Oven valve circuit and thermostatic actuator thereof
United States Patent 3925746
A gas oven having one thermostat switch operated main burner solenoid valve utilizing a pilot heated thermoelectric generator for electrical power to energize the valve solenoid. The generator, the solenoid and the one switch are thus connected in series in a closed loop. A timer and another thermostat switch are connected in a series circuit that shunts the solenoid or thermoelectric generator or both. Opening and closing of the one switch controls baking temperature. The timer switch disables the other switch until the baking period of time has ended. The timer switch then closes and permits the other switch to act as an override and periodically to provide a short circuit to keep the oven warm, but at a temperature, e.g. 170° F., less than the baking temperature. The short circuiting of the solenoid or thermoelectric generator is an outstanding advantage because expensive gold or platinum plated contacts and unreliability are avoided and the time-control circuit is simplified. Inexpensive high resistance contacts in the closed loop are undesirable because of the low e.m.f. provided by the thermoelectric generator. A single thermostatic actuator operates both of the thermostat switches. The single thermostatic actuator makes possible a substantial equipment and cost saving. The actuator is also of an economical construction because leaf springs and a diphragm bias spring are employed.
US Patent References:
Dual action thermostat having plural adjustment means
Levinn - December 1963 - 3114812
ELECTRICAL THERMOSTAT AND PARTS THEREFOR OR THE LIKE
Wolfe et al. - September 1969 - 3469779
FUEL CONTROL SYSTEM AND PARTS THEREFOR OR THE LIKE
Branson et al. - November 1971 - 3617972
Dual action thermostat having plural adjustment means
Levinn - December 1963 - 3114812
ELECTRICAL THERMOSTAT AND PARTS THEREFOR OR THE LIKE
Wolfe et al. - September 1969 - 3469779
FUEL CONTROL SYSTEM AND PARTS THEREFOR OR THE LIKE
Branson et al. - November 1971 - 3617972
Application Number:
05/291705
Publication Date:
12/09/1975
Filing Date:
09/25/1972
View Patent Images:
Export Citation:
Assignee:
International Telephone and Telegraph Corporation (New York, NY)
Primary Class:
Other Classes:
337/323, 337/320
International Classes:
F24C3/12; G05D23/19; H05B1/02; H01H37/40; H01H37/28
Field of Search:
337/309,320,323,340
Primary Examiner:
Kozma, Thomas J.
Assistant Examiner:
Tone, David A.
Attorney, Agent or Firm:
Stolzy, Donald A.
Parent Case Data:
This is a division of copending application Ser. No. 176,139, filed Aug. 30, 1971 now U.S. Pat. No. 3,754,704. The benefit of the filing date of said copending application is, therefore, hereby claimed for this application.
Claims:
What is claimed is
1. A thermostat comprising: a base; first switch means including a first conductive contact; first means on said base to hold said first contact in a first predetermined position; a member movably mounted on said base; temperature sensitive second means mounted on said base to move said member in accordance with temperature; overtravel means spring biased against said member in a direction toward said second means, said member being movable by said second means toward and away from said first contact, said first switch means including a second condutive contact carried by said overtravel means in a position to engage said first contact when said member is close enough to said first means, said overtravel means moving relative to said member when said member is moved toward said first means during engagement of said contacts; and second switch means fixed relative to said base, said second switch means being actuable in response to movement of said member, said second switch means including a leaf fixed at one end relative to said base, said leaf running approximately in a plane parallel to that of said member on the side thereof opposite the side of said member on which said second means is located; and a stop mounted on said base in a position to be engaged by the other end of said leaf on the member side of said leaf, said second switch means having third and fourth conductive contact portions engageable with each other and fixed to said member and said leaf, respectively, said third and fourth contact portions being mutually engaged at and above a predetermined temperature sensed by said second means, said leaf always being spring biased toward said member from a position on one side of said member opposite the side on which said second means is positioned, said leaf being spring biased into engagement with said stop when it engages the same, said third and fourth contact portions being disengaged from each other when said member moves away from said leaf below the stabilized position thereof which it reaches when it engages said stop.
