Induction driven power supply for circuits accompanying portable heated items

Abstract

An induction heating system having an induction source, a heating element heated from the induction source and a circuit energized by the induction source. The circuit can be a controller which includes a temperature sensor for measuring a temperature of the heating element, and a feedback loop formed between the temperature sensor and the induction source. The heating element can be mounted within a housing to form an induction heated container for holding items to be heated. Such a container can be used in commercial food warming and holding.

Description

BACKGROUND OF THE INVENTION

Induction heating technology is well known and in wide spread use in industrial and commercial applications. One of the advantages of induction heating is the “non-contact” aspect of the technology. In particular, an induction heater uses magnetic fields to energize a heating element formed of a suitable radiation-sensitive material. The magnetic field generator need not be in contact with the heating element or even the item which is itself to be elevated in temperature. This arrangement makes induction heating a wise choice in applications where the heated item must easily be moved. These include industrial applications such as assembly lines or branding irons, as well as commercial food and plate warming. Other applications involve containers for take out food, such as pizza delivery bags, for example. These containers have typically been made with an external temperature indicator and a heating element heated by an AC source. These containers include an AC cord which can potentially entangle a user, creating safety issues when the container is transported.

There is a problem however with some of these applications. A plate warmer for example, needs to maintain the temperature of the plate below some defined allowable value. This is especially important if the plate is to be handled by a person, or if the plate is constructed of a plastic/metal composite.

One way to control the final temperature of the plate can be to apply the induction heating to the plate for a specific time duration. This method can provide poor results, unless the temperature of the plates was controlled before the start of the heating process. For example, if the same plate was exposed to an induction heater twice in a row, one time right after another, the plate can rise to a much higher temperature.

Another method of controlling the final temperature of the plate uses an external temperature sensor to measure the temperature of the plate before, and/or during the induction heating process. The sensor can be a “contact” or “non-contact” type. The “contact” type of temperature measurement spoils the inherent “non-contact” nature of the induction heating process. Additionally, it can be difficult to get the sensor to contact the correct surface of the heating element while providing a reliable, robust design. The “non-contact” type of temperature measurement is better, but more costly.

A completely different solution might involve a specially formulated metal heating element that only “couples” (i.e., allow currents to be induced) with the induction field if the temperature of the metal is below some pre-determined value. These metals have a Curie point that prevent the metal from overheating, even though the induction field is still present.

The problem with the above methods is that none provide the capability of temperature indication, status monitoring, or other electronic functions without a power supply within the container or a wired, physical connection between the container and an external heater. These methods also do not provide electronic functions after the heated item is removed from the induction heating device.

SUMMARY OF THE INVENTION

A solution to this problem is to place an induction-driven power supply within the electromagnetic field used to heat the heating element. The power supply can, for example, include an induction coil across which is induced a current. In an alternate embodiment, this can be provided by an opening or slot formed on the heating element, the opening having a first lead and a second lead, wherein the opening creates a voltage differential transferred to the first lead and the second lead.

The power supply is used to provide power to various electrical circuits which accompany the heating element. For example, these circuits may include a control system having a temperature sensor, a temperature indicator, and a communication link, such as an RF, light or sound link, which electronically controls the operation of induction source. The controller can communicate to the inductor, via the communication link, if more heating power is necessary and to indicate the desired temperature has been reached. The temperature indicator indicates when the element has reached an acceptable temperature and the unit is ready to be used.

Additionally, the circuits may include energy storage devices, such as rechargeable batteries or capacitors, which are charged while the device is subjected to the electromagnetic field during the induction heating process. These energy storage devices permit the circuit to continue operating even when the container is removed from the electromagnetic field source.

In the case of the controller, the stored energy permits the monitoring of the temperature of the heating element with status LEDs even after the device has been removed from the inductor.

The induction driven circuit and heating element are preferably used in conjunction with a container for heating of food items.

The electromagnetic field can be generated by a single induction source. The induction source can also include a plurality of induction sources. A first induction source and a second induction source can be utilized where the first induction source heats a heating element and the second induction source powers a circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.

FIG. 1 illustrates an induction heating system comprising a power supply.

FIG. 2 illustrates an alternate embodiment of the heating system.

FIG. 3 illustrates a cross sectional view of a heating element housing where the heating element has a coil.

