transient response of switching solenoid

Thread Starter

tapiiiii

Joined Feb 16, 2024
2
Hi,
I am switching a solenoid (24 V, 27 W) with a electromechanic relay (srd-05vdc-sl-c). I have a flyback diode in parallel to the solenoid. I measured the switch-on event with a scope and the result confuses me. I am measuring the voltage over the solenoid.
photo_2024-02-17_00-26-28.jpg
The voltage drops back to 0 V for a moment, then comes back to 24 V and stays there.
Why does the voltage drop to 0 V?
The second rising edge looks like this:
photo_2024-02-17_00-26-16.jpg
What could be the cause for these ripples and the overshoot?
Initially I also wanted to use the 24 V power supply to power a sensor parallel to the solenoid. Now I am worried the sensor may gets damaged when exposed to these transients from the switching of the solenoid.
 

Papabravo

Joined Feb 24, 2006
21,225
You may be seeing multiple problems. Mechanical contacts will bounce depending on the details of the activation and/or return mechanisms. Does the solenoid have a return spring or does the solenoid get driven in both directions. The diode will only catch the voltage transient when the coil is OPENED.
 

crutschow

Joined Mar 14, 2008
34,455
As PB noted, you are likely seeing contact bounce from the relay contacts.
It bounces between 24V (contacts making the connection) and back to 0V (contacts bouncing open).
Note that the return to zero volts is a ramp from the inductive current, as determined by diode forward drop, the solenoid inductance, and the solenoid resistance.

I don't see how that would bother your sensor, but a large (few hundred µF) electrolytic capacitor from the sensor voltage supply to common should minimize any possibility of a problem.
You could eliminate the bounce, of course, if you used a DC output SSR instead of the mechanical relay to switch the solenoid current.
 

Thread Starter

tapiiiii

Joined Feb 16, 2024
2
You may be seeing multiple problems. Mechanical contacts will bounce depending on the details of the activation and/or return mechanisms. Does the solenoid have a return spring or does the solenoid get driven in both directions. The diode will only catch the voltage transient when the coil is OPENED.
Yes, the solenoid does have a return spring.
 
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