Electro magnetic interferences (EMI) generated by power circuit reset low voltage integrated circuits (ICs)

Electrical Engineering Asked by Krzych on December 27, 2020

The main question here is (explanation will follow):
Is there a way to filter out low voltage oscillations (>10MHz) on pins that have to stay high for the system to function? Oscillations are induced by EMI emitted by power circuit during relay bouncing). RC filters 100Ohm/100nF did not work. Traces are ~50cm long.


I’m currently struggling with proper EMI filtering for the low voltage control circuit that is responsible for switching on/off high voltage capacitors using relays.

I realize that the low level part of the circuit has a bunch of design flaws (e.g. the logic signal between uC and shift-reg could’ve been arranged into a differential pair). But currently I’m trying to figure out a way to modify the circuit with as little PCB modifications as possible.

Here is the essential part of the setup:
enter image description here

Power circuit is operating on fairly high voltage and current, but I don’t care about the quality of power of this circuit, moreover I cannot modify it.

The only issue I have to deal with looks like this (the description will follow):

enter image description here

  • Yellow – Power circuit voltage
  • Blue – Power circuit current
  • Green – logic signal (EN or CLR, see schematics)

Green signal represents EN or CLR; it is a CMOS 3.3V open-drain pin. When voltage falls below certain treshold (I guess 1.8V) it is no longer high, therefore the shift-reg resets. It cannot reset.

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