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I am building a circuit that gets power from AC. I have a center tapped 220V transformer, so I added a switch for 220V or 110V (the three wire sockets)

I added two MOVs, one S20K230 for 220V and a S20K140 for 127V, but when the circuit is running with 220V, the S20K140 will have voltage across it from the central tap, I don't think that is a problem since the voltage will be below the limit, but is this ok? I never worked with AC Mains before, thanks and sorry for the silly question.

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  • \$\begingroup\$ What are the MOVs intended to protect in your circuit? \$\endgroup\$
    – Andy aka
    Jul 16, 2020 at 21:05
  • \$\begingroup\$ A linear voltage regulator controlled by an LM723 and a LM7805 that will drive an Arduino, they have their own TVS diodes after being rectified. The voltage regulator will be a general power supply for some testing and stuff. \$\endgroup\$ Jul 16, 2020 at 21:10
  • \$\begingroup\$ If the regulators have TVS diodes, what do the MOVs bring to the party? They will need fuses if you do need to use them (not demonstrated yet). \$\endgroup\$
    – Andy aka
    Jul 16, 2020 at 21:22
  • \$\begingroup\$ I added them to act in case anything behind them shorts, like transformers, input capacitors, rectifiers etc. Do you think it is unnecessary? \$\endgroup\$ Jul 16, 2020 at 22:04
  • \$\begingroup\$ MOVs act as over-voltage limiters and they can go short-circuit if they have to handle too much over-voltage hence you need fuses and, they offer no protection against down-stream shorts. This leads me back to my original comment: What are the MOVs intended to protect in your circuit? \$\endgroup\$
    – Andy aka
    Jul 16, 2020 at 23:02

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In general it's not a problem, but 127VAC * 2 = 254V plus tolerance (and the increase on the 127V will be doubled). So you should go higher than 230VAC rating most likely.

You might also consider using two MOVs the same in series, saving on a BOM item and giving you a 280V rating rather than 230.

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  • \$\begingroup\$ Thanks for your comment Spehro, it's good to know it will not have problems. About the voltage, in Brazil we have three phases that are 120 degrees out of phase, so the when you have two 127V phases, actually your looking at 127*sqrt(3) = 220V. If I am going to be fine with those two varistor, it should be fine then! \$\endgroup\$ Jul 16, 2020 at 21:54
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    \$\begingroup\$ I have worked with 30 mm and 40 mm MOV's for decades. 2 of the same lower voltage saves money and space, plus there are more voltage options at the lower voltages. For countries with un-stable power feeds, we shipped suppressors with double-voltage MOV's. Less clamp protection, but the MOV's survived. \$\endgroup\$
    – user105652
    Jul 17, 2020 at 2:21
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    \$\begingroup\$ @Agusto The transformer, as you’ve shown it, will double the applied voltage so 127->254 across the MOV, not 220V. If the transformer is wound as 230:127:0 then it is maybe not a problem, but still worth checking. Is that clear enough? \$\endgroup\$ Jul 17, 2020 at 2:28
  • \$\begingroup\$ Yep, that's it, 230:127:0, thank you for the tip, luckly I think I'm good. \$\endgroup\$ Jul 17, 2020 at 11:18

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