Can you reverse the voltage on the coil of a non-latching, one-coil relay without
a)damaging the relay
or
b)energizing the relay (the relay stays in the Normally closed position)
I’m talking about a relay like this (Link goes to a PDF file)
If you’ve got a standard mechanical style relay, the coil on a relay basically acts like an electromagnet. When you energize it, it attracts the metal part that pulls the switch and turns the relay contacts on or off. It doesn’t really matter if you reverse the voltage. As long as there is voltage applied, current will flow, the metal will be attracted to the coil, and the relay will turn on.
There are exceptions to this, though.
Some relays have a built in snubber diode. A relay stores energy in the coil’s magnetic field. Turn off the relay, and that energy goes somewhere. If you don’t put a snubber diode on the relay, that somewhere may very well be backfeeding into your circuit and causing you all kinds of grief. Obviously, if you reverse the voltage on this type of relay, you are going to forward bias the diode, which isn’t going to be good. And by not good I mean not good in a “oh hell where’s that smoke coming from” kind of way.
The other exception to this is solid state relays, which don’t actually have a coil and therefore don’t work with reverse voltages. They can also make smoke if you reverse them.
The third exception is if the coil is bifilar wound and you only reverse one coil. It won’t actuate but will still heat up this way. Such relays were used on railroad cars when battery voltage was available but diodes weren’t invented yet and selenium rectifiers were for some reason undesireable (or maybe they weren’t invented yet either, I don’t know).
That’s what I was afraid of.
I knew it was an electromagnet, but then I wondered why they bother specifying the + and - pins.
But it could be a built in protection diode as you say, or just a bias that lets it work with less current in one direction I suppose.
Yes, whatever you do, you don’t want to release the magic smoke, 'cause that’s what makes it work!