Electric heating – Heating devices – With power supply and voltage or current regulation or...
Patent
1995-09-18
1998-09-08
Paschall, Mark H.
Electric heating
Heating devices
With power supply and voltage or current regulation or...
219548, 219537, 219483, 219553, 219543, 338 22R, H05B 102
Patent
active
058047972
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to a PTC planar heater used in applications related to aircraft, aerospace, automobile, shipping industries and others, wherein the heater must provide high output with limited weight and a method for adjusting the resistance of the same.
In general, PTC ceramic products have been manufactured by forming electrodes 2 on both sides of a PTC ceramic 1, sintered in the form of a rectangular sheet as shown in FIG. 24(a), for applying a voltage thereto. The output of the PTC ceramic 1 is not very high because of the limited surface area thereof. In order to increase the output, a metal releasing plate 17 is bonded thereto as shown in FIG. 24(b). According to this method, however, the thickness of the PTC ceramic 1 must be equal to or greater than a certain value and the heat releasing plate 17 must be quite large. This has resulted in a cost increase and problems in application where a limit is put on the weight.
Further, the increased output is limited to no-wind conditions, as the increase of the heat releasing coefficient is limited.
According to Japanese Unexamined Utility Model Publication No. Sho 55-105904, as shown in FIG. 23, such problems are addressed by forming a PTC thermistor 1 in the form of a thin plate, forming a pair of electrodes 2 on one side thereof, and causing the release of heat on the surface of a heat releasing plate 17 through an insulated substrate 3. This allows the output per unit area to be successfully increased.
However, in the structure disclosed in the above-described Japanese Unexamined Utility Model Publication No. Sho 55-105904, the PTC ceramic is sensitive to the atmosphere during sintering. This creates the problem that the resistance of the PTC ceramic significantly varies during mass production, which leads to the possibility of cost increases.
Further, the formation of the electrodes on one side of a thin plate can result in warping after printing and sintering.
Conventional methods for adjusting the resistance of such a device include the method disclosed in Japanese Unexamined Patent Publication No. Sho 51-109461, wherein an auxiliary electrode is formed on the rear side of a PTC thermistor substrate. According to this method, however, the surface area must be subjected to a significant change to accommodate the auxiliary electrode. This involves complicated techniques which reduce the feasibility of this method.
Further, in the case of the device disclosed in the above-described Japanese Unexamined Utility Model Publication No. Sho 55-105904, as shown in FIG. 22, resistance rapidly increases as a result of self-heating when a rush current Imax flows after the application of a voltage to attenuate the current therethrough which reaches a very low value of I0 when thermal equilibrium is reached. However, if the PTC thermistor is deteriorated by the conditions of the environment wherein the heating device is installed, the current is increased again as indicated by the curve (OS) in the thermal equilibrium wherein it should be low. This results in an overcurrent which creates an extremely dangerous state which can be triggered by as little as a spark from the PTC. Although a current fuse may be electrically connected in series to avoid this, this can increase the cost while still leaving the possibility of an accident if a current continues to flow at a level below the fusing current.
FIG. 21(a) and FIG. 21(b) show another conventional device wherein two PTC thermistors 1, having electrodes 2 on one side thereof, are connected together by a conductive connection portion 8 and are coated with an insulating film 4. This device breaks down under the application of a voltage of 520 V. When the breakdown occurs, sparks are generated and the resin and the like which encapsulates the device burns.
SUMMARY OF THE INVENTION
It is a first object of this invention to provide a PTC planar heater having a structure which is subject to less variation of resistance and less possibility of warpage in spite of the sheet-like
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patent: 5344591 (1994-09-01), Smuckler
Inenaga Koichi
Kaimoto Takashi
Nakano Osamu
Saito Masanori
Nippon Tungsten Co. Ltd.
Paschall Mark H.
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