DPDT Relay

**AltimateX** · 07-02-2008, 06:01 PM

I did a little searching and from what I understand, I can use a dpdt relay to shutdown my lcd when I turn off the ignition.

If so, how about this one? http://www.aquahub.com/store/12voltspdtrelay.html

I found a wiring diagram here: Wiring question.

If I splice the cig lighter adapter wire am I going to have two power wires? Does it matter which one I hook to "common" and "normally open"? And do I just ground the monitor somewhere on the chassis?

**pancit175** · 07-02-2008, 08:36 PM

First you have to think to yourself, why do you need to use a relay. Also why are you going to use a dpdt relay to power one device? You may as well use a standard spdt bosche style relay.
I'd suggest you check out http://www.bcae1.com/ for any relevant information on how relays work and are used. In summary, relays are used to isolate the activation wire from the circuit being activated. For instance, when the circuit/device you are powering up requires more current than the wire that activates it.

Furthermore, the cig power has both +12v (constant or accessory) and ground at those wires, and there is a reason for every wire so the relay is very specific as to how you wire it up. Are you planning on using the cig plug power as your power source or your activation source?

**AltimateX** · 07-02-2008, 08:53 PM

Are you saying an spdt relay is overkill? If that's the case, what would be a more practical alternative?

My cig lighter always has constant power going to it so I am guessing I will want to tap off it for power and use my accessory as an activation source.

**pancit175** · 07-02-2008, 09:41 PM

DPDT is overkill, you'll only effectively be using half of the relay. You want SPDT, which is more accessible. You can find those at any automotive parts store or radioshack near you.

[spoonfeed in process] SPDT relay pins:
85 - [coil] Accessory wire
86 - [coil] Ground (can be found at cig plug wires or straight to chassis)
87a- [normally closed] not used
87 - [normally oped] +12v constant (found at cig plug wires)
30 - [common] +12v ouput to device (active only when accessory is active)
[/spoonfeed in process]

**NiSlo** · 07-02-2008, 09:45 PM

Alot of people on here (myself included) actually splice into the molex connectors from the PSU that power syour hardrive, cd-rom's etc. The reason behind this is that the PSU supplies a clean feed of +12v while your vehicle electrical system typically runs at around 14v mark. The added benefit of this method is that the screen will only ever be turned on when the computer is so you don't have to worry about relays etc...

You just need to make sure your PSU can supply enough juice to power yoru computer and screen (which my m2-atx) does easily).

*edit* If you do go down the relay path, then an SPDT is all you need setup as below

*edit2* Beaten

**AltimateX** · 07-03-2008, 04:04 AM

Thanks very much guys! I appreciate the help very much. I will be going to the auto parts store after work today to pick up an spdt relay.

I do have one final question though. That diagram Nislo posted shows a fuse in my constant line. What size fuse would you recommend I use?