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I hate to draw things, but here is the system I am working on:

system image

In this system, as I switch on/off the AC pump through the contactor(with a flyback diode), the circuit(controller card) randomly shuts itself to the start state. So far, I have yet to pinpoint the issue.

I will not request you to find the issue from this crude schematic though. I did some research on removing noise from circuits, and stumbled upon this:

Capakor General Technical Information

Here on page 4, it looks like my scenario - common mode noise: main circuit isolated from earth ground, yet close to it - and as a suggestion, it advises to encase the circuit with a grounded metallic case and shunt the stray current through Y capacitors.

My board doesn't look like a simple "source and load" system though, it includes a 40+ pin microcontroller.

So my questions are:

  1. Are Y capacitors mandatory in this scenario, assuming the noise is caused by stray capacitances as on the document? Because everywhere else mentions they are mostly used in SMPS's and mains voltage related things.
  2. If they are necessary, where should I connect them? It seems to me that the noise is radiated type, because input voltage is already fed by an SMPS, which gives stable voltage (as far as my cheap non-True RMS multimeter says)
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  • \$\begingroup\$ This problem isn't about noise, it's about a dipping power supply. Replace that SMPS and try again. \$\endgroup\$ – Janka Aug 11 '17 at 20:47
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This is not an answer to your question - merely some help in asking it properly.

There is a proper schematic editor built into the post editor toolbar. Here is a schematic that might be close to your setup.

schematic

simulate this circuit – Schematic created using CircuitLab

You can edit this post, edit the schematic, copy and paste it into a schematic in your question. Modify it to match your actual setup.

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