Powering a 4 relay board (120 volt)

I am trying to power a relay board (4 relays at 120 volt) 4-Channel Relay Module Board w/ Optocoupler Isolation -Blue - Free shipping - DealExtreme

I can power the arduino with a small wall wart which in turn powers the relay board from the arduino's 5v output. I am using a USB phone charger rated at 1 amp. If I try to power the relay board directly with the wall wart, the board is receiving power (as evidenced by the onboard LED) but the relays will not switch.

I am not that educated in electronics. Do I need to incorporate a capacitor or voltage regulator (or anything else).

Thanks in advance.

I believe you said, if you power the relay board from the arduino, it works, but if you power the relay board from another power source it does not (just shows power light). If that is what you said, then you may be missing a ground between the arduino and the relay board, when using two power supplies. Connect grounds (common ground).
If I read that wrong, give us more details.

Hi Jack,

It is an optically isolated relay board so I don't think a common ground is necessary. There is a jumper on the board between COM and GND. I removed the jumper to see if it made a difference but it did not.


Even if it is optically isolated, there needs to be a return path (ground) for the signal that you send to the board.
If you just send 4 data signals without a common ground, then you will not get what you expect.


Running the Arduino ground to the COM on the relay board made it work.

Thanks so much!

Great! glad I could help.