texas Instruments incorporated,

Dallas Tex.

[221 Filed: Aug. 21, 1974 [211 Appl. No: 499.382

[75] Inventor:

[52] US. Cl. 4 .t 337/107; 337/1021337/377 [51] Int. Cl. 1. H0111 61/02; HOlH 63/013;

HOlH 71/16 [58] Field of Search .v 337/99, 100. 101 102.

Primary E.\'aminer.l. D. Miller Assistant Examiner-Fred E. Bell Attorney, Agent, or Firm lames P. McAndrews; John A Hang; Russell E. Baumann [57] ABSTRACT A timedelay switch comprising a base, a first flexible switch arm cantilevered from the base and carrying a first contact on the free end thereof and a second flexible switch arm cantilevered from the base and carry- 7.4 1 51 Nov. 11, 1975 ing a second contact on the free end thereof engageable and disengageahle with the first contact for making and breaking an electrical circuit The switch also includes a thermal actuator comprising a bimetallic arm cantilevered from the base for flexing toward and away from the switch arms in response to changes in temperature. the first and second switch arms and the bimetallic arm being substantially parallel to and spaced from one another for flexing movement in a common plane and about spaced transverse axes for each of the arms lying in a plane substantially perpendicular to the common plane contiguous to the base. A coupling mechanically links the bimetallic arm and at least one of the switch arms to effect movement of the free end thereof relative to the base and to the other switch arm in response to flexing of the bimetallic arm between a closed position in which the first and second contacts are in engagement with one another for making the above-stated electrical circuit and an open position in which the contacts are clear of one another for breaking this circuit. The switch further includes a self-regulating heater interposed between the fixed ends of the bimetallic arm and the second switch arm and having opposed current-carrying faces respectively in electrical and thermal contact with the fixed ends of the second switch arm and with the bimetallic arm for heating the latter thereby to cause the bimetallic arm to flex and to thus effect movement of the contacts between their closed and open positions in delayed response to energization of said heater.

13 Claims, 9 Drawing Figures TIME-DELAY swrrcn BAQKGROI'ND OF THE INVENTION This invention relates to a time-delay switch and more particularly to such a switch having a self regulating heater for heating a bimetallic thermal actuator which causes contacts to make or break in delayed re sponse to energization or deenergization of the heater.

Thermostatic switches of the stack-type (such as those manufactured and sold by Texas Instruments In- Corporated under the trade designations *22BT" and 23BT"I employing cantilevered switch arms and a cantilevered bimetal thermal actuator are low cost reliable units widely used in the heating and appliance industry. This type of switch is so widely accepted and produced in large numbers that it would be highly desirable if its components could be utilized in another and quite different field of application, viz., time-delay devices. without substantially increasing the cost thereof.

SUMMARY OF THE INVENTION Among the objects of this invention may be noted the provision ofa time-delay switch which may be actuated in delayed response either by the load current flowing therethrough or in delayed response to energization of another circuit independent of the load circuit; the provision of such a time-dclay switch which can handle relativcly high current loads (e.g.. IO amps at 250 VAC); the provision of such a time-delay switch in which the time delay may readily be adjusted; the provision of such a time-delay switch in which the contacts snap open and closed to minimize deleterious arcing of the contacts; the provision of such a timedelay switch which is inexpensive and can be fabricated from readily available components; and the provision of such a switch which is of rugged construction and is reliable in operation. Other objects and features of this invention will be in part apparent and in part pointed out hereinafter.

