relay diagram for car pc

Printed From: the12volt.com
Forum Name: Relays
Forum Discription: Relay Diagrams, SPDT Relays, SPST Relays, DPDT Relays, Latching Relays, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=111914
Printed Date: July 12, 2025 at 7:58 PM

Topic: relay diagram for car pc

Posted By: rsa_sean
Subject: relay diagram for car pc
Date Posted: February 25, 2009 at 10:35 AM

Could someone please validate this diagram, I am by no means an electronics expert but I think this will do what I am looking for:

The scenario:
I have a CarPC that has power supply unit (PSU) designed for car use, the supply has an auto power on ( based on a switched 12v line). I would like to begin the power up process by using my keyfob and the neg trigger from the dome lights to power the PSU's switched line, then when I turn the ignition the switched line would then power the PSU's switched line.

Does this look like a viable solution ??

posted_image

Other notes:

The dome lights turn on for a few minutes then power off. this is not an issue because the PSU I have has a shutdown controller.
Also the dome light turn off when the key is turned.

Thanks in advance.

Replies:

Posted By: KPierson
Date Posted: February 25, 2009 at 11:28 AM

I would put a diode across each coil. You could probably get away without one on the right relay, but the left relay will need one to protect the transistorized outout of the BCM that wasn't designed to handle inductive loads.

Other then that it looks good and I would think it would work.

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Kevin Pierson

Posted By: rsa_sean
Date Posted: February 25, 2009 at 12:35 PM

So you would recommend it like this ?

posted_image

Posted By: rsa_sean
Date Posted: February 25, 2009 at 12:37 PM

how many amps should the diodes be able to handle ?

would a 1 amp be sufficient or should I get something larger 6A ???

Posted By: KPierson
Date Posted: February 25, 2009 at 3:45 PM

a 1A diode would be fine.

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Kevin Pierson

Posted By: rsa_sean
Date Posted: February 26, 2009 at 7:39 PM

Could the diode just be placed on the negative trigger???

How would I place a diode across the coil ?? does anyone have a picture that shows what this physically looks like ?

Posted By: KPierson
Date Posted: February 26, 2009 at 8:17 PM

No, it has to go across the coil. You wire the striped side to the (+) side of the coil and the non striped side to the (-) side of the coil.

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Kevin Pierson