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I'm working on making a switch that can be turned on with a microcontroller.

I have it mostly wired on a protoboard, and it kind of works, except that even when the relay coil isn't energized and the relay is open, there is a little bit of current still flowing.

The relay I'm using is a PCJ-112D3M.

I hooked the black multimeter wire to the neutral AC wire, and the red multimeter wire to the hot AC wire after it goes through the relay. When the coil is energized the meter reads 120VAC as expected. But when the coil power is turned off, the meter reads 40VAC.

I'm not sure if it's just a bad relay, or if this is somehow expected behavior, but it doesn't seem right.

I just wanted to double check before I desolder the relay and replace it with a different one.

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  • \$\begingroup\$ We'll need to see your schematic and, in particular, details of any snubbers you have connected across the contacts. \$\endgroup\$
    – Transistor
    Commented Nov 17, 2020 at 0:23
  • \$\begingroup\$ The question presumably should be "Some voltage from relay even when open". The power is not "across" the relay. \$\endgroup\$
    – Kevin White
    Commented Nov 17, 2020 at 0:52
  • \$\begingroup\$ @KevinWhite Across, through, from... Not sure which word is most accurate, but thanks for the feedback. Figuring out the wording for the question is always a hard thing, especially in an area that I'm not especially strong in. \$\endgroup\$
    – AndyD273
    Commented Nov 17, 2020 at 0:56
  • \$\begingroup\$ @Transistor I hadn't heard of snubbers, so thanks for the heads up on that. I'll have to research those later. Your comment did remind me that I need to put a flyback diode along the coil, so thanks a lot! \$\endgroup\$
    – AndyD273
    Commented Nov 17, 2020 at 1:04

1 Answer 1

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Are you driving load? Even a simple load such as a lamp will do for testing.

I expect then that you will find the voltage drops to zero when the relay is off.

The voltage you are seeing is probably caused by capacitance either in the relay and wiring or in the cabling to the relay. (or a snubber as suggested by @Transistor).

Your meter is sensitive enough on the AC voltage ranges to detect even the few microamps of current that flows through the capacitance. It is negligible compared to the current required by normal AC loads.

I often see this in house wiring, especially where the live and switched circuit run together in the same cable.

Solid State Relays (SSRs) require a load to operate correctly and can pass a few milliamps when in the off condition. Conventional mechanical relays only pass a a few microamps due to the proximity of the contacts and wiring.

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  • \$\begingroup\$ Not driving a load yet. I was doing the testing before hand, just to make sure that I had the wiring working the way I thought I should. I'll wire a switch up to it so I can plug a lamp in. That'll also give me an idea if there is some current flowing, since an incandescent light will glow a little bit even at low voltage... \$\endgroup\$
    – AndyD273
    Commented Nov 17, 2020 at 0:29
  • \$\begingroup\$ Wired up the plug, and hooked the meter on it, got 28VAC, plugged a lamp in, and it instantly dropped to 0 (or essentially zero). I don't have any snubbers on it (hadn't heard of those before, so learning something new). I remember the house wiring capacitance stuff from watching the youtube video about the zombie Chinese lightbulb. I didn't think it would be a problem, since the wire from the relay to the meter is only a couple inches, but I guess you never know. \$\endgroup\$
    – AndyD273
    Commented Nov 17, 2020 at 0:57

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