FIG. 4 shows a block diagram of a circuit for a controller.

FIG. 5 illustrates a temperature controller circuit.

FIG. 6A illustrates a temperature indicator circuit.

FIG. 6B shows a state diagram that illustrates operation of the circuit of FIG. 6 A.

FIG. 6C shows a logic diagram that illustrates operation of the circuit of FIG. 6 A.

FIG. 7 shows a blinker circuit.

FIG. 8 illustrates a voltage controlled oscillation circuit.

FIGS. 9 and 10 illustrate an induction heating system for a food container having a plurality of induction sources.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates an induction powered heating system, given generally as 10 . The induction powered heating system 10 includes an induction source 20 and a heating element 22 . The heating system 10 also includes a power supply 42 which is energized by the induction source 20 . The heating element 22 can be formed of a material such that, when exposed to an induction source, a current is created within the heating element, thereby producing heat. The heating element 22 can be formed of a Curie point metal, for example. The heating element is typically mounted within a container or other housing 24 for the items to be heated (not shown).

The heating element 22 is mounted within a housing 24 . The heating element 22 and housing 24 form an induction heated container for holding items to be heated. The housing 24 includes a cavity defined by a top surface 11 , a bottom surface 15 and a side wall 19 . The side wall 19 attaches to an outer edge 13 of the top surface 11 with an outer edge 17 of the bottom surface 15 . A portion of the side wall 19 is moveably attached to the top surface 11 and the bottom surface 15 to allow user access to the cavity. The housing can be made of a thermally insulated material which can contain heat generated by the heating element 22 . The illustrated housing is a bag for storage of food, such as a pizza bag, for example.

The induction source 20 includes a field generator 26 and a power supply 28 . The field generator 26 has a core 56 and a ring 58 , where the core 56 and the ring 58 are made from ferrite, for example. The field generator 26 creates a magnetic flux which is used to induce a current in the heating element 22 , thereby creating heat. The power supply 28 can be a standard 120 VAC or a 240 VAC connection, for example.

The induction source 20 can produce an alternating magnetic flux. For example, at one instant, the core 56 can have a first polarity and the ring 58 can have a second polarity, thereby producing a radial magnetic field directed along the center axis of the core 56 and the ring 58 . At another instant the polarities of the core 56 and the ring 58 can switch such that the core 56 has a second polarity while the ring 58 has a first polarity. The resulting alternating magnetic flux induces a current in the heating element 22 to produce heat, provided that the heating element 22 is placed in close enough proximity to the induction source 20 .

The local power supply 42 is carried within the housing 24 . It can be as simple as an opening 46 on the heating element 22 , shown in FIG. 1 , such as a slot 46 formed in the heating element 22 , for example. Other geometries can also be used. Each side of the opening 46 can be coupled to leads 44 , such as a first lead and a second lead, which, in turn, can be coupled to an electronic circuit. When the heating element 22 is exposed to the induction source 20 , a current is created along the surfaces of the heating element 22 . The opening 46 creates a voltage drop; the leads 44 are placed on either side of the opening 46 draw the AC voltage created by this voltage drop. The voltage thus created is then used to power an electronic circuit.

FIGS. 2 and 3 illustrate an alternate embodiment of power supply 42 as a wire coil 50 . The coil 50 can be mounted in physical relationship within the container to be subjected to the magnetic field created by the induction source 20 . The coil 50 can be formed integrally with the heating element 22 . For example, the coil 50 can be etched or plated on to the heating element 22 . Alternately, the coil can be physically separate from the heating element 22 . Exposure of the coil 50 to a magnetic flux 52 created by the induction source 20 induces a current within the coil 50 . The coil 50 includes coil leads 54 which connect to an electronic circuit and provide power from the current created in the coil 50 to the circuit. In the preferred embodiment, the coil 50 is placed in a plane of the heating element 22 nearest the induction source 20 ; otherwise the material of the element 22 might interfere with the coil 50 receiving sufficient energy.

As mentioned previously, the supply 42 provides power to a circuit located within the housing 24 . The electronic circuit can be a heat control 30 . The controller 30 can include a temperature sensor 32 , which is arranged to measure the temperature of the heating element 22 . The controller 30 can also include a temperature indicator 34 which can be a light emitting diode, for example. The temperature indicator 34 can be used to indicate that the interior of the housing 24 is at a temperature appropriate for maintaining the warmth of its contents.

