do i need a relay or will a diode work?

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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=94905
Printed Date: April 27, 2024 at 10:26 AM

Topic: do i need a relay or will a diode work?

Posted By: DracosIgnis
Subject: do i need a relay or will a diode work?
Date Posted: June 18, 2007 at 5:57 PM

Hey Guys,
Hopefully a quick question here:

I have a small 12V air compressor that needs to be installed on a vehicle. I want to compressor to have switched/accessory power, so I can use relay and tap an existing accessory wire and I’m good to go, I understand how to do that.

BUT, I’d like to run a second 12V directly to the battery and have a rocker switch allowing me to turn the compressor on if the vehicle isn’t running, but I don’t want to chance back feeding the accessory wire that I’ve tapped if I forget to flip the switch back off.

Compressor is rated max at 18A, so I was thinking (3) 6A diodes between the compressor lead and the switched accessory wire.

Would that work,, am I over thinking it, is that the clean way to do it, or should I be thinking of doing this with relays?? Advice greatly appreciated.

Replies:

Posted By: dualsport
Date Posted: June 19, 2007 at 8:36 AM

When you say the air compressor needs to be installed, do you mean hardwired?
If you're going to run a separate wire to the battery anyway, is there a reason for connecting up to the switched accessory wire at all?
Only thing I would think you want to do it is to plug in the compressor on the accessory outlet, but that doesn't involve much installing.

Posted By: DracosIgnis
Date Posted: June 19, 2007 at 10:03 AM

Sorry, yes hardwaired. I would want it switched with accessory to make sure it doesnt run while I'm away from the truck (unless I forget to turn the manual switch off)

Posted By: cneubert42
Date Posted: June 19, 2007 at 11:10 AM

wire the relay like this 30 to compressor 87 to the battery 85 to ground 86 to constant 12 volts with switch inline and another 86 to acc. therefore your compressor will be on when the car is running and if ou want to turn it on when the car is off you can flip the switch.

Posted By: DracosIgnis
Date Posted: June 19, 2007 at 12:20 PM

Thanks for the reply and that wass part of my original thought, the problem is, if I turn the compressor on using the switch, I'll be back feeding 12 volts to the accessory wire would I not?

Posted By: DracosIgnis
Date Posted: June 19, 2007 at 12:38 PM

After some more thought, I think maybe a 3 position switch inline would solve my problem. It could be wired as above, 1 position of the switch would give it constatnt power, the middle posiiton would be 'OFF' completely (as an off override) and the third position would be 'ON' only if accessory is on. If correct, no way to backfeed... Makes sense to me, comments?

Posted By: cneubert42
Date Posted: June 19, 2007 at 1:54 PM

just throw a diode between the relay and the acc wire

Posted By: dualsport
Date Posted: June 19, 2007 at 9:41 PM

If you have your relay supply power only to the compressor, there won't be any backfeeding to worry about. The accessory power is just used to switch the relay on; not powering the compressor. Is there a reason you wanted to connect the accessory to the same wire as the compressor?