Varistors are nonlinear two-electrode semiconductor voltage-dependant resistors. The current in a varistor is proportional to applied voltage raised to a power. These devices are normally made of zinc oxide. Upon application of a high voltage pulse (such as lighting) they conduct a large current, thereby absorbing the pulse energy in the bulk of the material with only a relatively small increase in voltage, thereby protecting the circuit.
Important physical specifications to consider when searching for varistors include mounting options, lead types, and diameter. Mounting options include through hole and surface mount (SMT/SMD). Through hole varistors connect to a printed circuit board by inserting a terminal or lead through a hole in the board and soldering it to the opposite side. Surface mount components are a direct response to cost reduction efforts that center around improved circuit board production. Automatic or robotic pick and place equipment can pick up and place surface mount style components on a printed circuit board faster and more accurately than previous technology would allow. Instead of a pin or terminal passing through a printed circuit board and being soldered on the opposite side, surface mount components utilize a flat solderable surface that is soldered to a flat solderable pad on the face of the printed circuit board. The pad on the circuit board is usually coated with a paste like formulation of solder and flux. With careful placement, surface mount style components on solder paste will stay in position until elevated temperatures, usually from an infrared oven, melt the solder paste and solder the mount's flat terminals to the circuit board's pad. Lead types include axial leads, radial leads, and no leads (SMT). The diameter of the varistor is an important dimension to consider.
Performance specifications that should be considered for varistors include maximum AC RMS voltage, maximum clamping voltage, and operating temperature. The maximum RMS voltage is the maximum continuous sinusoidal RMS voltage that may be applied. The maximum clamping voltage is the peak voltage across the varistor measured under conditions of a specified peak pulse current and specified waveform.