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A varistor, VDR, (voltage dependent resistor) is a type of resistor. It controls the current depending on the voltage.
In these devices resistance drops sharply at a certain threshold voltage. Once the voltage across the device reaches the threshold, there is a breakdown of the electrical structure in the vicinity of the grain boundaries, which results in its electrical resistance dropping from several megohms down to a few hundred ohms. The major advantage of these is that they can dissipate a lot of energy, and they self reset–after the voltage across the device drops below the threshold, its resistance returns to being high.
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