Spikes on Relay Control

Hello Everyone!

I'm controlling a small motor with an Arduino Rele board. However, even though it is working, when checking the VCC and Control signal with the oscilloscope, I've found out huge negative spikes when it turns ON and OFF. The spikes go from 5V to -0.2V then back to 5V in 115us.

Also, you'll see that there is a little delay due to the relay being a mechanical device, so it takes around 5ms to toggle it.


  • 120V
  • 0.55A
  • 60Hz



Oscilloscope pictures

These relay boards should be connected as below:

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Nice that things work but as well as You that spike looks odd.
Can You post a wiring diagram of power and signal cables?
What takes place at the time the spike occurs some 5 ms later then the low going signal?
Is the motor having brushes?

Why the double supplies of 5 volt to the relay board? One from a separate supply and one from the Arduino? One would do I would guess.

To properly enable the relay board input optocoupler.

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Most looks good to me.
There's a cable between J1 and J2 I don't understand but it might be unimportant.
The LED1 will lower the current through the opto coupler light emitting diode. Skip it? The value of R1, 1 kOhm looks a little bit too high to me. Try 470 or less? Check the opto specs for max current!
How is the controller supplied? 5 or 12 volt? Must be 12 volt to Vin. 5 volt must be to Arduino +5 volt pin.

This is the actual components on a commercial 8 channel relay PCB.

The 4 channel relay board in question has similar circuit and component values.

The Arduino could be power by any of the 3 normal methods that we often see used.

These actual relay PCBs were poorly designed.

Unfortunately, there are 10,000s out there as the OP is experiencing. :frowning:

Current flow:

It happens due to the mechanical part of the relay. It takes around 5ms after the signal goes LOW to close the Normally Open output.

Did as instructed and it worked much better. The circuit wasn't using the optocoupler...

@LarryD , your drawing and explanation were very good! Thank you for your help.

I assume that those spikes happen mostly due to the load being inductance... right?

any switching of inductive loads causes spikes.
A recommended solution to reduce these spikes is the use of "snubbers".

RV mineirin


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