WO1997049264A1

WO1997049264A1 - Temperature controller

Info

Publication number: WO1997049264A1
Application number: PCT/US1997/010190
Authority: WO
Inventors: Phillipus Bernardus Mostert; Rudolf Barend Johannes Gouws
Original assignee: Denel (Proprietary) Limited; Handelman, Joseph, H.
Priority date: 1996-06-18
Filing date: 1997-06-17
Publication date: 1997-12-24
Also published as: AU3484797A

Abstract

A temperature controller (10) is provided for controlling temperature in heating apparatus heated by at least two heating elements. The controller is connectable to a power source and includes temperature sensing means (18, 20), voltage sensing means (28) and power control means (24, 26). The voltage sensing means senses an input voltage received from the power source. The power control means is operable selectively to control power fed to the at least two heating elements in response to the temperature sensed when the input voltage is less than a preselected voltage level, and operable selectively to interrupt power fed to at least one heating element when the input voltage exceeds the voltage level. The invention extends to a method of controlling temperature in heating apparatus including at least two heating elements.

Description

TEMPERATURE CONTROLLER

THIS INVENTION relates to a temperature controller. It relates in particular to a temperature controller for, and a method of, controlling temperature in heating apparatus including at least two heating elements.

Emergency vehicles such as road ambulances, aircraft ambulances and the like often have electrical systems which operate at different voltage levels e.g. a road ambulance may have a 1 2 volt electrical system and an aircraft ambulance may have a 24 volt electrical system. In such vehicles, it is often important to control the temperature of heating apparatus, e.g. a heating blanket or the like being applied to a patient, to a temperature within close ranges, e.g. of the order of about 1 °C. For the purposes of this specification, it is this application of the controller that should be predominantly, but not exclusively, borne in mind.

According to the invention, there is provided a method of controlling temperature in heating apparatus including at least two heating elements, the method including sensing a temperature of the heating apparatus; sensing an input voltage from a power source; selectively feeding power to the at least two heating elements when the input voltage is less than a preselected voltage level in response to the temperature sensed; and interrupting power fed to at least one of the heating elements when the input voltage exceeds the preselected voltage level.

The sensing means may sense when the input voltage is sourced from a 12 volt power source and when the input voltage is sourced from a 24 volt power source. Accordingly, the power may be interrupted when the preselected voltage level is about 1 6 volts.

The method may include interrupting power fed to the at least two heating elements When the temperature sensed exceeds a preselected maximum temperature. Accordingly, the device may be disabled when a malfunction occurs.

The method may include sensing the temperature by means of a first temperature sensor and controllably feeding power to the at least two heating elements in response thereto thereby to control the temperature in the heating apparatus; and sensing the temperature independently by means of a second temperature sensor to monitor when the temperature exceeds the preselected maximum temperature.

The method may further include monitoring when a low voltage condition is present on the power source; and generating an alarm signal in response to the low voltage condition.

Further in accordance with the invention, there is provided a temperature controller for controlling temperature in heating apparatus heated by at least two heating elements, the controller being connectable to a power source and including temperature sensing means for sensing temperature; voltage sensing means for sensing an input voltage received from the power source; power control means operable selectively to control power fed to the at least two heating elements in response to the temperature sensed when the input voltage is less than a preselected voltage level, and operable selectively to interrupt power fed to at least one heating element when the input voltage exceeds the preselected voltage level.

The voltage sensing means may be operable to sense when the power source is a 1 2 volt DC power source and when the power source is a 24 volt DC power source. The voltage sensing means may be a comparator and the preselected voltage level is typically about 1 6 volts. The voltage sensing means may include at least two comparators. The power control means may include disabling means. The temperature sensing means may be a first temperature sensor and the controller may include a second temperature sensor connected to the disabling means which is operable in response to a preselected maximum temperature sensed by the second temperature sensor to disable the at least two heating elements.