Briefly a time-delay switch of this invention comprises a base, a first flexible switch arm cantilevered from the base and carrying a first contact on the free end thereof and a second flexible switch arm cantilevered from the base and carrying a second contact on the free end thereofengageable and disengageable with the first contact for making and breaking an electrical circuit. The switch also includes a thermal actuator comprising a bimetallic arm cantilevered from the base for flexing toward and away from the switch arms in re sponse to changes in temperature, the first and second switch arms and the bimetallic arm being substantially parallel to and spaced from one another for flexing movement in a common plan and about spaced transverse axes for each of the arms lying in a plan substantially perpendicular to the common plane continguous to the base. A coupling mechanically links the bimetallic arm and at least one of the switch arms to effect movement of the free end thereof relative to the base and to the other switch arm in response to flexing ofthe bimetallic arm between a closed position in which the first and second contacts are in engagement with one another for making the above-stated electrical circuit and an open position in which the contacts are clear of one another for breaking this circuit. The switch further includes a self rcgulating heater interposed be tween the fixed ends of the bimetallic arm and the second switch arm and having opposed current-carrying faces respectively in electrical and thermal contact with the fixed ends of the second switch arm and with the bimetallic arm for heating the latter thereby to cause the bimetallic arm to flex and to thus effect movement of the contacts between their closed and open positions in delayed response to energization of said heater.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a perspective of a time-delay switch of this invention;

FIG. 2 is a side elevational view of a timedelay switch of this invention with its contacts closed and with its bimetallic thermal actuator arm at room tempcrature;

FIG. 3 is a view similar to FIG. 2 illustrating the thermal actuator arm in a heated position thereby effecting opening of the contacts;

FIG. 4 is a plan view of the switch of FIGS. 1-3;

FIG. 5 is a side elevational view of a modification of the timedelay switch shown in FIGS. 1-4;

FIG. 6 is a partial horizontal sectional view taken on line 66 of FIG. 2;

FIG. 7 is a vertical section taken on line 7-7 of FIG. 4;

FIG. 8 is a semidiagrammatic view of the timedelay switch of this invention serially connected to a load for making and breaking the load circuit in delayed response to energization and deenergization of the heater; and

FIG. 9 is a semidiagrammatic view of the time-delay switch of this invention in which the heater is energized and deenergized by means ofa second electrical circuit independent of the load circuit.

Corresponding reference characters indicate corresponding parts throughout the several views of the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings, a first embodiment of the time-delay switch of this invention is indicated in its entirety at 1 in FIGS. 1-4 and 6-9. More particularly the time-delay switch includes a base 3, and a first or upper flexible switch arm 5 cantilevered from base 3 and carrying a first or upper contact 7 on the lower face of its free or outer end. A second or lower flexible switch arm 9 is also cantilevered from base 3, this lower switch arm carrying a mating contact 11 on the upper face of its free end, these contacts being engageable with and disengageable from one another for making and breaking an electrical circuit. Preferably. these switch arms are made ofa resilient, flexible and electrically conductive metal or alloy.

Switch 1 also includes a thermal actuator. generally indicated at 13. comprising a bimetallic arm 15 canti' levered from base 3 for flexing toward and away from

switch arms

5 and 9 in response to changes in tempera ture of the bimetallic arm. As best shown in FIG. 2, switch

arms

5 and 9 and bimetallic arm 15 are substantially parallel to one another and are spaced from one another for flexing movement in a common plane and about axes. as indicated at X. Y and 7., for arms 5. 9 and 15, respectively. extending transversely of each respective arm contiguous to base 3. A ceramic insulator I7 carried by the free end ofbimetallic arm 15 couples the bimetallic arm to at least one of the

switch arms

5 or 9 to effect movement of the free end of the switch arms relative to base 3 in response to flexing of the himetallic arm between a first or closed position of the switch arms (see FIG. 2) in which contacts 7 and I] are in engagement with one another for making the electrical circuit between the switch arms and an open position (see FIG. 3) in which the contacts are separated from one naother for breaking the electrical circuit.

