Noob question about relays....

I was using the information in this forum to do some mods, and ran into an odd situation, and wondered if someone has some advice for me.

I'm putting in a socket, and I wanted it to be switched.

I have a fused always-on circuit (20A), and a fused on-with-ignition circuit (10A). I am trying to hook up a relay to the socket. I have the always-on circuit going to 30, and the power line to the socket coming from 87. I have the on-with-ignition circuit going to either 85 or 86 (can't recall, and not at my truck right now), and a ground going to the other (either 85 or 86). The power line is connected to the socket, and the socket is also grounded.

Seems like based on the descriptions here, that should work.

The odd thing is, whatever I'm doing seems to be "shorting" the relays.

I'm very new to wiring, so let me explain a bit. Prior to connecting the relay, I use an electrical meter, and test the resistance between 85/86. I measure high resistance. I connect up my wires and turn on the ignition. No relay click. Nothing inserted into the new power circuit. The fuse on the on-with-ignition circuit blows immediately. It's 10A.

I disconnect the relay, and run the same electrical meter test, no resistance at all between 85 and 86. Unfortunately, I seem to have burned through two relays so far. I don't know if that's possible, or that's the correct expression, but that's what it seems like.

Am I doing the wrong test? Has anyone seen anything like this? I had thought 85 and 86 were "directionless", but is it possible the relays I have care about the polarity?

When I remove the relay, and connect the socket to ground and the on-with-ignition circuit only, it works and doesn't blow the load, so I don't think it's as simple as a load issue.

Thoughts? Suggestions? Thanks for whatever help you can provide.


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Not sure if this helps, but this is what is shown on the relay. I don't understand the symbol below the coil.


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Looks like you have a diode across the coil. From what I remember, input your from your switched trigger the positive goes to 86 (cathode side with stripe) and ground to 85 (anode side).

X2 Rgardjr is exactly right. The diode in parallel over the coil requires the switched power to be 85(-) and 86(+) based on the schematic on that relay. That will force current flow thru the coil. Otherwise the diode will be forward biased and 85 & 86 will appear shorted.

Additionally, you can use 87 or 87a depending if you want the socket to be normally off or normally on (when relay is at rest)

Thanks, all. Much appreciated. Shall retrace my wiring and confirm before putting on another relay


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Just one other question, if I reverse the polarity like I did, does it destroy the relay, or is it only temporary?


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Reversing it will just be a short through the forward biased diode. Probably not hurt the relay as long as it is fused, but it probably would blow the fuse.

Looks like for my relays, they are permanently destroyed. Very very low resistance from both I hooked up incorrectly, even after disconnecting them all. Bummer. Hooked up one properly as per the advice here, and it works perfectly. Luckily, I had some spare . Lesson learned .


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Live and learn. Glad you figured it out.