Power Off Relay

paul330 · #1 01-12-2010, 05:48 AM

I am looking to set up a E-buss system with standby alternator and one battery. I would like to use a relay where if the power is OFF on the control circuit (Main Bus) the load circuit (E-buss) comes ON. Is there such a thing? Lamar Superswitch has a "negative" version but that closes if the control earths - not quite what I need.

fl-mike · #2 01-12-2010, 07:30 AM

Paul,
It sorta sounds like you are looking for a SPDT relay operating as a transfer switch with the coil connected to your main bus. If the main bus drops, the relay opens and connects to the NC contact.
That being said, I would consider a couple of steering diodes/rectifiers instead of a relay.

paul330 · #3 01-13-2010, 12:56 AM

I know that the Aeroelectric method is to use a diode but that requires the secondary wire to the E-buss to be permanently connected. For some reason, I just don't like that idea. It could be switched but then there would be a delay with no power momentarily. My idea would be the relay with an ON-AUTO-OFF switch to bypass the relay if needed. I know it would work well - it's just a case of finding a relay. They exist in the commercial world but I need to find one at experimental prices!

To clarify, with power in the switching circuit (in this case, the main buss) the contacts on the power circuit (E-buss standby wire) are held open. If power fails (ie loss of main buss) the contacts close and the E-buss is powered by its' secondary route.

If I knew how to do it, I could post a diagram!

n82rb · #4 01-19-2010, 05:39 PM

how many amps are we talking?

bob burns
N82RB

paul330 · #5 01-19-2010, 06:04 PM

25A. I think I have found what I need at Maplins (UK Radio Shack). It was "SPDT" that was the key to tracking it down. I just need to make sure it is continuous duty. Cost is less than $10

fl-mike · #6 01-20-2010, 10:51 AM

Quote:

Originally Posted by paul330

25A. I think I have found what I need at Maplins (UK Radio Shack). It was "SPDT" that was the key to tracking it down. I just need to make sure it is continuous duty. Cost is less than $10

There are plenty of the "ice-cube" SPDT relays that will handle that. Bosch/Tyco/Potter & Brumfield have some impressive offerings. You could likely pop one out of your car if you wanted to experiment.
http://order.waytekwire.com/productd...%2012V%20SPDT/
Sounds like you found something though.