In accordance with this invention, time-delay switch I further includes a self-regulating heater [9 interposed between the fixed ends of bimetallic arm and switch arm 9. Preferably, heater I9 is an annular, right cylindrical, pill-shaped ring having a central bore therethrough and opposed current carrying faces 21 and 23 (see FIG. 7) respectively in electrical and thermal contact with switch arm 9 and with bimetallic arm 15 for heating the latter, thereby to cause the bimetallic arm to flex and to thus effect movement of the contacts between their closed and open positions in delayed response to energization of the heater. Preferably, heater I9 is a resistor of a ceramic material, such as a doped barium titanate, having low initial resistance at room temperature and having a variable positive temperature coefficient (PTC) of resistance whereby, upon energization, the heater self-heats until it reaches an anomaly temperature at which its resistance increases abruptly in relation to its increase in temperature and thus becomes selfregulating at a temperature somewhat above the anomaly temperature with the current flowing through the heater generating heat at a rate commensurate with the rate at which heat is dissipated therefrom.

Bimetallic arm 15 is of a conventional bimetallic material which, as best shown in FIG. 7, has a first layer of metal 25 bonded in face-to-face relation with a second layer of metal 27. The first metal layer has a thermal coefficient of expansion lower than the second layer so that upon heating of the bimetallic arm it flexes away from its high-expansion side 27 and upon cooling it flexes toward its high-expansion side. With the bimetallic arm 15 at room temperature, it is substantially perpendicular with respect to base 3 as shown in FIG. 2.

Switch arm 9 is so structured as to be resiliently biased into engagement with an upwardly facing surface 29 of coupling insulator 17 so that, upon movement of bimetallic arm 15 toward switch arm 9, the free end of switch arm 9 is also flexed upwardly. As further shown in FIG. 2, with contact 7 in engagement with contact 11, upward flexing movement of the bimetallic arm also effects upward flexing movement of switch arm 5.

Switch arm 5 is so structured that it is abruptly movable over center between a first unrestrained stable position s'omewhat below its position as shown in FIG. 2 and a second stable position (shown in phantom in FIG. 2). More particularly, switch arm 5 is so structured as to constitute overcentering means comprising two longitudinal slots 31a, 3 lb formed therein extending lengthwise thereof intermediate contact 7 and base 3 defining outer strips 33a, 33b and an inner center strip 35. The outer strips have peakshaped deformations 36 formed therein so as to shorten the outer strips with respect to the inner strip and to thus place the inner strip under compression. As shown, center strip 35 is also deformed out of the plane of switch arm 5, but it is somewhat longer than the outer strips and thus is under compression loading. It will be understood that the unstable center position of switch arm 5 is between the position of switch arm 5, as shown in FIG. 2, and its po sition as shown in FIG. 3, and that, with switch arm 5 below its center position, outer strips 33a, 33b and inner strip 35 bias arm 5 toward its lower stable position and thus maintain contact 7 in engagement with contact 11 thereby completing the circuit; and with switch arm 5 above its center position. it is resiliently biased toward its second or upper stable position.

(shown in phantom in FIG. 2) and thus maintains contacts 7 and I1 spaced from one another thereby to positively break the circuit.

Coupler I7 has a lip 37 overlying surface 29 and the upper surface of switch arm 5. This lip is spaced from surface 29 a distance sufficient to permit switch arm 5 to abruptly move away from switch arm 9 and to permit contacts 7 and 11 to separate as switch arm 9 is flexed upwardly by bimetallic arm 15. Thus, the contacts 7 and II are abruptly separated from one another as switch arm 5 moves from its closed position (FIG. 2) through its unstable center position (not shown) to its open position (FIG. 3). Upon overcentering, lip 37 stops switch arm 5 from springing to its second or upper stable position. Upon flexing of bimetallic arm 15 in the opposite direction, as upon cooling of the bimetallic arm, lip 36 engages the upper face of the free end of switch arm 5 and moves it downwardly with respect to base 3. As heretofore mentioned, switch arm 9 is springbiased to remain in engagement with insulator face 29. Thus, contacts 7 and 11 remain separated from one another upon initial downward movement of bimetallic arm 15. Upon further downward movement of the arm 15, switch arm 5 is caused to move through its overcenter position from its second stable position and into engagement with contact 11 thereby to abruptly remake the circuit. This abrupt overcenter movement of the first switch arm 5 causes contacts 7 and II to snap open and closed thereby preventing contact chatter with consequent deleterious arcing of the contacts. It will be understood that, with suitable minor modifications readily apparent to those skilled in the art, switch arm 5 need not have the abovedescribed snap-acting make-and-break means, but rather may be a slow make-and-break-type switch.

