I designed a 3-state contactor control circuit.

EVSE_POWER_POSSIBLE, 3V3 from a microcontroller, basically enables the rest of the circuit.

When EVSE_POWER_POSSIBLE is HIGH a blue LED turns on in the button. When pressing the button it turns red (EVSE_POWER_ENABLED is HIGH) and the contactor turns on, ie CPC1972GSTR starts conducting. The load (contactor coil) is 230V.

Pressing the button again clears the EVSE_POWER_ENABLED and I can measure 0V on CPC1972GSTR pin 1, but the SSR doesn't reset or turn off. If I disconnect the board it goes back to the off state.

The button attached is this.

Note, there is a hardware pins 2 and 3 are switched on Q1, this is patched on the board.


  • \$\begingroup\$ Sounds like SCR or TRIAC latch-up. Try adding an RC snubber across the contactor coil, or perhaps across the output of the SSR. Something like 220 nF and 47 ohms. \$\endgroup\$
    – PStechPaul
    Commented Dec 16, 2022 at 21:39
  • \$\begingroup\$ Can we assume that the CPC1972 is switching a 120VAC or 220VAC 50/60Hz coil of a contactor? \$\endgroup\$ Commented Dec 16, 2022 at 21:55
  • \$\begingroup\$ @SpehroPefhany Yes, the contactor coil is 230V. \$\endgroup\$ Commented Dec 16, 2022 at 21:58

1 Answer 1


May I suggest that you try a temporary substitution of the contactor coil load with a simple resistive load that is non-inductive but with similar resistance so that the current load on the SSR is pretty much the same. If the SSR behaves properly with this resistive load then you will know that the problem you are seeing relates to the inductive nature of the contactor coil.

As stated in comments the inductive loads of a contactor coil often require the use of a properly characterized snubber circuit across the SSR output to curtail the noise produced by switching AC loads. Even though this small IC packaged SSR is touted to have close tolerance zero crossing switching the multi-milliseconds delay of the contactor itself to its output contacts making or breaking may very well be feeding major noise back into your circuit board and affecting the SSR.

Part of the solution to this may very well be taking a careful look at your wiring and packaging to make sure you use assembly techniques that keep contactor and motor wiring separate from the circuit board and the contactor coil wiring. Shielding of the contactor control wiring may even be investigated.

One last thing to ensure is that you are not over doing the specs of the SSR. These small IC packaged devices are only rated to a maximum load current of 250mA rms.


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