Mechanical relay output voltage doesn’t change


As a followup to my previous question, where a 2222 npn transistor wasn't beefy enough to handle the load I was putting on it to activate a solenoid valve, I decided to try a mechanical relay board kit instead (which I realize is overkill for what I need, but it wasn't too expensive, and I figure I can reuse it eventually).

Like my previous question, I'm driving the logic with an arduino. I see the LED on the relay toggle, and I can hear the relay click when it's turned on or off. However, the output voltage (NO to COM on the 'relay 2' outputs) doesn't change when I test it with a multimeter. The voltage between any pair of output pins stays zero (NC to COM as well, all the outputs on relay 1 as well).

Is it likely that I make a mistake when I soldered the board kit together? I could try to re-do the soldering, but visually it looks OK to me (although I'm new to this), and things are functioning enough that I can hear the relay click when it's toggled.

Could I have fried both of the relays? It's a 'Omron Relay G2RL-14-DC12, SPDT, 12A, 12VDC'. Is there a way I can diagnose what's wrong with the board? I've prodded various through-hole pins on the underside of the board, and I can see a voltage across the LED's pins, but not the output pins. Is there something else I should test? Since I can hear them click, I assumed they're working, but maybe something else on the board is busted.

The output voltage should be the same as the input voltage, right? The multimeter says the voltage across the input pins is 11.7 when the relay is on (open), 11.9 when the relay is off (closed), 12.01 when the arduino is off too (why would it be different? I thought it was an isolated circuit).

Thanks for fielding my newb question 🙂

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Best Answer

The relay contacts are not connected to the board 12 volt power supply - they are just isolated switch contacts.

If you measure the resistance between the COM (common) and NC (normally closed) terminals, you should see near zero ohms when the relay is released, and an open circuit (inifinite resistance) when the relay is operated.

Measuringing resisttance between COM and NO will give the opposite result - near zero ohms when the relay is operated.