1. A thermostat comprising: a base; first switch means including a first conductive contact; first means on said base to hold said first contact in a first predetermined position; a member movably mounted on said base; temperature sensitive second means mounted on said base to move said member in accordance with temperature; overtravel means spring biased against said member in a direction toward said second means, said member being movable by said second means toward and away from said first contact, said first switch means including a second condutive contact carried by said overtravel means in a position to engage said first contact when said member is close enough to said first means, said overtravel means moving relative to said member when said member is moved toward said first means during engagement of said contacts; and second switch means fixed relative to said base, said second switch means being actuable in response to movement of said member, said second switch means including a leaf fixed at one end relative to said base, said leaf running approximately in a plane parallel to that of said member on the side thereof opposite the side of said member on which said second means is located; and a stop mounted on said base in a position to be engaged by the other end of said leaf on the member side of said leaf, said second switch means having third and fourth conductive contact portions engageable with each other and fixed to said member and said leaf, respectively, said third and fourth contact portions being mutually engaged at and above a predetermined temperature sensed by said second means, said leaf always being spring biased toward said member from a position on one side of said member opposite the side on which said second means is positioned, said leaf being spring biased into engagement with said stop when it engages the same, said third and fourth contact portions being disengaged from each other when said member moves away from said leaf below the stabilized position thereof which it reaches when it engages said stop.
Description:
BACKGROUND OF THE INVENTION
This invention relates to cooking apparatus, and more particularly, to a low voltage system to keep a gas oven at one or more approximately constant temperatures.
In the past, it has been the practice to utilize the available e.m.f. from a thermoelectric generator heated by a gas pilot. The main burner valve solenoid is thus connected to the generator periodically by a thermostat to keep the oven at a preset baking temperature. Generally, the thermostat switch contacts are gold or platinum plated to reduce the contact resistance. Thus, the introduction of further series contacts, even when gold plated, can thus cause further contact resistance and load the generator so that it will not reliably energize the main valve solenoid. Prior art keep warm systems are complicated. It has, therefore, been impossible to economically control prior art ovens to bake food or to keep food warm after it has been cooked.
SUMMARY OF THE INVENTION
In accordance with the device of the present invention, the above-described and other disadvantages of the prior art are overcome by providing an economical thermostatic actuator.
According to one feature of the invention, a single thermostatic actuator is provided for a first switch that periodically short circuits the generator or the solenoid winding for keep warm temperature control, and a second switch which connects the generator and winding for baking temperature control.
The first switch contacts are thus not connected in series with the generator and solenoid winding to introduce unreliability by the addition of more contact resistance.
It is also a feature of the invention that the selfsame thermostatic actuator may be employed to operate both of the first and second switches. A substantial equipment and cost saving are, therefore, achieved.
Another feature of the invention resides in the use of leaf springs which are less expensive than levers employed in the prior art.
Still a further feature of the invention resides in an economical bias spring for a diaphragm in the thermostatic actuator.
The above-described and other advantages of the present invention will be better understood from the following detailed description when considered in connection with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings which are to be regarded as merely illustrative:
FIG. 1 is a vertical sectional view through an oven which may be employed with the present invention;
FIG. 2 is a vertical sectional view through a portion of a thermostat constructed in accordance with the present invention;
FIG. 3 is a horizontal sectional view of the thermostat portion taken on the line 3--3 shown in FIG. 2;
FIG. 4 is another vertical sectional view through the thermostat portion taken on the line 4--4 shown in FIG. 2;
FIG. 5 is a perspective view of a diaphragm spring shown in FIGS. 2, 3 and 4;
FIG. 6 is an inverted vertical sectional view of the thermostat portion taken on the line 6--6 shown in FIG. 3; and
FIG. 7 is a block diagram of the system of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In FIG. 1, an oven is indicated at 10 having a housing 11, a gas fuel inlet conduit 12 connected to a pilot burner 13, a main burner 14 and a solenoid valve 15. Solenoid valve 15 receives gas by a connection 16 from inlet 12. Valve 15 controls whether or not gas is admitted to main burner 14. A thermoelectric generator 17 is positioned adjacent pilot burner 13 to be heated by gas burning thereat. A metal bulb 18 contains a thermally expansible liquid. The bulb 18 and the liquid thus form a part of the thermostat, to be described. A hollow tube 19 is sealed to bulb 18.
