MOV connected through Diode

I am interfacing multiple AC wall switches to activate relays that control appliances. For surge protection, instead of connecting a MOV for each switch, thinking of connecting one MOV through diodes from each switch. Pls see the diodes D2 & D4. I have total of 8 switches is each relay controller board.

  1. Is this an efficient way or should connect individual MOVs without diode and across the AC terminals?

  2. Pls see the Zener Diode (ZD1) for constant voltage & the diode (D1) for reverse voltage protection, which are connected parallel. Is this more efficient that having only the Zener diode?

As I felt so and as Andy aka had mentioned, here is the updated circuit. Please advise if any mistakes or anything more to be considered.Updated Circuit:Multiple AC inputs protected by two MOVs

  • \$\begingroup\$ What happens to a negative pike? Are you going to tell the universe to just not cause any anymore? \$\endgroup\$ – Asmyldof Jul 6 '17 at 8:37
  • \$\begingroup\$ Why have you added the diagram that shows the extra protection thus rendering 50% of my answer as looking stupid. Please consider retracting that edit. \$\endgroup\$ – Andy aka Jul 6 '17 at 8:39
  • \$\begingroup\$ @Andy: very sorry, I have just updated my question and only then to see that you have answered already. You have said right. \$\endgroup\$ – John Jul 6 '17 at 8:41

Those diodes will have to be have a rather large single cycle surge rating or they will die to protect the MOV.

Why do you think you need MOVs anyway, just make R3,R7 out of a series string of a few resistors so it will survive 3kV or so (Resistors have a voltage rating!) for 20us (Check the standards for required ratings), and use butch ZD and reverse polarity diodes.

Also, how is your circuit protection set up, if S1 and S2 are fed from different circuits I would worry about failure of one diode shunting power into unexpected places. How is the common neutral return done?

I would either arrange to forgo the MOVs, or use one per circuit and make each isolated input have its own phase and neutral connections so as to avoid any risk of something unfortunate if someone hard of thinking wires the thing between two circuits having separate protection.

  • \$\begingroup\$ @Dan_Mills: Fantastic, that you foresaw potential issues with the diodes. One MOV per circuit makes more sense. S1 & S2 are the output from wall switches that belong to the same mains supply. Sense 1 & Sense 2 goes to microcontroller to ON/OFF the relay. \$\endgroup\$ – John Jul 6 '17 at 14:30

Sharing one MOV via diodes only limits one half of the AC cycle from potential over-voltages. So, I would say "no" this won't work unless you use two MOVs and diodes connected the other way round to limit the other half cycle.

However, if R3 and R7 are appropriately voltage rated to take potential blips of high voltages (that would otherwise have been clamped by an MOV) then I don't see any need to use MOVs at all.


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