Varistor is short for ‘varying resistor’ but is currently used only for voltage variable resistors also known as voltage dependent resistor (VDR). The resistance of this two-terminal device varies with voltage applied across its terminals. The resistance variation is nonlinear, similar to a zener diode. For voltages below a threshold, known as the ‘knee’, the resistance is very high, but above the threshold the resistance falls sharply. However, the varistor is not polarity sensitive. It can be considered as a series combination of two back-to-back zener diodes. This operational characteristic is utilized extensively in protecting costly electric circuits against excessive voltage. In one application, the varistor may reside across the circuit to be protected. An overvoltage will cause a large current, which will trigger a series connected protective device and isolate the protected equipment. The protective device can simply be a fuse or even a relay. Alternatively, the varistor may be used as a voltage clamp where it offers a low resistance to voltages above the clamp level. Since the varistor is not polarity sensitive, it is suitable for protection in both AC and DC voltages. It is used extensively in high power transmission circuits as well as low power mobile chargers. The basic varistor consists of a layer of oxide on a metal sheet, typically zinc. For this reason, varistors are also called Metal-Oxide Varistors (MOV). Stacking many varistors in a series as one varistor can increase the clamp voltage of a MOV.Varistors are available for a broad range of specifications and are classified by maximum voltage, clamp voltage, peak current, surge energy and response time. Read more Read less