The temperature controller may include alarm generating means responsive to, for example, the second temperature sensor and be operable to generate an alarm signal when the preselected maximum temperature is sensed.

The alarm generating means may be responsive to the voltage sensing means and may be operable to generate an alarm signal when a low input voltage from the power source is sensed. In certain embodiments, the disabling means may be connected to the voltage sensing means and be operable to disable the heating apparatus when the low input voltage from the power source is sensed. Accordingly, for example, the load on a power source in the form of a battery may be reduced so that the remaining power in the battery may be used for more critical apparatus.

The alarm signal may be an audible alarm, e.g. a buzzer, and/or a visual alarm, e.g. a LED or the like.

The power control may means include a microprocessor connected, for example, to the first temperature sensor for monitoring the temperature sensed by the first temperature sensor, and switching means connected to the microprocessor and operable under control of the microprocessor to switch power form the power source to at least one heating element in response to the temperature sensed.

Typically, two heating elements are connected in parallel.

The microprocessor may be a first microprocessor and the switching means may be first switching means. The power control means may include a second microprocessor connected to the second temperature sensor; and second switching means connected to the second microprocessor and operable under control of the second microprocessor to interrupt power to the at least two heating elements when the preselected maximum temperature is sensed by the second temperature sensor.

Accordingly, in the event of the first microprocessor malfunctioning, the second microprocessor may independently monitor the heating apparatus to sense when the preselected maximum temperature is sensed. Typically, the temperature controller controls the temperature of the heating apparatus at about 36°C to 37°C and the preselected maximum temperature is between about 39°C and about 42 °C.

The first and second temperature sensors are typically temperature sensitive integrated circuits operable to provide a digital output signal to the first and second microprocessors. For example, each temperature sensitive integrated circuit may be a DS1 620 or DS1821 digital thermometer and thermostat available from Dallas Semiconductor Corporation.

The temperature controller may include adjustment means for selectively adjusting a desired or preselected temperature of the heating apparatus. The adjustment means may be defined by a microprocessor, e.g. the first microprocessor, and associated switches e.g. membrane switches. The microprocessor may accordingly be programmed selectively to enable at least one heating element in response to a comparison between the preselected temperature and the sensed temperature.

The temperature controller may include display means for displaying at least the temperature sensed by the sensing means. Preferably, in addition, the display means is operable to display the desired or preselected temperature. The display means may comprise a plurality of seven-segment LED displays or the like. The first and second microprocessors are typically PIC16C54 chips or the like.

The invention is now described, by way of example, with reference to the accompanying drawings in which: Figure 1 shows a schematic circuit diagram of an embodiment of a temperature controller in accordance with the invention;

Figure 2 shows a schematic circuit diagram of a microprocessor and display means of the temperature controller of Figure 1 ;

Figure 3 shows a schematic block diagram of a further embodiment of a temperature controller in accordance with the invention;

Figure 4 shows a schematic diagram of a heating element switching arrangement of the controller of Figure 3;

Figure 5 shows a schematic circuit diagram of the controller of Figures 3 and 4; and Figure 6 shows a schematic partly cut-away top plan view of the heating elements of Figure 4 fitted to a heating blanket.

Referring to Figure 1 , reference numeral 10 generally indicates a temperature controller particularly intended for use in emergency vehicles such as a road ambulance or aircraft ambulance. Often in such vehicles the voltage supplied from the vehicle battery varies from vehicle to vehicle, e.g. road ambulances normally have a 1 2 volt battery while aircraft ambulances normally have a 24 volt battery. It is however desirable to have a standard temperature controller for controlling the supply of heat, e.g. via a heating blanket to a patient in the vehicle.

The temperature controller illustrated in Figure 1 includes connection means in the form of a jack plug 1 2 for connecting an earth to line 14 and a variable DC voltage from the vehicle battery or from a separate battery to line 1 6.