The oven 10, and all parts thereof thus far described in connection with FIG. 1, may, by themselves only, be entirely conventional. The same is true of all the structures shown in FIG. 1.
The baking temperature of the oven 10 is set by adjusting the rotational position of first means 21 which includes a shaft shown in FIG. 2. First means 21 is threaded through a base 22 approximately in the shape of a plate. Base 22 has an upper cover 23 fixed thereto by any conventional means such as screws. Base 22 also has a lower cover 24 fixed to base 22 by any conventional means. Base 22 and cover 23 may be made of, for example, metal. Cover 24 may be made of a dielectric material.
First means 21 has a calibration screw 25 threaded into the lower end thereof.
Tube 19 in FIG. 1 carries the said fluid to second means 20 including a flexible diaphragm shown in FIG. 2 which moves downwardly or upwardly in FIG. 2 depending upon the temperature of the fluid in bulb 19. Second means 20 may, by itself, be entirely conventional.
Second means 20 has a button 26 fixed thereto. A U-shaped spring 27 biases second means 20 upwardly as viewed in FIG. 2. It is a feature of the invention that leaf spring 27 has legs 28 with leaf spring tines 29 that are slidable upwardly in base 22 through slots 30 therein, as shown in FIG. 4. This construction makes possible an easy assembly because there is a snap fit. Tines 29 are again shown in FIG. 5. Note will be taken that the purpose of spring 27 is to preload second means 20.
As shown in FIG. 2, an actuating member 31 which is a main conductive leaf spring has a concave hemispherical portion 32 to which is bonded a dielectric sphere 33 that bears against the lower surface of spring 27. An auxiliary member 34 which is a conductive metal leaf spring has a hemispherical electrical contact 35 which touches member 31.
An overtravel member 36 which is a conductive metal leaf spring acts as an overtravel spring. It is spot welded to member 31 at 37. When unstressed, before spot welding at 37, member 36 may be supplied with an appropriate bend at 38 so that end 39 will touch member 31 when not lifted away therefrom by engagement of an electrical contact 40 with a contact 41 on member 36.
Contacts 40 and 41 both may be made of gold or a gold alloy and bonded to conductive metal leaf springs 42 and 36, respectively.
As shown in FIG. 3, member 31 has a hole 43 through which an electrical connector pin 44 projects. Member 31 is thus maintained in compression between a flange 45 integral with pin 44 and an insulator 46 having a flange 47. Pin 44 is press-fit into insulator 46.
Members 31 and 36, and spring 42 may be, if desired, mounted in an identical manner with pins identical to pin 44 and insulators identical to insulator 46.
As shown in FIG. 2, a spherical dielectric body 48 is bonded to the right end of spring 42 in the same manner that body 33 is bonded to member 31. Bodies 48 and 33 are thus in the shape of balls. Balls 48 and 33 bear against calibration screw 25 and spring 27, respectively.
As shown in FIG. 3, members 31 and 34, and spring 42 may be, if not otherwise positioned to do so, bent, for example, on lines 49, 50 and 51 shown in FIG. 3 to be "prestressed." That is, they are either shaped or positioned or both normally to lie in pressure contact with other corresponding structures. For example, member 31 is positioned and shaped in a manner such that ball 33 lies in pressure contact with spring 27. Similarly, member 34 is shaped and positioned in a manner such that contact 35 lies in engagement with member 31 so long as the end 52 of member 34 does not lie in engagement with stop 53 which is a set screw, shown in FIG. 6.