Switch 1 is shown to include terminals 39, 4! and 43 for switch arm 5, switch arm 9 and bimetallic arm IS, respectively, for connection of the switch in an electrical circuit as will appear. Insulating means comprising a plurality of ceramic

insulating rings

45a, 45b and 45c are interposed between the switch arms, heater l9 and bimetal arm 15, as shown in FIG. 7, to isolate the

switch arms

5 and 9 from one another. Base 3 further comprises a metal support 47 overlying switch arm 5. Each

switch arm

5 and 9, bimetallic arm 15, each terminal 39, 41 and 43, and support 47, as well as heater 19, has a bore therethrough for reception of a fastener 49 (e.g., a rivet) which traverses these bores for positively securing the various parts together in assembled relation. As shown in FIG. 7, each

insulator

45a, 45b and 45c has an axially protruding flange 50 adapted to separate or electrically insulate

switch arms

5 and 9 from one another and to isolate the bimetallic arm 15 and the switch arms from fastener 49.

Support 47 overlies switch arm 5 and carries an insu lated fulcrum pin 51 engageable with center strip 35 of switch arm 5 from above to serve as an abutment for the center strip. Pin 51 has a rounded outer end and is carried by a manually adjustable screw 53 threaded in an opening (not shown) in support 47 for selectively moving pin 51 toward and away from switch arm 5. By running adjustment screw 53 in and out relative to switch arm 5, switch arm Sis engaged by the pin at dif ferent positions of flexurc of the switch arm and the amount of movement between switch arm 5 and base 3 required to move the switch arm through its overcenter position is varied. Thus, the amount of deflection of bimetallic arm required to effect ovcrcenter movement of switch arm 5 may readily be varied, which in turn effects a change in the time delay of switch 1. The more the deflection of the bimetallic arm 15 required to move switch arm 5 over center, the greater the time delay. As previously mentioned, screw 53 may readily be manually adjusted. Support 47 has an upturned stop 55 struck therefrom and adjustment screw 53 carries a stop arm 57 engageable with stop 55 to limit rotation of the adjustment screw to less than one full turn.

In FIG. 5, a modification of switch 1 is shown, this modified switch being indicated in its entirety at la to distinguish it from switch 1, which is essentially identical to switch 1 but for adjustment screw 53. In switch la the adjustment screw is a stub screw, indicated at 59. threaded in support 47. This stub screw is preset so that the time delay for the switch is a desired interval as may be established by calibrating the switch to open and close in a desired time interval. Once this time interval has been established, stub screw 59 may be secured in its calibrated position by means ofa dab of ceramic cement 61 applied to the screw locking it to support 47. Thus, once initially calibrated, the time delay of switch 1a is not selectively adjustable.

Referring to FIG. 8, a switch 1 of this invention is shown to have its terminal 39 connected to a load L and its terminal 43 connected to a power source E so that heater 19 is serially connected with the switch contacts, the laod and the power source. With contacts 7 and 11 closed and with the bimetallic arm 15 at room temperature, current flows through load L and heater 19 thereby causing the heater to self-heat and thus causing bimetallic arm 15 to deflect upwardly. Due to the downward spring bias of switch arm 9, the bottom face of switch arm 9 remains in engagement with face 29 of insulator l7 and contact 7 is biased into engagement with contact 11 upon initial upward deflection of the bimetallic arm. As switch arm 5 moves through its center position, it abruptly breaks contact and moves into engagement with lip 37 of insulator l7 and thus maintains the circuit open. Thus, both load L and heater 19 are simultaneously deenergized. Heater 19 and bimetallic arm 15 immediately begin to cool and thus effect downward deflection of the bimetallic arm. With lip 39 in engagement with the upper face of switch arm 5. the latter moves downwardly with the bimetallic arm and switch arm 9 remains in springbiased engagement with the upwardly facing face 29 of insulator 17. As switch arm 5 moves through its unstable center position, contacts 7 and 11 abruptly close remaking the circuit and thus again energizing load L and heater [9. Heater 19 again self-heats and, after a time delay, again opens the circuit. This intermittent or flasher operation of the circuit will continue so long as power is supplied to load L.