The temperature controller 10 further includes temperature sensing means and, in this embodiment, a first temperature sensing means is in the form of a DS1 620 integrated circuit chip 1 8 and a second DS1 620 integrated circuit chip 20 available from Dallas Semiconductor Corporation. The first chip 1 8 senses temperature and will emit low temperature or high temperature signals if the temperature sensed is either more than -1 °C or more than + 1 °C relative to a desired temperature of, for example, 36 °C. The second integrated circuit chip 20 is arranged to sense in this embodiment, an over or maximum temperature of, for example, 4 °C above the desired temperature, i.e. above about 40 °C.

The temperature sensing chips 18 and 20 are connected to driver transistors 21 and 22 respectively, which in turn control MOSFET transistors 24 and 26 respectively. The transistors 24 and 26 form power control means, the transistor 24 being switched on when a temperature below 35 °C is sensed. The transistor 26 is permanently on unless a temperature in excess of about 40 °C is sensed.

The controller further includes voltage sensing means implemented in this embodiment by a zener diode 28. When the voltage on line 1 6 applied to the zener diode 28 is above 18 volts, a pair of voltage control MOSFET transistors 30 and 32 are arranged to isolate a first heating element 34 to render it inoperative.

Thus all power supplied across lines 14 and 1 6 is applied to a second heating element 36. If however the voltage applied across lines 14 and 1 6 drops below 1 8 volts, the MOSFET transistor 32 conducts thereby placing the heating element 34 in parallel with the heating element 36.

The controller further includes a pair of voltage regulating integrated circuit chips 38 and 40 which provide a fixed DC voltage to the temperature sensing chips 18 and 20 and their associated circuitry.

When a malfunction of the controller occurs, i.e. when the MOSFET transistor 26 is switched off by the temperature sensing chip 20, a signal is sent via a diode 42 and via the jack plug 1 2 to a buzzer (not shown) to give an audible warning of the malfunction. 7

Referring now to Figure 2, circuitry is shown whereby the desired temperatures can be varied. The adjustment of the temperature is implemented by way of a microprocessor 44 having an oscillator circuit 46. Power is supplied to the circuit via a jack plug 48 and the voltage is regulated by a voltage regulator 50.

When it is desired to increase the desired temperature, a pair of switches 52 and 54 are depressed simultaneously. Likewise, when it is desired to lower the desired temperature, the switch 52 is depressed simultaneously with a switch 56.

The Figure 2 circuit also includes display means in the form of three

LED displays 58, 60 and 62. The LED display 62 displays tens, the LED 60 displays units and the LED display 58 displays " °C".

The LED displays 60 and 62 are driven by a pair of decimal counters 64 and 66, respectively, and which are controlled by the microprocessor 44 to display the desired temperature.

The microprocessor 44 is connected via lines 68 and jack plug 70 to lines 72 in Figure 1 via the jack plug 12.

Referring to Figures 3 to 6 of the drawings, reference numeral 100 generally indicates a further embodiment of a temperature controller in accordance with the invention. The controller 100 is similar to the controller 10 and, accordingly, like reference numerals have been used to indicate the same or similar features unless otherwise indicated.

The controller 100 includes a first or primary microprocessor 102 (see

Figures 3 and 5), typically a PIC1 6C54, connected via driver circuitry 104.1 (used when powered by a 1 2 volt power source) and 104.2 (used when powered by a

24 volt power source) and a primary relay 106 to a first or primary heating element

34. The primary microprocessor 102 is also connected to associated oscillator circuitry 108, and via line 1 10 and connector 1 12 to a first or primary temperature sensor 18. The primary temperature sensor 1 8 is typically a programable digital thermostat such as a DS1821 available from Dallas Technologies.

