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Minimizing 120VAC relay EMI

Haberkern

Feb 10, 2022
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Hello all. I have attached a portion of my circuit for a pcb that regrettably has to be mounted above a low voltage board with the MCU. The relays are driving 120VAC relays that draw around 1Arms. The 5V isolator should protect the digital signals, but due to proximity I'm assuming, when a relay disconnects power the ringing sometimes causes havoc with the digital circuits that are underneath this board. I have replaced the AC TVS diodes with 180V ones (ya I screwed up and forgot about the whole sqrt(2) rms of 120VAC and blew up the first diode). But I would also like a relatively inexpensive low pass to further protect my digitals from these spikes. Any suggestions? Thank you
 

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Haberkern

Feb 10, 2022
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Here's a screen grab of the ~300v ringing when a valve closes and the relay disconnects for reference
 

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Harald Kapp

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Where's the driving circuitry (NFETD1, NFETD2) for the relays? I see no flyback diodes across the relay coils to minimize flyback voltage at turn-off. The problem occurring at "disconnect' suggests that this may be one (the?) cause for your issues.
To avoid EMI due to arcing across the relay contacts: if you can tolerate a small leakage current, place e.g. a 2.2 nF / 250V X1/Y1 capacitor across the relay contacts.
 

Haberkern

Feb 10, 2022
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Thank you. I didn't realize I could use the flybacks on an AC relay. I'll give that and the caps a shot.
 

bertus

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Nov 8, 2019
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Hello,

I see two relays in your schematic RLY1 and RLY2.
As the coil is 5 Volts DC a flyback diode should be installed.

Bertus
 

Haberkern

Feb 10, 2022
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I see what you mean. Thank you for clarifying. I even put notes on the schematic to add flybacks. I will do so.
 

Minder

Apr 24, 2015
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I use 1N400x versions, the 1n4007 covers up to 1kv.
 
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