Still further, spring 42 is shaped and positioned in a manner such that ball 48 lies in pressure contact with calibration screw 25.
As shown in FIG. 3, member 31 has a hole 54 therethrough through which contact 40 makes with contact 41. See also FIG. 2.
As indicated previously, the system of the present invention may be employed to keep an oven warm, for example, at 170° F., after food in the oven has cooked. The temperature setting to keep the oven warm may be achieved by turning stop 53, shown in FIG. 6.
Pin 44' identical to pin 44 is also shown in FIG. 6 with an insulator 46' identical to insulator 46. The pin and insulator for spring 42 may be longer or shorter than pin 44 and insulator 46, if desired to keep spring 42 spaced from member 31, as shown in FIG. 2. The same is true of the location of that portion of plate 22 surrounding the insulator employed in mounting spring 42.
In FIG. 7, the switch which is formed by contacts 40 and 41 is indicated at 55. The switch which is formed between contact 35 and member 31 is indicated at 56. All of the structure required to operate switches 55 and 56 is identified by the legend "thermostatic actuator" and given the reference numeral 57. Thermoelectric generator 17 is connected from ground to contact 40. Valve 15 has a solenoid winding 58 which is connected from ground to both contact 41 and member 31. A manual-automatic switch 59 and a timer switch 60 are connected in that order from contact 35 to ground. A timer 61 operates timer switch 60.
OPERATION
In the operation of the invention, assume that first means 21 is turned to its off position. In that case, the contacts 40 and 41 will break, spring 27 keeping members 31 and 36 spaced from spring 42 and thereby preventing contact 41 from touching contact 40. Generator 17 will thus not be connected to winding 58, and switch 55 will open. Valve 15 will thus be deenergized, and main burner 14 will receive no gas for combustion.
The normal position for switch 59 will be the "manual" position. In this way, the oven can be operated without the keep warm feature. This is true because the closure of all three of the switches 56, 59 and 60 will cause winding 58 and/or generator 17 to be short circuited.
With switch 59 in the manual position, first means 21 may be turned to a baking temperature, and food cooked without the benefit of timer 61 for any length of time desired. During this mode of operation, contact 40 is placed in engagement with contact 41 and generator 17 energizes winding 58 through the closing of switch 55 until the oven comes up to temperature. When the oven slightly exceeds the set baking temperature, spring 27 will be expanded downwardly by second means 20, as shown in FIG. 2, and member 31 will engage spring end 39 and lift contact 41 out of engagement with contact 40. As the oven cools, contacts 40 and 41 will make again and the process will be repeated. First means 21 may then eventually be set to its off position.
Timer 61 is of the type such that when it is set to time a predetermined period, switch 60 is open. When the timer 61 runs down to zero, switch 60 closes. Thus, for timed cooking, switch 59 is moved to its automatic position, and timer 61 is turned to time a predetermined period. Switch 60 is then open. First means 21 is then turned to a desired baking temperature. So long as timer 61 keeps switch 60 open, baking takes place in a manner exactly the same as that previously described herein. When switch 60 closes, note will be taken that, in general, first means 21 will be set to a baking temperature substantially above the keep warm temperature. For this reason, contact 35 will always be in engagement with member 31 when switch 60 closes. Switch 59 stays in the "automatic" position until the oven is shut completely off. Thus, all of the switches 56, 59 and 60 are closed. The same is true of switch 55 when switch 60 closes. Hence, generator 17 and winding 58 are both short circuited through the switches shown in FIG. 7. Valve 15, therefore, closes and oven 10 cools down until slightly below the keep warm temperature. In this case, member 31 will follow spring 27 upwardly in FIG. 6 and move out of engagement with contact 35. This will open switch 56 and remove the short circuit. Winding 58 will then again be energized, and valve 15 will be opened. Oven 10 will then be again heated to slightly above the keep warm temperature, and spring 27 will move member 31 downwardly into engagement with contact 35. Again, generator 17 and winding 58 will be short circuited. The process will be repeated continuously until switch 59 is moved to the manual position and first means 21 is turned to the off position.
