Simulating Varistor

[Viktar Tatsiankou]

Hi all, I am having difficulties simulating the varistor. I am using V430MA7B from MISC family ----> Voltate Supressor group. The circuit is simle enough consisting of a varistor in parallel with the DC supply and a 10 Ohm load. According to the datasheet the maximum DC voltage is 365 VDC, hence any spike above this value should be clamped to 365 VDC. I placed a source of 380 VDC in parallel with the varistor and the load. The simulation step size is set to a fraction of the microseconds, so that I can observe the clamping action, as it can only sustain over the max. voltage for some number of microseconds. Anyways, I am attaching the file. The circuit is extremely simple, but I am puzzled as to why the clamping action is not achieved.

Thanks.

Regards,

Viktar

[OStep · Accepted solution]

A couple of issues with your circuit:

1. You are probing the same voltage node using each channel of the scope. You should not expect the voltage to be different! If you hope to see voltage clamping behaviour by probing the varistor voltge, you need to insert a series source resistance.

2. The varistor is unidirectional and you have it hooked up the wrong way (Yes, the symbol unfortunately does not make this obvious to see).

In the attached circuit I am testing the varistor by running a Dc sweep analysis on the input source connecting in parallel with the varistor. Notice that I am probing the currrent. You can see that there's a knee around 430V.

Hope that helps.