The induction powered heating system 10 can also include a communication link 40 . Preferably, the communication link 40 is an infrared link. The communication link 40 , however, can be an ultrasound communication link or a radio communication link. The communication link 40 can include a transmitter 36 and a receiver 38 . The transmitter 36 can be in electrical communication with the controller 30 and the receiver 38 can be in electrical communication with the induction source 20 . The communication link 40 can help form a feedback loop between the temperature sensor 32 and the induction source 20 . In this manner, when the heating element 22 is exposed to a magnetic flux created by the induction source 20 , the temperature of the heating element 22 rises. The temperature sensor 32 then measures the temperature of the element 22 and relays this data to the controller 30 . If the temperature of the heating element 22 is low, the controller 30 sends a signal to the induction source 20 by way of the communication link 40 . This signal causes the inductor 20 to continue to provide a magnetic field, thereby increasing the temperature in the element 22 . If the temperature of the plate 22 rises above pre-determined level or temperature, the controller 30 can send by way of the communication link 40 a signal to the induction source 20 . This signal causes a reduction in power of the magnetic flux produced by the induction source 20 . This same signal can also be used to eliminate the presence of a magnetic flux by placing the induction source in an off mode of operation. By reducing the strength of the magnetic flux or eliminating the magnetic flux, the temperature of the heating element 22 can be reduced. Therefore, the feedback loop can control the temperature of the plate 22 , thereby controlling the temperature within the housing 24 .

In an alternate embodiment, the heating element 22 can be formed of a Curie point metal. By using a Curie point metal for the heating element 22 , a communication link 40 and feedback loop between the temperature sensor 32 and the induction source 20 are not needed. Curie point metals have the property that they will heat only up to a certain temperature and not beyond.

The electronic circuit or controller 30 can have a backup or chargeable power supply which is charged by the power supply 42 . The backup power supply can be a battery or can be a capacitor, for example. When the heating element 22 is placed near the induction source 20 , the magnetic flux energizes the power supply 42 , which can thereby provide energy to charge it.

FIG. 4 shows a block diagram of a circuit 92 for a controller 30 . The controller circuit 92 can be connected to the power source 42 . The controller circuit 92 includes a rectifier 90 , a backup power supply 88 connected to the rectifier 90 , a temperature sensor circuit 60 , a temperature indicator circuit 80 and a blinker circuit 100 . Temperature indicators 34 and a transmitting portion 36 of a communication link 40 are also connected to the circuit 92 .

FIG. 5 illustrates the rectifier circuit 90 in more detail. It converts an AC input signal to a DC output signal and also charges the chargeable power source 88 . The circuit includes input diode bridge 84 which acts to rectify the incoming signal. The chargeable power source 88 includes super capacitors in the illustrated embodiment. The circuit 90 can also include zener diodes 94 which regulate the output voltage, as well as a voltage regulator in circuit U 1 .

FIG. 6A illustrates the temperature controller circuit 60 and the temperature indicator circuit 80 . The temperature controller circuit 60 includes one or more thermostats 62 and a transmitter 36 , which is an infrared diode in the illustrated embodiment. The temperature controller circuit 60 also includes a latch component 102 , formed of a resistor 104 and a diode 106 as well as logic inverters U 1 A and U 1 B. The temperature indicator circuit 80 includes light emitting diode (LED) drivers 96 and one or more visual temperature indicators 34 .

The thermostats 62 include a first thermostat 74 and a second thermostat 76 . In a non-activated state, the first thermostat 74 is closed, thereby grounding a portion of the controller circuit 60 . The first thermostat 74 opens when the temperature of an associated heating element 22 rises above a preset high temperature of the thermostat 74 . The second thermostat 76 is also closed when in a non-activated state and opens when the temperature rises above a preset level. As will be more fully explained below, the primary purpose of the second thermostat 76 is to close when the temperature of the heating element 22 falls below a preset low temperature.

