Emergency Lights Learning Project

[dcbrown73]

Hello all,

I'm getting started in learning electronics and deciding on a project that isn't too deep to start or just too simple (I've already done the LED stuff)  I decided a nice little emergency lighting for my home would be a nice thing to have, but I had a question concerning relays.

My thought was I would plug the device into a 120v socket and use a relay to control it's function.  What I was thinking was the relay would use the house power to lock the relay in the NO state and the absents of the house power would flip (release) to NC which would engage the battery power.

My dumb question is, are relays meant to function with the magnet always engaged under normal use?  Does it not matter or does it stress the relay to the point of failure at some point?

So far I haven't fully designed the circuit, but I suppose it would be the 120v -> transformer to voltage level for relay -> rectifier to convert to DC -> relay?   I suppose I should get some software to design it.  I will look for some.

Thanks,
David

[Seekonk]

Relays do heat up as well as consume power.  Most DC relays will pull in at about half voltage.  Once pulled in it only takes about 15% power.  I'd use an opto isolator pulling down a FET gate to switch on lights.

[dcbrown73]

Thank you, I will look up a  opto isolator.

[james_s]

A relay will work though, so long as the coil voltage supplied is within the rated range it can be left energized continuously. Small relays do not consume very much power, probably a bit more than the LED in an opto coupler but even those consume some power.

[edavid]

My dumb question is, are relays meant to function with the magnet always engaged under normal use?

Yes.  Solenoids often have on time limitations, but not relays.

So far I haven't fully designed the circuit, but I suppose it would be the 120v -> transformer to voltage level for relay -> rectifier to convert to DC -> relay?

You can do it that way if you want, but it's much simpler to buy a relay with a 120VAC coil.

A relay will work though, so long as the coil voltage supplied is within the rated range it can be left energized continuously. Small relays do not consume very much power, probably a bit more than the LED in an opto coupler but even those consume some power.

I looked at some AC relay datasheets, and they have coil ratings of 0.5-1VA, so not much power at all.