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Test setup: I am looking at this relay G5V-2-DC12. The test is simple. I power it up with 12V and connect a PWM signal (0V-5V 125kHz) to the common pin. I probe at the NO pin of the relay, and I expect to get exactly the same PWM signal back when 12V is ON. That works completely fine when 12V is ON (relay is energized).

enter image description here

Problem: However, when 12V is off, I expect to see nothing at the NO pin, but I got a PWM signal with the same frequency 125kHz but lower amplitude, about 400mV peak to peak.

Experimental Result: The image below shows the results I got when the relay is NOT energized. The top waveform is the input PWM signal (connect to COMMON pin), and the bottom is the signal probed at the NO pin of the relay.

Question: My question is if there is a name for this kind of problem and how to solve it.

enter image description here

Solution: Connect a resistor to the NO pin. I select the 1kR because with the 50R or 100R, the voltage drops as we draw current via the resistor.

Alex
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    Capacitive coupling across the relay? Can you add some resistive load to just dampen it out? Also, how does your measurement setup look? – winny Jul 21 '23 at 20:34
  • I added the image for the test setup. It's messy, and I can tell it is one reason why I get that unwanted signal. But yeah, I will try your suggestion. – Alex Jul 21 '23 at 20:45
  • 125 kHz isn’t terribly high, but your oscilloscope leads are very long, so you might see a slightly exaggerated version of the signal. Always aim for short scope leads. Please report back after loading it. – winny Jul 22 '23 at 06:20
  • The resistor helps. Thanks – Alex Jul 24 '23 at 21:45

1 Answers1

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That is capacitive coupling.

The signal is picked up by the scope because it has high impedance, and there is stray/parasitic capacitance between relay terminals, after all they are just metal conductors running at close proximity and with air as insulation in between.

How to solve it depends on what you want to do with it. If you apply a load like 1kohm or 50 ohms from the disconnected pin to ground, it will surely attenuate the effect of capacitive coupling between relay terminals.

Justme
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