If desired, timer 61 may be of the type which incorporates the function of switch 59 by having both a zero-time and an off position spaced from the zero-time position. In the off position, switch 60 is open. If this is the case, switch 59 may be omitted.
Although the system of FIG. 7 has been shown to have a ground reference, an ungrounded system is also within the scope of the present invention.
It is also a feature of the invention that member 31 may be solid without aperture 54, and overtravel member 36 may be eliminated if the invention is to be operated without the keep warm feature.
This invention relates to cooking apparatus, and more particularly, to a low voltage system to keep a gas oven at one or more approximately constant temperatures.
In the past, it has been the practice to utilize the available e.m.f. from a thermoelectric generator heated by a gas pilot. The main burner valve solenoid is thus connected to the generator periodically by a thermostat to keep the oven at a preset baking temperature. Generally, the thermostat switch contacts are gold or platinum plated to reduce the contact resistance. Thus, the introduction of further series contacts, even when gold plated, can thus cause further contact resistance and load the generator so that it will not reliably energize the main valve solenoid. Prior art keep warm systems are complicated. It has, therefore, been impossible to economically control prior art ovens to bake food or to keep food warm after it has been cooked.
SUMMARY OF THE INVENTION
In accordance with the device of the present invention, the above-described and other disadvantages of the prior art are overcome by providing an economical thermostatic actuator.
According to one feature of the invention, a single thermostatic actuator is provided for a first switch that periodically short circuits the generator or the solenoid winding for keep warm temperature control, and a second switch which connects the generator and winding for baking temperature control.
The first switch contacts are thus not connected in series with the generator and solenoid winding to introduce unreliability by the addition of more contact resistance.
It is also a feature of the invention that the selfsame thermostatic actuator may be employed to operate both of the first and second switches. A substantial equipment and cost saving are, therefore, achieved.
Another feature of the invention resides in the use of leaf springs which are less expensive than levers employed in the prior art.
Still a further feature of the invention resides in an economical bias spring for a diaphragm in the thermostatic actuator.
The above-described and other advantages of the present invention will be better understood from the following detailed description when considered in connection with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings which are to be regarded as merely illustrative:
FIG. 1 is a vertical sectional view through an oven which may be employed with the present invention;
FIG. 2 is a vertical sectional view through a portion of a thermostat constructed in accordance with the present invention;
FIG. 3 is a horizontal sectional view of the thermostat portion taken on the line 3--3 shown in FIG. 2;
FIG. 4 is another vertical sectional view through the thermostat portion taken on the line 4--4 shown in FIG. 2;
FIG. 5 is a perspective view of a diaphragm spring shown in FIGS. 2, 3 and 4;
FIG. 6 is an inverted vertical sectional view of the thermostat portion taken on the line 6--6 shown in FIG. 3; and
FIG. 7 is a block diagram of the system of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In FIG. 1, an oven is indicated at 10 having a housing 11, a gas fuel inlet conduit 12 connected to a pilot burner 13, a main burner 14 and a solenoid valve 15. Solenoid valve 15 receives gas by a connection 16 from inlet 12. Valve 15 controls whether or not gas is admitted to main burner 14. A thermoelectric generator 17 is positioned adjacent pilot burner 13 to be heated by gas burning thereat. A metal bulb 18 contains a thermally expansible liquid. The bulb 18 and the liquid thus form a part of the thermostat, to be described. A hollow tube 19 is sealed to bulb 18.
The oven 10, and all parts thereof thus far described in connection with FIG. 1, may, by themselves only, be entirely conventional. The same is true of all the structures shown in FIG. 1.
