The previous two contributions showed there is a strong correlation between metal-oxide varistor (MOV) current and resistance. This means there is also a strong correlation between MOV current and voltage making an MOV a current dependent resistor rather than being a voltage dependent resistor. This misconception probably came about as we are used to applying a variable voltage to components to measure their VI characteristic rather than a variable current. Previously, a high current MOV example was used, which gave a relationship of V = 815 + 4.32*(I)0.51 over a current range of 300 A to 50 kA. Predominately, the voltage varies as the square root of the current and the resistance varies as the reciprocal of the square root of the current. The steps in analyzing the 14 mm 275 V MOV example here are; log the data sheet VI characteristic values, unify those values, curve fit the resultant values to produce the characteristic equation valid over an extended current range of 10 µA to 6 kA.
The previous “There’s an R in varistor” post showed how the varistor is really a current controlled component. In modelling the varistor resistance or voltage, it is logical that current should be the controlling parameter used in the data set curve fitting equations. The IEC 61051-1 ED3 includes a 2-term power law equation, V = BxIC , in its varistor definition. However, this is a misdirection as it will be shown this equation is only valid over a small range of currents.
The IEC definition of varistor is a resistor, the resistance of which is strongly varying with the applied voltage. This is made even more blatant by the other commonly used term VDR, standing for voltage dependent resistor. Recent Chinese work on varistors contradicts this idea and maintains that what we know as a varistor is a current dependent resistor. This post examines the variation of the varistor resistance parameter with applied current and voltage. The conclusion is that the Chinese are right.