Protect circuits from electrical spikes and surges. You’ll often find these varistors in surge protectors for programmable logic controllers (PLCs) and sensors. Also known as voltage-dependent resistors (VDRs), they change their response based on the voltage. When the voltage is high, their resistance drops to route excess current away from the primary flow. But when voltage is low, their resistance increases, sending current through the primary circuit flow instead of the varistor.