There’s an R in vaRistor

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.

The Chinese varistor data presented is shown in the following table:

The varistor resistance, R, is simply the division of the table voltage by the table current. A logarithmic plot of the table voltage versus current shows the typical varistor characteristic. 

Plotting resistance against voltage and then resistance against current clearly shows that the resistance against current has a better defined relationship than the resistance against voltage.

    

This explains why the Chinese are saying that the varistor resistance/conductance is related to current rather than voltage. In turn, the traditional varistor definition needs changing. Taking the IEC 61051-1, ED 3, definition:

varistor (voltage dependent resistor), VDR: component, whose conductance, at a given temperature range, increases rapidly with voltage within a given current range

This should be modified to something more modern like

metal oxide varistor, MOV: non-linear resistor made of a sintered mixture of zinc and other metal oxides whose conductance, at a given temperature and within a given current range, increases rapidly with current

Material for this post was extracted from "There’s an “R” in Varistor" by M J Maytum, presented at the  annual ATIS Protection Engineers Group (PEG):  Electrical Protection of Communications Networks Conference held at UL LLC Corporate Office in Northbrook, Illinois on 5-7 March 2019.