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Old 10th November 2012, 11:15 PM   #1
fualcr is offline fualcr  Canada
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Default Can MOV (varistors) be wired to each primary in a universal dual-primary transformer?

***Disclaimer: AC line level wiring is dangerous and should not be done by anyone***

Hi Guys,

With universal dual-primary (115V+115V) transformers, does anybody know if metal-oxide varistors (aka MOVs) can be wired directly to each of the primaries such that:

1. In 115V operation, the 130V varistors are in parallel, just like the 115V primaries
2. In 230V operation, the 130V varistors are in series, just like the 115V primaries

The reason for doing this is so that the device can have a voltage selector switch which reconfigures the primaries depending on the AC line voltage, without needing an extra set of contacts to reconfigure the MOVs.

Here is a drawing demonstrating what I would like to do. I have omitted the voltage selector switch for clarity. The left side is for 115V operation, the right side is for 230V operation. Notice also the phase-markings on the transformers as well as the ugly red disclaimer:

Click the image to open in full size.

Some of the literature I've found online says MOV's in series/parallel are OK, but I just wanted to run it by the people here since I haven't found real-world examples of this particular use-case.

Looking forward to your thoughts...
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Old 11th November 2012, 12:12 AM   #2
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It will be fine, no matter what happens the MOVs will clamp either primary voltage before it reaches a high enough value to cause insulation breakdown.
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Old 11th November 2012, 01:34 AM   #3
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You can also add an MOV that is designed for the 230vac line, since it won't do a thing until it sees a high enough voltage anyway!

The other thing is that once "hit" MOVs may no longer function.

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