Electronics > Microcontrollers

DC 24V Ringing Waveform due to external AC220V Cord plug (without load)

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JinhoAhn:
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, 24Vdc was ringing (tens of nano seconds, almost down to 0~5V).
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?

fourtytwo42:
Hello
A picture of your circuit with the ESP32 & relay would help.
What are you using to drive the relay ?

JinhoAhn:
it's not relaed to MCU and Relay Drive.
if there's only one 24V SMPS pulged in AC outlet and i check the output of DC line using Oscillocope.
at that time, i plug a another AC Cord without load, 24V DC voltage is ringing during short period of time (hundreds of ns)
will it be possible due to voltage reflection?

I used another EMC Filter, X-Capacitor, decoupling capacitor for DC line.
there's no effectiveness.

does my potable oscillocope have problem? (40Mhz, china, MDS8207)

BennoG:
You are probably picking up a little EMC with your scope wires.
They act like a antenna when not shielded correctly.

Benno

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