As shown in FIG. 9, load L and a primary power source E are serially connected across

terminals

39 and 41 of switch 1 ofthis invention so that the load current is carried through contacts 7 and 11. Heater 19 is selectively energized by means of a second power E and a switch S. Thus, with contacts 7 and 1] closed and with bimetallic arm 15 at room temperature, the load current will continue to flow, In delayed response to closing of switch S and energizing heater I9, bimetallic arm 15 flexes upwardly and opens the contacts. The contacts will remain open so long as switch S is closed and heater 19 remains energized. Upon opening switch S, the heater is deenergized thereby allowing it and the bimetallic arm to cool so as to permit the bimetallic arm to flex in the opposite direction and to thus close the contacts in delayed response to deenergization of the heater.

As shown in the drawings, bimetallic arm 15 is dis posed in switch 1 with its high expansion side or layer 27 facing away from switch arms 5 and 9 (Le, it faces downwardly). Thus upon heating the bimetallic arm, its free end moves upwardly (as shown in FIG. 3). At room temperature, contacts 7 and II are normally closed and heating of bimetallic arm 15 causes the contacts to open. It will be understood that by disposing bimetallic arm 15 in the switch so its high expansion side 27 faces the switch arms and by suitably modifying bimetallic arm 15 so contacts 7 and 11 are open when the bimetallic arm is at room or other reference temperature (i.e., so that switch 1 is normally open), heating bimetallic arm 15 will cause it to flex away from the switch arms. Lip 39 of insulator 17 is movable with the bimetallic arm and it will effect downward movement of switch arm 5 through its unstable center position thus causing switch arm 5 to abruptly move toward switch arm 9 closing contacts 7 and 11. Subsequent deenergization of heater 9 and consequent cooling of the bimetallic arm will cause the contacts to reopen.

In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

As various changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

l. A time-delay switch comprising a base, a first flexible switch arm cantilevered from said base and carrying a first contact on the free end thereof, a second flexible switch arm cantilevered from said base and carrying a second contact on the free end thereof engageable and disengageable with the first contact for making and breaking an electrical circuit, thermal actuator means comprising a bimetallic arm cantilevered from said base for flexing toward and away from said switch arms in response to changes in temperature, said first and second switch arms and said bimetallic arm being substantially parallel to and spaced from one another for flexing movements in a common plane and about spaced apart transverse axes for each of said arms lying in a plane substantially perpendicular to said common plane contiguous to said base, means coupling said bimetallic arm and at least one of said switch arms to effect movement of the free end thereof relative to said base and to the other switch arm in response to flexing of said bimetallic arm between a closed position in which said first and second contacts are in engagement with one another for making said electrical circuit and an open position in which the contacts are clear of one another for breaking said circuit. and self-regulating heating means interposed between the fixed ends ofthe bimetallic arm and the second switch arm and having opposed current carrying faces respectively in electrical and thermal contact with the fixed ends of the first switch arm and with the bimetallic arm for heating the latter thereby to cause said bimetallic arm to flex and to thus effect movement of the contacts between their closed and open positions in delayed response to energization of said heater means.

2. A time-delay switch as set forth in claim 1 wherein said self-regulating heater is a self-heating positive coefficient resistor having a relatively low initial resistance which increases abruptly as its temperature rises above a given level 3. A time-delay switch as set forth in claim 2 further comprising means for maintaining said first and second contacts in engagement with one another during initial flexing of said bimetallic arm and for abruptly opening said contacts upon flexure of said first switch arm beyond a predetermined position.

