I have two power supplies, both 12v for the car (one is actually a shutdown controller that regulates 12v). My question is I'd like to use the 12v out on the shutdown controller to power an incar PC, but also I'd like to make it the switch for the relay.

If I run a line from the 12v out on the shutdown controller just for on/off on the relay, will it consume any major amount of current? Basically I need full power coming off of the shutdown controller for the in car PC...

Depends on what you mean with major... How much current can the device that turns on the relay supply?

Current drawn depends on the resistance of the coil in the relay - a typical automotive relay (aka Bosch relay) will typically pull a lot (relatively speaking) more current than some other relays (like for PCB mount)

Check out an electronics supplier and browse the relays available. Find the one with the highest coil resistance that provides the switching options you need and and can handle the current required. Current drawn is then I=U/R where U=12V and R is the resistance of the coil.

You'll probably want to add a diode (1N400x series works nicely) wired in reverse across the coil in order to snuff the inductive kickback as you turn off the relay.

Edit: but compared to the current drawn by a PC, the relay draws very little power

/MIchael

if you pop open a common bosch relay you'll see that the coil winding is itty-bitty. the amount of power it uses when energized is also, technically speaking, itty-bitty. nearly insignificant.

basically the constant 12v in is coming straight from the care, so I don't car about that. The current coming from the shutdown controller for the switched 12v is rated at 80 watts? but it's also feeding a mini-itx mobo with a 2.5 in HD, and a laptop optical drive...

so I don't car about that

pun intended?

damn you got it before I could change it , yeah I normally have to edit a few times to get all the spelling worked out...

OK... 80W @12V means 80/12=6.66A, so say 20mA (0.02A) shouldn't be a problem to tap off...

A current no higher than 20mA @ 12V means the coil resistance must not be below 12/0.02=600 Ohm. I definately recall seeing 12V relays with coils in the 600 to 800 Ohm range...

/Michael

Thanks, so I should be all set..