| 摘要 |
A self-limiting electric heater comprising a PTC element in the form of a body having first and second faces opposite one another and first and second ends opposite one another and at least one series element in the form of a layer (preferably a coating) on the first face so as to be in intimate thermal contact with it. This layer is in effective electrical contact with the PTC material only in the neighbourhood of the first end and external connections are made to the PTC material and to the layer, both in the neighbourhood of the second end. Preferably the external connection of the PTC material in the neighbourhood of the second end is made by a further series element in the form of a similar resistive layer in the second face of the PTC element and in effective electrical contact with it only in that neighbourhood and an external electrical connection to this further series element in the neighbourhood of the said first end; the structure thus becomes at least in general terms rotationally symmetrical. Insulating the series resistance layers electrically from the PTC layers in this way (but maintaining thermal contact) stops the PTC material from tending to short out the series resistances when it is cold and has low resistance, minimising inrush current. It also allows the current path through the PTC material to be long and of small cross-sectional area, increasing the high-temperature electrical resistance of the PTC element and minimising the effect of inhomogeneity.
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