Image credit: Digi-Key | HV Components/CKE
Metal oxide varistors (MOV) are semiconductors that protect electronic components and systems from transient voltages.
Construction and Operation
These clamping devices contain a matrix of zinc oxide grains sandwiched between two metal plates which serve as electrodes. The boundaries between grains form diode junctions which allow current to flow in only one direction. Applying low or moderate voltages causes reverse leakage through the diode junctions and a small flow of current. Higher voltages trigger the avalanche effect and cause the diode junctions to break down. As a rule, metal oxide varistors exhibit highly non-linear current-voltage characteristics. They have high resistance at low voltages and a low resistance at high voltages. Unlike transient voltage suppression (TVS) diodes, however, metal oxide varistors degrade as they absorb repeated transients.
Performance specifications for metal oxide varistors (MOV) include maximum working voltage, varistor voltage, maximum clamping voltage, surge current, surge shift, energy absorption, capacitance, and leakage current. Maximum working voltage is the maximum steady-state, direct current (DC) voltage where the typical leakage current is less than a specified value. Maximum clamping voltage is the maximum peak voltage measured across the device with a specific pulse current and waveform. Surge shift is the change in voltage after the application of the surge current. Energy absorption is the maximum amount of energy that can be dissipated with a specified waveform without damage. Additional specifications for metal oxide varistors include response time and maximum alternating current (AC) root mean square (RMS) voltage. The maximum AC RMS voltage is the maximum continuous sinusoidal RMS voltage that may be applied.
Form Factor and Applications
Product specifications for metal oxide varistors (MOV) include mounting style, lead type, and application. Through hole technology (THT) mounts components on a printed circuit board (PCB) by inserting component leads through holes in the board and then soldering the leads in place on the opposite side of the board. Surface mount technology (SMT) adds components to a PCB by soldering component leads or terminals to the top surface of the board. There are three lead styles for metal oxide varistors: axial, radial, and lead-less. In terms of applications, some metal oxide varistors are designed for cell phones, digital cameras, personal digital assistants (PDA), MP3 players, and notebook computers. Others telecommunications devices, power supplies, data systems or power systems. Both types of MOVs provide fast response times, low working voltages, low capacitance, low leaking current, and low clamping voltage.
Metal oxide varistors may be manufactured, tested, and used based on various standards. Some common standards are listed below.
IEC 61643-331 (Components for low-voltage surge protective devices - MOV specification)
MIL-PRF-83530 (General specification for metal-oxide varistors)