FIG. 6B shows a state diagram that illustrates the operation of the thermostats 74 , 76 . When the heating element 22 is cold, both the first thermostat 74 and the second thermostat 76 are closed 140 . When the thermostats 74 , 76 are initially closed, a ground or logic zero voltage is fed to the inverters U 1 A and U 1 B. This, in turn, activates the “not ready” indicator 34 and deactivates the “ready” indicator. As the heating element 22 is inductively heated, the second thermostat 76 eventually opens when the temperature of the element 22 reaches the preset low temperature value 156 , shown at point 142 . Opening of the second thermostat 76 does not activate any portion of the circuits 60 , 80 at this point in the process. This is because when at least one of the thermostats is closed, the connection to ground prevents a voltage J 1 (5V) from appearing across capacitor CA and the input to logic gate U 1 A remains a logic low.

As the temperature of the heating element 22 continues to rise and reaches the preset high temperature value 158 of the first thermostat 74 , the first thermostat then opens, shown at point 144 . The combination of the first thermostat 74 opening along with the second thermostat 76 already being open allows a voltage J 1 (5V) to appear across capacitor CA and at the input of logic inverter U 1 A. The consecutive inverters U 1 A and U 1 B then present a logic high voltage, thereby causing the indicator 34 to switch to a “ready” indication 160 , as shown in FIG. 6 C. This indicates to a user that the heating element is at a proper temperature for use.

The thermostats 74 and 76 also control the voltage across the resistor 70 and parallel infrared diode, forming transmitter 36 . While the heating element 22 is in proximity to an induction source, the transmitter 36 forms a feedback loop with the induction source. The transmitter 36 sends an infrared light signal to the induction source which, in turn, controls the inductor source to either increase or decrease the magnetic field, thereby either increasing or decreasing the temperature of the heating element 22 . This maintains the temperature of the heating element within a narrow range. For example, at point 144 , the temperature of the element 22 reaches a preset maximum temperature. The transmitter 36 provides a signal to the induction source to decrease the magnetic field strength, thereby decreasing the temperature of the element 22 below the preset maximum. At point 146 , the temperature of the element 22 has reached a preset minimum temperature. The transmitter 36 then provides a signal to the induction source to increase the magnetic field strength, thereby increasing the temperature of the element 22 above the preset maximum temperature. This hysteresis or fluctuation in temperature of the element 22 is given generally as 148 .

During this fluctuation 148 , at point 146 , the first thermostat 74 closes because the temperature of the element 22 is below the preset high temperature value 158 of the first thermostat 74 . While the second thermostat 76 remains open, however, resistor 104 and diode 106 maintain the latch component 102 in an active or “latched” state. The latch component 102 is therefore able to continue to provide a “ready” indication 162 , shown in FIG. 6 C.

When the heating element 22 is removed from proximity of the induction source at point 150 , the temperature of the heating element 22 starts to further decrease. At point 152 , the first thermostat 74 is again closed and the latch component 102 continues to display a “ready indication” 164 . As the temperature falls below the preset low temperature value for the second thermostat 76 , the second thermostat 76 closes, shown at point 154 . This closure disengages the latch component 102 , thereby causing the indicator to produce a “not ready” indication 166 , shown in FIG. 6 C.

Another possible circuit is shown in FIG. 7 . This is a circuit 100 which provides a blinking visual indication as long as the power supply 42 is connected. Preferably, the circuit 100 produces a blinking visual indication in LED 34 when the LED 34 provides a “ready” indication, as shown in FIG. 6 A. Such flashing or blinking can continue until the voltage source providing power to the circuit is terminated. For example, when the heating element 22 is removed from the induction source 20 , the chargeable power supply 88 is used to power the blinker circuit 100 . The LED 34 can flash until the power from the chargeable power source is drained. The chargeable power source can, for example, provide power to the circuit for approximately 30 minutes, thereby allowing flashing of the LED 34 for that amount of time. This time frame is the typically expected “hot” time for a pizza delivery.

FIG. 8 illustrates a voltage controlled oscillation circuit, given generally as 110 . The circuit 110 creates a feedback loop between the power supply 42 and the induction source 20 based upon the voltage generated by the power supply 42 . The voltage feedback loop can be used, for example, to increase the field strength from the induction source 20 if the power supply is improperly positioned over the source 20 . The circuit 110 controls the transmitter 36 , such as an infrared LED, such that the transmitter 36 flashes at a particular rate based upon the voltage produced by the power supply 42 . For example, the closer the power supply 42 is to the induction source 20 , the greater the voltage generated within the power supply.