The controller 100 further includes a secondary microprocessor 1 14 with its associated oscillator circuitry 1 16. The secondary microprocessor 1 14 is connected via driver circuitry 104.3 (used when powered by a 12 volt power source) and 1 04.4 (used when powered by a 24 volt power source) to a secondary relay 1 1 8. The secondary microprocessor 1 14 is also connected via driver circuitry 104.5, 104.6, 104.7 to a voltage select relay 1 20, alarm generating means in the form of a buzzer 1 22, and an LED warning light 124, respectively. The secondary micro-processor 1 14 is also connected via line 1 90 to a secondary temperature sensor 20 and via lines 1 26 to voltage sensing means 1 28 (see Figures 3 and 5). The voltage sensing means 1 28 is connected via lines 1 30 to the primary microprocessor 102. As will be described in more detail below, the controller 100 is operable to control heating of medical apparatus such as a heating blanket 176 ( see Figure 6) by the primary and secondary heating elements 34 and 36 in response to a temperature sensed by the primary sensor 1 8.

Referring in particular to Figure 5 of the drawings, power to the controller 100 is fed in via a jack plug 12 to a ground line 14 and to a positive supply line 1 6. A 5 volt voltage regulator 1 32 is connected via diode 1 34 and via a fuse 1 36 to the positive supply line 1 6. The voltage regulator 1 32 provides a regulated 5 volt output on line 138 which is fed to the various chips of the controller 100. Microprocessor reset circuitry 140, 142 is connected to the primary and secondary microprocessors 102, 1 14 respectively in order to reset the primary and secondary microprocessors 102, 1 14 when the controller 100 is switched on.

The voltage sensing means 1 28 is connected via line 144 to the positive supply line 1 6 and a potential divider 146 is arranged to divide the input voltage fed in from the power source by a factor of ten which is then fed via line 148 to positive inputs of four comparators 1 50, 1 52, 1 54, and 1 56. The comparators 1 50 to 1 56 have their negative inputs 1 50.1 , 1 52.1 , 154.1 , and

1 56.1 , set at 2.1 volts, 1 .6 volts, 1 .1 5 volts, and 0.9 volt respectively. Outputs

1 50.2, 1 52.2, 1 54.2, and 1 56.2 of the comparators 1 50 to 1 56 are connected to the primary and secondary microprocessors 102 and 1 14 thereby to enable them to monitor the input voltage.

The controller 100 further includes a membrane switch arrangement 1 58 comprising a "set" switch 1 60, an "increase temperature" switch 162 and a "decrease temperature" switch 164. The membrane switch arrangement 1 58 is connected via lines 166 to a pull-down resistor bank 168 and to the primary microprocessor 102. In use, the primary microprocessor 102 scans the membrane switch arrangement 158 to enable a user to set a desired reference temperature, typically 36 °C, by means of the switches 160, 162 and 1 64. The primary microprocessor 102 is also connected to display drivers 170, and via resistor banks 1 72 to seven-segment LED displays 174. When the set switch 160 is depressed and adjustment of the desired reference temperature takes place, the displays 1 74 display the current setting of the desired or preselected temperature.

The temperature controller 100 is typically used to regulate the temperature of the heating blanket 176 (see Figure 6). The heating blanket 1 76 includes a planar base 178 of plastics material to which the primary and secondary heating elements 34, 36 are mounted. The heating blanket 1 76 includes a connector 180 which is connected to the primary and secondary heating elements 34, 36 and, via lines 182, to primary and secondary connectors 1 84, 186 (see Figure 5). The primary and secondary temperature sensors 18, 20 are also securely mounted to the base 178 (see Figure 6) and are connected via lines 1 88 to the connectors 1 1 2, 1 1 3.

