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Voltage Dependent Resistor VDR

The resistance of certain semiconductors (e.g. silicon carbide) depends notably on the applied voltage. Such materials are termed varistors or VDRs (Voltage Dependent Resistor).

Variable Resistors !

VDRs are voltage-dependent semiconductors whose resistance decreases notably as the applied voltage increases.

Above a certain threshold voltage specific to each varistor, its resistance drops sharply, i.e. the varistor's switching characteristic exhibits a distinct bend at this point. The characteristic is symmetric with respect to the voltage, i.e. polarity does not have any influence. Illustrated below are a varistor's circuit symbol (left) and characteristic (right). 

Variable Resistors !vdr-graph.jpg

Varistors are suitable for protection against overvoltage. Their resistance is extremely high under normal operating conditions but drops abruptly to a minimal level on application of an overvoltage, thus permitting charge to be drained easily. VDRs are therefore used for protecting sensitive electronic circuits as well as high-tension networks.

At present, varistors are usually based on zinc oxide (ZnO). In combination with other metal oxides such as bismuth oxide, chromium oxide or manganese oxide, the semiconductor powder is pressed and sintered into tablets. The resultant blank is bonded on both sides with silver or aluminium and furnished with connections.

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