Electric heating – Heating devices – With heating unit structure
Patent
1997-05-09
2000-03-28
Walberg, Teresa
Electric heating
Heating devices
With heating unit structure
338306, H05B 306, H01L 1012
Patent
active
060434678
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to thick film resistive heating elements such as can be used particularly but not exclusively in liquid heating appliances such as water boilers, kettles and the like.
BACKGROUND ART
Owing to the low thermal mass of such elements and their generally low vaporization temperature, it is necessary to protect them from overheating in the event of incorrect use of appliances to which they are fitted or malfunction of the element itself.
Conventionally, a mineral insulated element is protected by an electromechanical device such as a domed bi-stable, bi-metallic blade which is arranged so that it adopts a stable position in contact with a part of the element and thereby retains a switch in the electrical supply circuit to the element in a position which maintains the electrical supply. However, should the temperature of the element rise above a predetermined threshold temperature which is above the normal operating range, then the blade will move into its other stable position and cause the switch to operate to cut off the electrical supply to the element. As soon as the temperature of the blade drops below the threshold temperature then it will revert back to its original stable position to enable the electrical supply to be once more restored to the element.
As a back-up to the blade in the event that it should fail to function correctly, part of the device is made of a fusible or thermoplastic material which is designed to melt or to soften if a second predetermined threshold temperature higher than the aforesaid first temperature is reached. This is intended to cause the switch to disconnect and thereby permanently cut off the electrical supply to the element.
However, as thick film resistive heating elements have a low thermal mass, the rate of rise of temperature under fault conditions is so high that it is not sufficient simply to arrange an electromechanical control device as described above in contact with such an element in the same way as with a mineral insulated element to protect it from damage and to ensure that it will operate efficiently.
It is, therefore, an object of the present invention to provide a thick film resistive heating element which is adapted for use with a conventional electromechanical controller similar to the type described above.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention there is provided a thick film resistive heating element comprising a thick film resistive track applied to the surface of an electrically insulative substrate and over which is applied an encapsulating insulating layer to protect the track, and characterized in that an area of the element is left uncovered by the encapsulating layer to define a window through which a temperature sensitive control device can be placed in direct contact with the track and/or the electrically insulative substrate.
Preferably, the power density of the track is increased in said window area over the average power density of the rest of the track.
Preferably also, in the area of the window and beyond, the resistive track comprises a plurality of parallel tracks which are concentrated in the area of the window to provide a uniform temperature distribution.
Preferably also, the lengths of the parallel tracks are balanced so that adjacent tracks are substantially at equipotential.
If a portion of the temperature sensitive control device is placed in direct contact with the track, then preferably the lengths of the tracks in direct contact with the temperature sensitive control device are made substantially equal along their center line. Alternatively, if a portion of the temperature sensitive control device is placed in direct contact with the electrically insulative substrate, then preferably at least two parallel tracks loop around each side of window portion in close proximity thereto.
Preferably also, the plurality of tracks are arranged to cover that area of the element adjacent the location of the control device to increase the heat transfere
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Otter Controls Limited
Robinson Daniel
Walberg Teresa
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