In use, the primary and secondary heating elements 34, 36 are selectively connected in parallel (see Figure 4) and connected in series with a primary relay contact 106.1 , of the primary relay 106, and a secondary relay contact 1 18.1 , of the secondary relay 1 18. The secondary heating element 36 is also connected in series with a voltage select relay contact 1 20.1 and the secondary heating element 36 is operatively selectively enabled dependent upon the supply voltage fed to the temperature controller 100 via the jack plug 12. When the supply voltage is less than about 1 6 volts, the voltage select contact relay 120.1 is closed, as will be described in more detail below, thereby to allow heating by both the primary and the secondary heating elements 34, 36. However, when the controller 100 is powered by a 24 volt power source and the input voltage is proximate 24 volt, the voltage select relay contact 120.1 is open as only one of the heating elements 34, 36 is required to maintain the heating blanket 176 at the desired preselected temperature.

In use, the primary microprocessor 102 monitors the temperature of the heating blanket 176 via the primary temperature sensor 18. The primary temperature sensor 18 provides a digital read-out of the sensed temperature along lines 1 10 to the primary microprocessor 102. The primary microprocessor 102 then compares the sensed temperature with the preselected temperature stored therein and/or set by the switch arrangement 1 58. If the sensed temperature exceeds the preselected temperature, the relay 106 is de-energized via the driver circuitry 1 04.1 or 104.2, as the case may be, and accordingly the primary relay contact 106.1 is opened thereby interrupting the flow of power to the primary and secondary heating elements 34,36.

If, however, the sensed temperature is below the preselected temperature, the primary microprocessor 102 energizes the relay 106 via the driver circuitry 1 04.1 or 104.2, as the case may be, and depending upon whether or not the secondary relay contact 1 1 8.1 and the voltage select relay contact 1 20.1 are closed (see Figure 4), power is fed to the primary and secondary heating elements 34, 36 thereby to heat the heating blanket 1 76.

The secondary microprocessor 1 14 has a preprogrammed reference temperature in the range of about 39 °C to about 42 °C which is continually and independently of the primary microprocessor 102 compared with the temperature sensed by the secondary temperature sensor 20 and fed into the secondary microprocessor 1 14 via lines 1 90. Although the secondary temperature sensor 20 could be coupled to the primary microprocessor 102, the secondary microprocessor 1 14 is included to provide a redundancy feature so that, in the event of the primary microprocessor 102 malfunctioning, protection against excessive temperature is still provided by the secondary microprocessor 1 14 in conjunction with its associated circuitry.

The secondary microprocessor 1 14 is also connected via the driver circuitry 104.6 and a driver transistor 192 to the buzzer 122, and via the driver circuitry 104.7 to the LED warning light 124. Dependent upon whether or not the voltage sensed by the voltage sensing means 128 is associated with a 24 volt or 12 volt power source, the secondary microprocessor 1 14 is also operable selectively to close voltage select relay contacts 120.1 (when a 1 2 volt source is sensed) thereby selectively to connect the secondary heating element 36 so that it may be selectively powered in addition to the primary heating element 34 at low voltage.

When the voltage sensing means 128 senses an input voltage fed into the controller 100 which exceeds 16 volts and also exceeds 22 volts, the secondary microprocessor 1 14 de-energizes the voltage select relay 1 20 via the driver circuitry 104.5. Accordingly, the secondary heating element 36 is disconnected. In use, the primary microprocessor 102 then senses the temperature of the heating blanket 1 76 via the primary temperature sensor 18 and selectively heats the heating blanket 1 76 by means of the primary heating element 34 only (as discussed in more detail below). If the voltage sensing means 1 28 senses that the input voltage is between about 16 volts and about 22 volts, the secondary microprocessor 1 14 also disables the secondary relay 1 1 8 thereby to disable the primary heating element 34 as this is indicative of a low battery condition. Simultaneously, the secondary microprocessor 1 14 also switches on the transistor 1 92 via the driver circuitry 104.6 thereby to enable the buzzer 122 to advise of the low battery condition and the fact that the heating elements have been disabled.

