GB2272619A

GB2272619A - Temperature control in a heater assembly

Info

Publication number: GB2272619A
Application number: GB9223587A
Authority: GB
Inventors: Raheel Butt
Original assignee: Central Research Laboratories Ltd
Current assignee: Central Research Laboratories Ltd
Priority date: 1992-11-11
Filing date: 1992-11-11
Publication date: 1994-05-18
Also published as: GB9223587D0

Abstract

A heater assembly 2 includes a thick film heating element 10 mounted on a substrate layer 8 of ceramic material covering a metallic plate 4. To prevent the assembly from reaching a dangerously high temperature the leakage current between electrode 16 and plate 4 through the ceramic layer 8 is sensed, and the element 10 is switched off should the current reach a predetermined level. <IMAGE>

Description

A HEATER ASSEMBLY This invention relates to a heater assembly, and in particular to such an assembly comprising a metal plate, a layer of ceramic material provided on a major surface of the plate, a thick film heater element provided on the layer of ceramic material, and a switch which is electrically connected in series with the heater element.

In a known such assembly the heater element comprises a thick film electrically resistive track of a base metal powder and glass frit. The track is deposited on a substrate comprising a ceramic layer provided on a metal core plate. A further ceramic layer on the opposite major surface of the metal plate forms the heated surface.

The maximum operating temperature of such a heater is limited by the electrical characteristics of the substrate. At high temperatures, for example above about 56O0C, there may be a dangerously high current leakage between the heater track and the metal core, which could lead to breakdown. Therefore accurate temperature limitation is required.

A known means of limiting the temperature of heaters comprises a switch including a bi-metal disc circumferentially restrained within a body in thermal contact with the heater.

When a predetermined maximum temperature is reached, the disc is forced to 'dish' or become bowed, pushing a plunger which separates two spring contacts thus breaking the electrical connection to the heater element. A disadvantage of this device is that the body has a large thermal mass compared with the small thermal mass of the heater element, which causes a high degree of operational hysteresis.

According to the present invention, there is provided a heater assembly comprising a metal plate, a layer of ceramic material provided on a major surface of the plate, a thick film heater element provided on the layer of ceramic material, and a switch which is electrically connected in series with the heater element, characterised in that an electrode is provided on the layer of ceramic material, voltage supply means is connected between the electrode and the plate via current sensing means, a comparator is provided for comparing the output of the current sensing means with a reference value, and the output of the comparator is coupled to a control input of the switch for applying a control signal thereto such as to open the switch should the output of the current sensing means exceed the reference value.

The control signal may also be arranged to close the switch should the output of the current sensing means subsequently fall below the reference value.

The invention makes use of the fact that a small current may leak across the ceramic layer the size of which current increases with the temperature of the ceramic layer. The leakage current that exists at a given temperature, at which the element should be switched off, can be determined by experiment.

In order that the invention may be more readily understood, reference will now be made, by way of example, to the accompanying drawing, in which: Figure 1 is a diagram of a heater assembly in accordance with the invention; and Figure 2 is a diagram of a possible implementation of a current to voltage converter included in the assembly of Figure 1.

In Figure 1 a heater assembly includes a hotplate 2 which is shown in cross-section. The hotplate 2 comprises a metal plate core 4 coated on each of its upper and lower major surfaces or faces with a layer 6,8 of crystalline ceramic material. A thick film heater element 10 is provided on the lower ceramic layer 8, the upper layer 6 being intended for use as the heated surface. Opposite ends of the element 10 are connected to respective terminals of a mains supply 12, one via a switch in the form of a relay 14, and the metal core 4 is earthed.

An electrode 16 is also provided on the lower ceramic layer 8 adjacent the heater element 10. The primary winding of a transformer 22 is connected across the mains supply and the electrode 16 is connected to the metal core 4 via voltage supply means in the form of the secondary winding of the transformer 22 and current sensing means 18. The current sensing means 18 takes the form of a current to voltage converter, the output of which is connected to the non-inverting input of a comparator 20. A reference voltage is obtained by means of a tapping on the secondary winding of the transformer 22, and is connected to the inverting input of the comparator 20. The output of the comparator 20 is coupled to a control input (the energising winding 23) of the relay 14.

In use of the hotplate 2, the relay 14 is closed, supplying power to the heater element 10. As the temperature of the ceramic layer 8 rises, the output voltage of the current sensing means 18 also rises. If the temperature rise is sufficient to cause this output voltage to become greater than the reference voltage, the output of the comparator 20 opens the relay 14, breaking the current supply to the element 10.

Thus, with the reference set at an appropriate level, the heater may be prevented from reaching a temperature at which a dangerously high leakage current exists.

The current sensing means 18 may comprise, for example, a simple resistor the voltage across which is fed to the noninverting input of the comparator. Preferably, however, it is constructed as shown in Figure 2.

In Figure 2 the current sensing means comprises a high gain amplifier 24, the inverting input of which is connected to the electrode 16. A first resistor 26 is connected across the amplifier 24 from the inverting input to the output, and a further resistor 28, of the same value as the first resistor 26, is connected between the non-inverting input of the amplifier 24 and the secondary winding of the transformer 22. The output of amplifier 24 is connected to the energising winding 23.

The power supply for the comparator 20 and the amplifier 24 may be obtained from the secondary winding of the transformer 22 via a conventional rectifier circuit (not shown).

It will be evident that many modifications are possible to the embodiment described, within the scope of the invention defined by the claims. For example the secondary winding of the transformer 22 may be replaced by d.c. voltage sources of appropriate value connected between the plate 4 and the current sensing means 18, and to the inverting input of the comparator 20 respectively. As another example the mechanical relay 14 may be replaced by a semiconductor switch.

In a practical embodiment in which the current sensing means 18 was constructed as described with reference to Figure 2, the voltages across the secondary winding of the transformer 22 and at the tapping thereon were 20V and 14V respectively. The layer 8 of ceramic material was 150 microns thick, and was composed of about 30% each of silica and calcia, about 168 magnesium oxide, about 5% lead oxide, and about 1-3% each of alumina, phosphorous oxide, barium and vanadium oxides. The area of the electrode 16 was lcm2 and the value of the resistors 26 and 28 was 2500fl.

Claims

1. A heater comprising a metal plate, a layer of ceramic material provided on a major surface of the plate, a thick film heater element provided on the layer of ceramic material, and a switch which is electrically connected in series with the heater element, characterised in that an electrode is provided on the layer of ceramic material, voltage supply means is connected between the electrode and the plate via current sensing means, a comparator is provided for comparing the output of the current sensing means with a reference value, and the output of the comparator is coupled to a control input of the switch for applying a control signal thereto such as to open the switch should the output of the current sensing means exceed the reference value.

2. A heater assembly as claimed in claim 1, wherein the voltage supply means is an a.c. voltage supply means.

3. A heater assembly as claimed in Claim 1 or 2, wherein the current sensing means comprises a current to voltage converter.

4. A heater assembly substantially as described herein with reference to Figure 1 of the accompanying drawings or to Figures 1 and 2 of said drawings.