Hi, I’m using a few AND/OR logic gates to generate enable signals before they enter a microcontroller. The logic works normally, but sometimes outputs change briefly when a nearby relay switches. The relay and logic ICs are on the same board. Questions: Is this more likely EMI coupling or supply disturbance? Should I add more decoupling or flyback suppression first? On the PCB, is physical separation between relay components and logic ICs usually enough?

Logic Gate Output Changes When Nearby Relay Activates

eva_402 7 days ago

Hi,
I’m using a few AND/OR logic gates to generate enable signals before they enter a microcontroller. The logic works normally, but sometimes outputs change briefly when a nearby relay switches.

The relay and logic ICs are on the same board.

Questions:

Is this more likely EMI coupling or supply disturbance?
Should I add more decoupling or flyback suppression first?
On the PCB, is physical separation between relay components and logic ICs usually enough?

Top Replies

0 acdc90 7 days ago

is the gates powered on 5volts

is the relays on 5v or 12v

does the relay have diode across coil

can we have a circuit ?
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0 robogary 6 days ago

Depends. Is the relay switching an inductive load ?

If possible, you could remove the relay's load and cycle to see if that is related to misoperation.

Relay coils should have a flyback diode (or a shottkey double back diode tvs) unless it effects any critical timing. Larger coils should also have RC suppression.

If your relay switches an inductive load like a motor or solenoid, that load should also have suppression. The suppression will also ensure the relay contact life.
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0 colporteur 5 days ago

I've experienced the interference from relays in digital electronics. The components don't even have to be on the same board. Mostly working with Arduino's and raspberry pi. My fix was to code to simulate key bounce. relay contacts under heavy load do tend to generate emf. Its never seems to be good for the contacts and the surround environment. I can give you empathy but not a practical solution.
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0 DAB 3 days ago

Never underestimate the value of metal shielding.

Also, ferrite cores can be very useful in reducing induced spikes.
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