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I have designed yet another custom PCB with an ESP32 and some relays that I will use for some IoT projects.

I am using one of these relays to switch a 230 V AC contactor which then switches a 230V AC pump.

I am using the COM-NO contacts of the relay to switch the 230 V AC phase to the contactor. For transient protection, I have added an RC snubber across the relay COM-NO contacts (on the 230 V AC side) and a diode (on the 5 V DC side). Please find a simplified schematic below:

circuit

Here's the thing: the relay switches fine, but the contactor remains powered ON even when I turn OFF the relay. I have measured the voltage across the A1 and A2 contacts of the contactor: when the relay is ON, I measure 230 V AC, which is correct. When the relay is OFF, I measure around 168 V AC, which makes no sense to me.

I have tried to switch the NO contact with the NC contact on the relay (that is: I switch the contactor using the COM-NC contacts of the relay): in this case, the circuit works fine and the contactor is switched ON or OFF according to the relay status. The only difference is that the COM-NC circuit doesn't have an RC snubber on the 230 V AC side, the rest is exactly the same.

I am puzzled... is it the RC snubber that is causing this "parasitic voltage" which keeps the contactor ON even when the relay is OFF? Or do you have another opinion?

These are the components I have used, for your reference:

  • Relay: OMRON G5LE-1 DC5 - SPDT relay - datasheet here
  • RC Snubber: ROXBURGH RE1202 0.2 µF, ± 20%, 120 ohm, ± 30%, 500 mW, 275 V - datasheet here

Regarding the RC snubber selection: I am an amateur, I haven't even tried to size the snubber - I wouldn't even know where to start. I know this is "bad", but I have selected this specific snubber primarily because I have seen it used in other projects similar to mine.

For reference, please find below the schematic of the relay switching circuit.

Thanks in advance!

schematic

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  • \$\begingroup\$ What are the specs of the contactor ? \$\endgroup\$
    – Antonio51
    Commented Sep 24, 2023 at 17:54
  • \$\begingroup\$ It's an ABB, model is ESB25-40N-06 \$\endgroup\$
    – mrcsrplln
    Commented Sep 24, 2023 at 18:00

1 Answer 1

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... is it the RC snubber that is causing this "parasitic voltage" which keeps the contactor ON even when the relay is OFF? ...

Yes. The snubber is wired "serial" with the contactor A1-A2.
If you will use a snubber (probably to be changed), wire it between A1 and A2.
Nothing to be used on the relay.

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  • \$\begingroup\$ Thanks, that makes sense! \$\endgroup\$
    – mrcsrplln
    Commented Sep 24, 2023 at 18:01

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