4. A time-delay switch as set forth in claim 2 wherein said bimetallic arm has its higher expansion layer facing away from switch arms whereby with said bimetallic arm substantially .at a reference temperature said contacts are closed and upon energization of said heater to effect heating of said bimetallic arm the latter flexes toward said switch arms thereby to effect opening of said contacts in delayed response to energization of said heater.

5. A time-delay switch as set forth in claim 2 wherein said bimetallic arm has its higher expansion layer facing toward said switch arms whereby with said bimetallic arm substantially at a reference temperature said contacts are open and upon energization of said heater to effect heating of said bimetallic arm the latter flexes away from said switch arms thereby to effect closing of said contacts in delayed response to energization of the heater.

6. A time-delay switch as set forth in claim 2 wherein said base comprises electrical insulation means separating said first and second switch arms from one another.

7. A time-delay switch as set forth in claim 6 wherein said insulating means comprises a plurality of annular insulators having a bore therethrough wherein each of said switch arms and said bimetallic arm has a bore therethrough. and wherein said heater is an annular member having a bore therethrough said bores being coaxially aligned. and means traversing said bores for retaining the foregoing components assembled.

8. A time-delay switch as set forth in claim 3 wherein said first switch arm is movable from a first stable position relative to said base through an unstable center po' sition and to a second stable position. wherein said first switch arm is reversibly movable from its second stable to its first stable position. and wherein said means for maintaining said contacts in engagement with one an' other and for abruptly breaking said contacts includes overcentering means for abruptly moving said first switch arm from its unstable center position to one of its stable positions upon movement from its other stable position through its center position.

9. A time-delay switch as set forth in claim 8 wherein said overcentering means comprises a pair of slots in said first switch arm extending longitudinally thereof but terminating short of its free end and defining a pair of outer strips and a center strip, said outer strips each having a deformed portion so as to shorten said outer strips with respect to said center strip and to thus place said center strip under compression. whereby upon dcfleeting said first switch arm from one stable position through its center position it abruptly moves towards its other stable position.

10. A time-delay switch as set forth in claim 9 wherein said coupling means comprises an insulative member interposed between the free end of said bimetallic arm and said second switch arm so that with said first switch arm in its first stable position and with said first and second contacts in engagement with one an other. flexing of said bimetallic arm towards said second switch arm causes said insulative member to engage said second switch arm and to thus effect movement of said first switch arm from its first stable position toward its second stable position whereby upon movement of said switch arm through its center position it abruptly moves toward its second stable position.

11. A time-delay switch as set forth in claim 10 wherein said insulative member further comprises a portion engageable with said first switch arm as the latter moves from its center position toward its second stable position after said first contact has moved clear of said second contact. said portion of said insulative member effecting movement of said first switch arm toward its first stable position upon flexing of said bimetallic arm in the opposite direction away from said second switch arm whereby upon movement of said first switch arm from its second stable position through its said center position it abruptly moves toward its said first stable position thereby to remake said first and second contacts.

12. A time-delay switch as set forth in claim 1 further comprising means for selectively adjusting the time delay between energization and deenergization of said heater and movement of said contacts between their open and closed positions.

13. A time-delay switch as set forth in claim 9 further comprising means for selectively adjusting the time delay between energization and deenergization of said heater and movement of said contacts between their open and closed positions said adjustment means including a stop carried by said base and engageble with said center strip intermediate the ends thereof. said stop being selectively movable toward and away from said center strip thereby to vary the amount of move ment of the free end of said first switch arm required to effect movement thereof between its first and second stable positions.