With a relatively high voltage generated by the power supply 42 , the circuit 110 sends a signal to the transmitter 36 which causes the transmitter 36 to flash at a relatively high rate. Conversely, with a relatively low voltage generated by the power supply 42 , the circuit 110 sends a signal to the transmitter 36 which causes the transmitter 36 to flash at a relatively low rate. The signal sent by the transmitter 36 is received by the receiver 38 on the induction source 20 .

The circuits shown here are by way of example only. Many other uses of the supply voltage generated by the supply 42 are possible. For example, the feedback loop formed between the power supply 42 and the induction source 20 could also include a microprocessor to control the loop. Such a microprocessor can be mounted to the housing 24 which holds the heating element 22 and power supply 42 .

FIGS. 9 and 10 illustrate an alternate embodiment of the induction powered heating system 10 . In this embodiment, the induction source 20 includes a plurality of induction sources. Preferably, the induction source 20 includes a first induction source 120 and a second induction source 122 where the first induction source includes a first induction coil 130 and the second induction source includes a second induction coil 132 . The first induction source 120 is used to heat the heating element 22 while the second induction source 122 is used to power the circuit 30 .

The circuit 30 located in the container 24 includes a power source 42 that is energized by the second induction source 122 , a transmitter 36 and a temperature sensor 32 . The first induction source includes a receiver 38 that, together with the transmitter 36 , forms a communication link 40 .

The communication link 40 forms a feedback loop between the temperature sensor 32 and the first induction source 120 . When the heating element 22 is exposed to a magnetic flux created by the induction source 20 , the temperature of the heating element 22 rises. The temperature sensor 32 then measures the temperature of the element 22 and relays this data to the controller circuit 30 . If the temperature of the heating element 22 is low, the controller 30 sends a signal to the first induction source 120 by way of the communication link 40 . This signal causes the first inductor 120 to continue to provide a magnetic field, thereby increasing the temperature of the element 22 . If the temperature of the heating element 22 rises above pre-determined level or temperature, the circuit 30 sends, by way of the communication link 40 , a signal to the first induction source 120 that causes a reduction in power of the magnetic flux produced by the induction source 120 , thereby reducing the temperature of the heating element 22 .

The signal from the communications link 40 of the circuit 30 can also be used to eliminate the magnetic flux generated by the first induction source 120 by placing the first induction source 120 in an “off” mode of operation. By using both a first induction source 120 and a second induction source 122 as part of the induction heating system 10 , the circuit 30 can be used to stop the magnetic flux generation of the first induction source 120 while continuing to be powered by the second induction source 122 . For example, in the case where a single induction source is used to power both the heating element 22 and the circuit 30 , when the circuit 30 provides a signal to stop the magnetic flux generation of the induction source, the circuit 30 is then reliant upon power from a backup source. By using a first induction source 120 and a second induction source 122 , power form a backup power source for the circuit 30 is not required when the first induction source 120 is disabled. The circuit 30 continues to receive power form the second induction source 122 while the first induction source in inoperative.

FIG. 9 illustrates the first induction source 120 and the second induction source 122 as being electrically separate. In this configuration, the first induction source 120 includes a first voltage source 124 while the second induction source 122 has a second voltage source 126 . Alternately, FIG. 10 illustrates the first 120 and second 122 induction sources as being electrically connected. The induction sources 120 , 122 share a common voltage source 128 and can be arranged in either a series or a parallel wiring configuration.

While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.

Claims

1. A device for heating food in a container comprising:an induction source external to the container; a circuit located within the container, the circuit inductively powered by the induction source; a heating element, the heating element being heated by the induction source; and the induction source comprising a first induction source and a second induction source, the first induction source heating the heating element and the second induction source powering the circuit.

2. The device of claim 1 wherein the first induction source and the second induction source are electrically connected.

3. The device of claim 1 wherein the circuit comprises a power supply.

4. The device of claim 3 wherein the power supply comprises an induction coil charged by the induction source.

5. The device of claim 1 wherein the circuit comprises a feedback loop formed between the circuit and the induction source.

6. The device of claim 5 wherein the feedback loop comprises a communication link between the circuit and the induction source.