The baking temperature of the oven 10 is set by adjusting the rotational position of first means 21 which includes a shaft shown in FIG. 2. First means 21 is threaded through a base 22 approximately in the shape of a plate. Base 22 has an upper cover 23 fixed thereto by any conventional means such as screws. Base 22 also has a lower cover 24 fixed to base 22 by any conventional means. Base 22 and cover 23 may be made of, for example, metal. Cover 24 may be made of a dielectric material.
First means 21 has a calibration screw 25 threaded into the lower end thereof.
Tube 19 in FIG. 1 carries the said fluid to second means 20 including a flexible diaphragm shown in FIG. 2 which moves downwardly or upwardly in FIG. 2 depending upon the temperature of the fluid in bulb 19. Second means 20 may, by itself, be entirely conventional.
Second means 20 has a button 26 fixed thereto. A U-shaped spring 27 biases second means 20 upwardly as viewed in FIG. 2. It is a feature of the invention that leaf spring 27 has legs 28 with leaf spring tines 29 that are slidable upwardly in base 22 through slots 30 therein, as shown in FIG. 4. This construction makes possible an easy assembly because there is a snap fit. Tines 29 are again shown in FIG. 5. Note will be taken that the purpose of spring 27 is to preload second means 20.
As shown in FIG. 2, an actuating member 31 which is a main conductive leaf spring has a concave hemispherical portion 32 to which is bonded a dielectric sphere 33 that bears against the lower surface of spring 27. An auxiliary member 34 which is a conductive metal leaf spring has a hemispherical electrical contact 35 which touches member 31.
An overtravel member 36 which is a conductive metal leaf spring acts as an overtravel spring. It is spot welded to member 31 at 37. When unstressed, before spot welding at 37, member 36 may be supplied with an appropriate bend at 38 so that end 39 will touch member 31 when not lifted away therefrom by engagement of an electrical contact 40 with a contact 41 on member 36.
Contacts 40 and 41 both may be made of gold or a gold alloy and bonded to conductive metal leaf springs 42 and 36, respectively.
As shown in FIG. 3, member 31 has a hole 43 through which an electrical connector pin 44 projects. Member 31 is thus maintained in compression between a flange 45 integral with pin 44 and an insulator 46 having a flange 47. Pin 44 is press-fit into insulator 46.
Members 31 and 36, and spring 42 may be, if desired, mounted in an identical manner with pins identical to pin 44 and insulators identical to insulator 46.
As shown in FIG. 2, a spherical dielectric body 48 is bonded to the right end of spring 42 in the same manner that body 33 is bonded to member 31. Bodies 48 and 33 are thus in the shape of balls. Balls 48 and 33 bear against calibration screw 25 and spring 27, respectively.
As shown in FIG. 3, members 31 and 34, and spring 42 may be, if not otherwise positioned to do so, bent, for example, on lines 49, 50 and 51 shown in FIG. 3 to be "prestressed." That is, they are either shaped or positioned or both normally to lie in pressure contact with other corresponding structures. For example, member 31 is positioned and shaped in a manner such that ball 33 lies in pressure contact with spring 27. Similarly, member 34 is shaped and positioned in a manner such that contact 35 lies in engagement with member 31 so long as the end 52 of member 34 does not lie in engagement with stop 53 which is a set screw, shown in FIG. 6.
Still further, spring 42 is shaped and positioned in a manner such that ball 48 lies in pressure contact with calibration screw 25.
As shown in FIG. 3, member 31 has a hole 54 therethrough through which contact 40 makes with contact 41. See also FIG. 2.
As indicated previously, the system of the present invention may be employed to keep an oven warm, for example, at 170° F., after food in the oven has cooked. The temperature setting to keep the oven warm may be achieved by turning stop 53, shown in FIG. 6.
