Hello.
I'm now designing the I/O Control Board by using ESP32 MCU.
24Vdc Relay will be used for Contact Output.
During Turning ON the relay for the AC 220V load by using 24V relay, 5V, 3.3V, 24V DC Power is ringing fastly (tens of nano seconds).
There's a freewheeling diode for coil. so, it's not a switching noise.
So, I thought it's due to capacitive coupling between relay contact(AC220V) and relay coil.
(Kind of common mode noise)
But, I was not true.
When just I connect the one 24V SMPS(for Protective Relay[EMC Passed]) and connect the external ac cord (without or with small load).
and I just directly check DC 24V line without load using oscilloscope.
But, i was ringing like the below.
I used each long AC line to seperate phigically.
MCU was not malfunctioned for a long time.
(there hasn't been no problem)
But. I'm not sure what it is and what could be happend by this ringing wave form.

Did you have this kind of experience?

 

Are you sure this is actual power supply level changes or just signal pickup (from relay switching RFI/EMI) from unshielded and randomly positioned meter probes being displayed superposed on the DC signal?

 

Almost certainly scope artefact, if that is a picture of the probes.

 

Are you sure this is actual power supply level changes or just signal pickup (from relay switching RFI/EMI) from unshielded and randomly positioned meter probes being displayed superposed on the DC signal?

I don't know why. But. there's no problem during MCU runnig.
At first, I found that every DC power(3.3V, 5V, 12V, 24V) in our board was ringing during relay switching.
There was no change when I use decoupling capacitor & EMI Filter, Common mode choke, etc.

This waveform also was detected during AC wire cord (cord reel) connection without load.
So, I thought it's related to voltage reflection through the cable.......

I hope, it's not real waveform.

 

Almost certainly scope artefact, if that is a picture of the probes.

I hope. it's nor real waveform.
Waveform was bad.
but, there's no malfunctions during the runnign of MCU.

I don't know why SMPS respond when i plug the just AC Cord without load........

 

It is entirely possible that the power supply has a very fast response time and not enough stability. It might also be a much more subtle thing like an inadequate return connection causing a momentary oscillation in the digital output. OR it could also be a measurement issue, as some have stated.