When the voltage regulating means 1 28 senses that the input voltage is above 1 6 volts, regulation of temperature takes place as follows. The primary and secondary microprocessors 102, 1 14 selectively switch or energize the primary relay 106, and secondary relay 1 18 via the driver circuits 104.2 and 104.4 respectively which, in turn, are connected to relay coils 106.3 and 1 18.3 of the primary and secondary relays 106, 1 1 8 via 1 2 volt zener diodes 1 94. The zener diodes 1 94 provide a 12 volt voltage drop since the relay coils 106.3, 1 18.3 are typically 1 2 volt coils. The relay coils 106.3, 1 1 8.3 are thus protected from the input voltage which is in excess of 1 2 volts.

When the controller is supplied with power from a 12 volt power source, the secondary microprocessor 1 14 senses this via the voltage sensing means 1 28 and energises the voltage select relay 120. Accordingly, voltage select relay contacts 120.1 (see Figures 3 and 5) close and the secondary heating element 36 is connected in parallel with the primary heating element 34. Thus, when a low voltage is supplied to the controller 100, both the primary and secondary heating elements 34, 36 are used and, when a high voltage is applied, only the primary heating element 34 is used.

In the 1 2 volt mode of operation, the primary and secondary microprocessors 102, 1 14 selectively switch or energise the primary relay 106, and the secondary relay 1 18 via the driver circuits 104.1 and 104.3 as no 1 2 volt voltage drop is required for the relay coils 106.3 and 1 1 8.3 via the zener diodes.

When the voltage sensing means 1 28 senses that the input voltage is below 1 6 volts and above 1 1 .5 volts, the controller 100 operates in a normal fashion in which the primary microprocessor 102 selectively controls heating of the heater blanket 1 76 by the primary heating element 34 in response to the temperature sensed by the primary temperature sensor 1 8. Simultaneously, the secondary microprocessor 1 1 4 continually via the secondary temperature sensor 20 monitors when an over temperature condition arises and, in response thereto, disables or de-energises the secondary relay 1 1 8 thereby to disconnect power from the primary heating element 34 and it also generates an alarm signal via the buzzer 1 22 as hereinbefore described. The over temperature condition is however monitored in both the 12 volt and 24 volt mode of operation.

However, should the input voltage drop below about 1 1 .5 volts but still remain above about 9 volts, the controller 100 remains operative but the secondary microprocessor 1 14 monitors this low battery condition via the voltage sensing means 1 28 and, in response thereto, powers the LED warning light 124 via the driver circuitry 104.7 to indicate the low battery condition. The controller 100 nevertheless continues to function in a normal fashion selectively heating the heating blanket 1 76 via the primary heating element 34.

If however the input voltage drops below 9 volts, power supplied to the controller 100 is too low to ensure proper heating of the heating blanket 1 76 and a warning is thus generated. In particular, the secondary microprocessor 1 14 senses that the input voltage is below 9 volts via the voltage sensing means 1 28 and enables the buzzer 1 22 via the driver circuitry 104.6. Simultaneously, the secondary microprocessor 1 14 intermittently powers the LED warning light 124 so that it flashes. The primary microprocessor 102 disables the primary relay 106 and, accordingly, both the primary and secondary heating elements 34, 36 are disabled. Accordingly, the load on the battery may be reduced in order to allow the battery to power apparatus which is more critical.

As can clearly be seen from Figure 6 of the drawings, only the primary and secondary heating elements 34, 36 and the primary and secondary temperature sensors 18, 20 are mounted to the base 178 of the heating blanket 176. The electronic circuitry of the controller 100 is mounted in an external housing which is located externally of the heating blanket 1 76 and is connected to the primary and secondary heating elements 34, 36 and the primary and secondary temperature sensors 1 8, 20 via lines 1 82, 188 and the connector 180.

The invention illustrated provides a temperature controller 1 0, 100 which can be readily connected to a standard voltage source on a vehicle such as an ambulance and which can be used accurately to control a desired temperature irrespective of the voltage of a battery in the vehicle. If necessary, the temperature controller 10, 100 can also be powered via a suitable step down transformer from a mains source if required. The controller can also be powered from an independent battery, e.g. during mountain rescue operations, or the like.

