A chip varistor (variable resistor) is a static electricity countermeasure component for today's devices. Devices such as smartphones, tablets, notebook computers, digital cameras, require high-speed data processing capacities, in addition to being even more compact and low-profile. As devices become more compact, low-profile and sophisticated, they have begun to use high-performance IC components, which has made the improvement of electromagnetic immunity and withstand voltages even more difficult. In particular, an electrostatic discharge (ESD) countermeasure is absolutely necessary for protecting devices from malfunctions or IC damage.
Varistors are voltage dependent nonlinear resistive elements with a resistance that decreases rapidly when a voltage surge is applied exceeds a given value. The varistor is typically placed between an IC (needed to be protected from ESD) and power/signal line. The varistor will allow the ESD surge to pass through it to ground , instead of continuing to the IC it is protecting. A varistor is equivalent to a series of back to back Zener diodes and therefore, does not have polarity.