Pin 44' identical to pin 44 is also shown in FIG. 6 with an insulator 46' identical to insulator 46. The pin and insulator for spring 42 may be longer or shorter than pin 44 and insulator 46, if desired to keep spring 42 spaced from member 31, as shown in FIG. 2. The same is true of the location of that portion of plate 22 surrounding the insulator employed in mounting spring 42.
In FIG. 7, the switch which is formed by contacts 40 and 41 is indicated at 55. The switch which is formed between contact 35 and member 31 is indicated at 56. All of the structure required to operate switches 55 and 56 is identified by the legend "thermostatic actuator" and given the reference numeral 57. Thermoelectric generator 17 is connected from ground to contact 40. Valve 15 has a solenoid winding 58 which is connected from ground to both contact 41 and member 31. A manual-automatic switch 59 and a timer switch 60 are connected in that order from contact 35 to ground. A timer 61 operates timer switch 60.
OPERATION
In the operation of the invention, assume that first means 21 is turned to its off position. In that case, the contacts 40 and 41 will break, spring 27 keeping members 31 and 36 spaced from spring 42 and thereby preventing contact 41 from touching contact 40. Generator 17 will thus not be connected to winding 58, and switch 55 will open. Valve 15 will thus be deenergized, and main burner 14 will receive no gas for combustion.
The normal position for switch 59 will be the "manual" position. In this way, the oven can be operated without the keep warm feature. This is true because the closure of all three of the switches 56, 59 and 60 will cause winding 58 and/or generator 17 to be short circuited.
With switch 59 in the manual position, first means 21 may be turned to a baking temperature, and food cooked without the benefit of timer 61 for any length of time desired. During this mode of operation, contact 40 is placed in engagement with contact 41 and generator 17 energizes winding 58 through the closing of switch 55 until the oven comes up to temperature. When the oven slightly exceeds the set baking temperature, spring 27 will be expanded downwardly by second means 20, as shown in FIG. 2, and member 31 will engage spring end 39 and lift contact 41 out of engagement with contact 40. As the oven cools, contacts 40 and 41 will make again and the process will be repeated. First means 21 may then eventually be set to its off position.
Timer 61 is of the type such that when it is set to time a predetermined period, switch 60 is open. When the timer 61 runs down to zero, switch 60 closes. Thus, for timed cooking, switch 59 is moved to its automatic position, and timer 61 is turned to time a predetermined period. Switch 60 is then open. First means 21 is then turned to a desired baking temperature. So long as timer 61 keeps switch 60 open, baking takes place in a manner exactly the same as that previously described herein. When switch 60 closes, note will be taken that, in general, first means 21 will be set to a baking temperature substantially above the keep warm temperature. For this reason, contact 35 will always be in engagement with member 31 when switch 60 closes. Switch 59 stays in the "automatic" position until the oven is shut completely off. Thus, all of the switches 56, 59 and 60 are closed. The same is true of switch 55 when switch 60 closes. Hence, generator 17 and winding 58 are both short circuited through the switches shown in FIG. 7. Valve 15, therefore, closes and oven 10 cools down until slightly below the keep warm temperature. In this case, member 31 will follow spring 27 upwardly in FIG. 6 and move out of engagement with contact 35. This will open switch 56 and remove the short circuit. Winding 58 will then again be energized, and valve 15 will be opened. Oven 10 will then be again heated to slightly above the keep warm temperature, and spring 27 will move member 31 downwardly into engagement with contact 35. Again, generator 17 and winding 58 will be short circuited. The process will be repeated continuously until switch 59 is moved to the manual position and first means 21 is turned to the off position.
If desired, timer 61 may be of the type which incorporates the function of switch 59 by having both a zero-time and an off position spaced from the zero-time position. In the off position, switch 60 is open. If this is the case, switch 59 may be omitted.
Although the system of FIG. 7 has been shown to have a ground reference, an ungrounded system is also within the scope of the present invention.
It is also a feature of the invention that member 31 may be solid without aperture 54, and overtravel member 36 may be eliminated if the invention is to be operated without the keep warm feature.