The volt-ampere characteristics of the varistor are characterized at the level in which current suddenly starts to flow through the device at a certain voltage.
The varistors are used to protect semiconductor devices in electronic and electric circuits from over-voltage. A varistor is inserted in parallel with a circuit to be protected. When an impulse is applied to the circuit, pulse current “ls”, which is determined by pulse voltage “Vs” and pulse impedance “Zs”, flows to limit the pulse voltage to the varistor limit voltage Vclamp.
The relation can be expressed by the equations as follows:
Vs = Is × Zs + Vclamp (1)
Vclamp = Vs – Is × Zs (2)
The pulse current ls is easily obtained by the following equation because of Vs >> Vclamp.
Thus, the circuit can be protected from being damaged by pulse voltages as long as it has a “withstand” voltage larger than the maximum limit voltage.
Owing to the characteristic, the varistors are extremely effective at protecting electronic and electric equipment through the absorption of abnormal voltages and lightening pulses.