Claims

1. A time-delay switch comprising a base, a first flexible switch arm cantilevered from said base and carrying a first contact on the free end thereof, a second flexible switch arm cantilevered from said base and carrying a second contact on the free end thereof engageable and disengageable with the first contact for making and breaking an electrical circuit, thermal actuator means comprising a bimetallic arm cantilevered from said base for flexing toward and away from said switch arms in response to changes in temperature, said first and second switch arms and said bimetallic arm being substantially parallel to and spaced from one another for flexing movements in a common plane and about spaced apart transverse axes for each of said arms lying in a plane substantially perpendicular to said common plane contiguous to said base, means coupling said bimetallic arm and at least one of said switch arms to effect movement of the free end thereof relative to said base and to the other switch arm in response to flexing of said bimetallic arm between a closed position in which said first and second contacts are in engagement with one another for making said electrical circuit and an open position in which the contacts are clear of one another for breaking said circuit, and self-regulating heating means interposed between the fixed ends of the bimetallic arm and the second switch arm and having opposed current- carrying faces respectively in electrical and thermal contact with the fixed ends of the first switch arm and with the bimetallic arm for heating the latter thereby to cause said bimetallic arm to flex and to thus effect movement of the contacts between their closed and open positions in delayed response to energization of said heater means.

2. A time-delay switch as set forth in claim 1 wherein said self-regulating heater is a self-heating positive coefficient resistor having a relatively low initial resistance which increases abruptly as its temperature rises above a given level.

3. A time-delay switch as set forth in claim 2 further comprising means for maintaining said first and second contacts in engagement with one another during initial flexing of said bimetallic arm and for abruptly opening said contacts upon flexure of said first switch arm beyond a predetermined position.

4. A time-delay switch as set forth in claim 2 wherein said bimetallic arm has its higher expansion layer facing away from switch arms whereby with said bimetallic arm substantially at a reference temperature said contacts are closed and upon energization of said heater to effect heating of said bimetallic arm the latter flexes toward said switch arms thereby to effect opening of said contacts in delayed response to energization of said heater.

7. A time-delay switch as set forth in claim 6 wherein said insulating means comprises a plurality of annular insulators having a bore therethrough wherein each of said switch arms and said bimetallic arm has a bore therethrough, and wherein said heater is an annular member having a bore therethrough said bores being coaxially aligned, and means traversing said bores for retaining the foregoing components assembled.

8. A time-delay switch as set forth in claim 3 wherein said first switch arm is movable from a first stable position relative to said base through an unstable center position and to a second stable position, wherein said first switch arm is reversibly movable from its second stable to its first stable position, and wherein said means for maintaining said contacts in engagement with one another and for abruptly breaking said contacts includes overcentering means for abruptly moving said first switch arm from its unstable center position to one of its stable positions upon movement from its other stable position through its center position.

9. A time-delay switch as set forth in claim 8 wherein said overcentering means comprises a pair of slots in said first switch arm extending longitudinally thereof but terminating short of its free end and defining a pair of outer strips and a center strip, said outer strips each having a deformed portion so as to shorten said outer strips with respect to said center strip and to thus place said center strip under compression, whereby upon deflecting said first switch arm from one stable position through its center position it abruptly moves towards its other stable position.

10. A time-delay switch as set forth in claim 9 wherein said coupling means comprises an insulative member interposed between the free end of said bimetallic arm and said second switch arm so that with said first switch arm in its first stable position and with said first and second contacts in engagement with one another, flexing of said bimetallic arm towards said second switch arm causes said insulative member to engage said second switch arm and to thus effect movement of said first switch arm from its first stable position toward its second stable position whereby upon movement of said switch arm through its center position it abruptly moves toward its second stable position.

11. A time-delay switch as set forth in claim 10 wherein said insulative member further comprises a portion engageable with said first switch arm as the latter moves from its center position toward its second stable position after said first contact has moved clear of said second contact, said portion of said insulative member effecting movement of said first switch arm toward its first stable position upon flexing of said bimetallic arm in the opposite direction away from said second switch arm whereby upon movement of said first switch arm from its second stable position through its said center position it abruptly moves toward its said first stable position thereby to remake said first and second contacts.

13. A time-delay switch as set forth in claim 9 further comprising means for selectively adjusting the time delay between energization and deenergization of said heater and movement of said contacts between their open and closed positions said adjustment means including a stop carried by said base and engageble with said center strip intermediate the ends thereof, said stop being selectively movable toward and away from said center strip thereby to vary the amount of movement of the free end of said first switch arm required to effect movement thereof between its first and second stable positions.