Electrical resistors – Resistance value responsive to a condition – Current and/or voltage
Patent
1996-07-12
1997-10-14
Berhane, Adolf
Electrical resistors
Resistance value responsive to a condition
Current and/or voltage
338225D, H01C 710
Patent
active
056776624
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a heat-sensitive resistive compound and to a method for producing it and using it, particularly for manufacturing electric devices, generally known as PTC (positive temperature coefficient) devices, that increase their internal resistance as temperature increases.
A typical example of the application of PTCs is constituted by relays for protecting against overcurrents in electric circuits. If a short circuit occurs, these devices limit the current to a preset value with a sudden increase in resistance and on the other hand restore normal flow when the short circuit is eliminated. These known devices are constituted by an electrically conducting material and optionally by a heat-conducting material which are mixed together and immersed in a polymeric matrix in the plastic or semisolid state. The compound is then stratified and highly compressed between pairs of flat electrodes which are connected to the terminals of a circuit. For low or normal operating temperatures, for example up to approximately 80.degree. C., the resistance of these devices is extremely low, for example a few hundredths of an ohm, and increases suddenly to tens or hundreds of ohms above these temperatures.
PTC devices are also used as heaters for liquids or solids or as temperature sensors.
A possible application of PTC heaters might be constituted by electrically heated rear-view mirrors for motor vehicles, which are meant to prevent the forming of condensation or ice on the cold reflecting surfaces. Conventional heating devices for mirrors are not based on the PTC effect but are generally constituted by a sheet of resistive material which is applied on an insulating layer. The resistive material adheres to the surface to be heated, and is etched by etching with acid (so-called "etched foil") so as to form electrical paths that have a preset geometry and length and are distributed over the support. A first drawback of these conventional heaters is that the etching process is highly polluting due to the disposal of the substances used in etching the metal layer. A second drawback of these devices is their low power, which however is sufficient for the intended purposes. Finally, since they do not use the PTC effect, these devices require thermostatic regulators inserted in the electric supply circuit.
Some heaters that use the PTC effect are known; they are constituted by a special electrically conducting ink which is deposited on an insulating layer with printing or screen-printing methods so as to produce an electric path that has a preset pattern. The ink is generally constituted by a solution of electrically conducting materials dispersed in a liquid. A characteristic of this type of device is the fact that at normal operating temperatures they have relatively low resistances which allow them to be supplied with currents at a voltage between 12 and 24 V and with relatively low power levels, for example under 10 W. Furthermore, their ratio between resistance at low or normal temperature and resistance at high temperature, that is to say above 110.degree. C., is less than 3, and this can be a considerable limitation in some technological fields.
A relatively recent application of PTC devices is constituted by heaters for aquariums or fishponds or for photographic baths. Such cases require a relatively high power level, on the order of 100-200 W for operating temperatures on the order of 27.degree. C. For biological or chemical reasons, it is also necessary to ensure that the operating temperature is definitely constant. In case of accident, for example if the level of the liquid decreases and the heater operates in air, it is desirable that the dissipated power drop to levels around 15-20 W, which correspond to temperatures on the order of 100.degree.-120.degree. C., to avoid overheating and thus irreversible damage to the device.
Due to the polluting characteristics of the "etched foil" process, this method for manufacturing electric heaters has been discarded in favor of the use of electrically cond
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Bresolin Valerio
Ragazzon Daniele
Berhane Adolf
Hydor S.R.L.
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