Claims

CLAIMS:

1 . A method of controlling temperature in heating apparatus including at least two heating elements, the method including sensing a temperature of the heating apparatus; sensing an input voltage from a power source; selectively feeding power to the at least two heating elements when the input voltage is less than a preselected voltage level in response to the temperature sensed; and interrupting power fed to at least one of the heating elements when the input voltage exceeds the preselected voltage level.

2. A method as claimed in Claim 1 , in which the sensing means senses when the input voltage is sourced from a 12 volt power source and when the input voltage is sourced from a 24 volt power source.

3. A method as claimed in Claim 2, in which the power is interrupted when the preselected voltage level is 16 volts.

4. A method as claimed in Claim 1 , which includes interrupting power fed to the at least two heating elements when the temperature sensed exceeds a preselected maximum temperature.

5. A method as claimed in Claim 4, which includes sensing the temperature by means of a first temperature sensor and controllably feeding power to the at least two heating elements in response thereto thereby to control the temperature in the heating apparatus; and sensing the temperature independently by means of a second temperature sensor to monitor when the temperature exceeds the preselected maximum temperature.

6. A method as claimed in Claim 1 , which includes monitoring when a low voltage condition is present on the power source; and generating an alarm signal in response to the low voltage condition.

7. A temperature controller for controlling temperature in heating apparatus heated by at least two heating elements, the controller being connectable to a power source and including temperature sensing means for sensing temperature; voltage sensing means for sensing an input voltage received from the power source; power control means operable selectively to control power fed to the at least two heating elements in response to the temperature sensed when the input voltage is less than a preselected voltage level, and operable selectively to interrupt power fed to at least one heating element when the input voltage exceeds the preselected voltage level.

8. A temperature controller as claimed in Claim 7, in which the voltage sensing means is operable to sense when the power source is a 1 2 volt DC power source and when the power source is a 24 volt DC power source.

9. A controller as claimed in Claim 8, in which the voltage sensing means is a comparator and the preselected voltage level is 1 6 volts.

10. A temperature controller as claimed in Claims 7, in which the power control means includes disabling means and in which the temperature sensing means is a first temperature sensor and the controller includes a second temperature sensor connected to the disabling means which is operable in response to a preselected maximum temperature sensed by the second temperature sensor to disable the at least two heating elements.

1 1 . A temperature controller as claimed in Claim 10, which includes alarm generating means responsive to the second temperature sensor and operable to generate an alarm signal when the preselected maximum temperature is sensed.

12. A temperature controller as claimed in Claim 1 1 , in which the alarm generating means is responsive to the voltage sensing means and is operable to generate an alarm signal when a low input voltage from the power source is sensed.

1 3. A temperature controller as claimed in Claim 10, in which the power control means includes a microprocessor connected to the first temperature sensor for monitoring the temperature sensed by the first temperature sensor, and switching means connected to the microprocessor and operable under control of the microprocessor to switch power form the power source to at least one heating element in response to the temperature sensed.

14. A temperature controller as claimed in Claim 1 3, in which the microprocessor is a first microprocessor and the switching means is first switching means, the power control means including a second microprocessor connected to the second temperature sensor; and second switching means connected to the second microprocessor and operable under control of the second microprocessor to interrupt power to the at least two heating elements when the preselected maximum temperature is sensed by the second temperature sensor.

1 5. A temperature controller as claimed in Claim 13, in which the first and second temperature sensors are temperature sensitive integrated circuits operable to provide a digital output signal to the first and second microprocessors.

1 6. A temperature controller as claimed in Claim 7, which includes adjustment means for selectively adjusting a desired temperature of the heating apparatus.

1 7. A temperature controller as claimed in Claim 7, which includes display means for displaying at least the temperature sensed by the sensing means.