7. The device of claim 1 wherein the circuit comprises a controller having a temperature sensor for measuring a temperature of the heating element and a feedback loop formed between the temperature sensor and the first induction source.

8. The device of claim 7 wherein the controller further comprises a temperature indicator.

9. The device of claim 7 wherein the feedback loop comprises a communication link between the first induction source and the controller.

10. The device of claim 1 wherein the container is thermally insulated.

11. The device of claim 1 wherein the container comprises a cavity, the cavity defined by a top surface, a bottom surface and a side wall, the side wall attaching an outer edge of the top surface with an outer edge of the bottom surface and wherein a portion of the side wall is moveably attached to the top surface and the bottom surface.

12. The device of claim 1 wherein the heating element is formed of a Curie point metal.

13. The device of claim 1 wherein the induction source comprises a ferrite material.

14. A device for heating food in a container comprising:an induction source external to the container; a circuit located within the container, the circuit inductively powered by the induction source; a heating element, the heating element being heated by the induction source; and the circuit comprising a power supply, the power supply comprising an opening on the heating element, a first lead and a second lead wherein the opening creates a voltage differential between the first lead and the second lead.

15. A device for heating food in a container comprising:an induction source external to the container; a circuit located within the container, the circuit inductively powered by the induction source; and a backup power supply, wherein the backup power supply is charged by the circuit.

16. The device of claim 15 wherein the backup power supply comprises a battery.

17. The device of claim 15 wherein the backup power supply comprises a capacitor.

18. A method for monitoring the temperature of an inductively heated device comprising:providing a first induction source, a second induction source, an inductive heating element heated by the first induction source, a circuit having a temperature sensor attached to the heating element, a temperature monitor and a power supply energized by the second induction source; placing the heating element within a magnetic field generated by the first induction source; placing the power supply for the circuit within a magnetic field generated by the second induction source; heating the heating element from the first induction source; energizing the circuit from the second induction source; and monitoring a temperature of the heating element.

19. The method of claim 18 further comprising:providing a backup power supply; charging the backup power supply from the power supply; removing the heating element and circuit from the magnetic fields; allowing the backup power supply to power the temperature sensor and temperature monitor; and monitoring the temperature of the heating element.

20. A method of controlling the temperature of an inductively heated device comprising:providing a first induction source, a second induction source, a heating element heated by the first induction source, a circuit energized by the second induction source, the circuit having a temperature sensor attached to the heating element and a feedback loop formed between the temperature sensor and the first induction source; placing the heating element within a magnetic field generated by the first induction source; placing the circuit within a magnetic field generated by the second induction source; measuring the temperature of the heating element with the temperature sensor and a controller; and communicating to the first induction source to adjust the strength of the magnetic field of the source.

21. The method of claim 20 further comprising increasing the power of the magnetic field of the first induction source to increase the temperature of the heating element.

22. The method of claim 21 further comprising turning off the magnetic field of the first induction source when the temperature of the heating element reaches a predetermined temperature.

23. A method of powering a circuit within a heated food container comprising:providing a food container having a power supply energized by a first induction source and a circuit powered by the power supply; providing a heating element within the food container, the heating element heated by a second induction source; and placing the food container in proximity to the first induction source and the second induction source thereby energizing the power supply and heating the heating element.

24. A device for heating food in a container comprising:a first induction source external to the container; a second induction source external to the container; a circuit located within the container, the circuit inductively powered by the first induction source; and a heating element wherein the heating element is heated by the second induction source.

25. The device of claim 24 wherein the first induction source and the second induction source are electrically connected.

26. The device of claim 24 wherein the circuit comprises a power supply.

27. The device of claim 24 wherein the circuit comprises a feedback loop formed between the circuit and the first induction source.

28. The device of claim 24 wherein the circuit comprises a controller having a temperature sensor for measuring a temperature of the heating element and a feedback loop formed between the temperature sensor and the first induction source.

29. The device of claim 28 wherein the controller further comprises a temperature indicator.

30. The device of claim 24 wherein the heating element is formed of a Curie point metal.

31. A device for heating food in a container comprising:a first induction source external to the container; a second induction source external to the container; a circuit located within the container, the circuit inductively powered by the first induction source; and a backup power supply wherein the backup power supply is charged by the circuit.