The typical curve for current vs voltage for a varistor:
looks a lot like the 2 reverse voltage regions of a normal diode:
Is this equivalent the following, and if not, why not?
simulate this circuit – Schematic created using CircuitLab
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2 Answers
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A Varistor isn't two diodes in series, it's a chaotic mesh of thousands of little diodes in all sorts of orientations.
Courtesy of wikipedia:
The most common type of varistor is the metal-oxide varistor (MOV). This type contains a ceramic mass of zinc oxide grains, in a matrix of other metal oxides (such as small amounts of bismuth, cobalt, manganese) sandwiched between two metal plates (the electrodes). The boundary between each grain and its neighbour forms a diode junction, which allows current to flow in only one direction. The mass of randomly oriented grains is electrically equivalent to a network of back-to-back diode pairs, each pair in parallel with many other pairs
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A reasonable model might be more like two zener diodes with a relatively high resistor in series.
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\$\begingroup\$ I like to two zeners, but don't understand the high resistor. \$\endgroup\$ Commented Dec 15, 2017 at 20:37
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\$\begingroup\$ My thinking was that the knees of a varister is much less sharp than that of a zener. \$\endgroup\$ Commented Dec 16, 2017 at 1:05