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Discussion Starter · #1 ·
Here's an odd question for the electrical gurus.

Am looking for a way to supply power to a device when the ignition is off then stop power to it when driving. Sort of like a backwards relay.

Any ideas?
 

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Most relays have a NC and NO contacts. Are you looking at adding a relay, or tapping off an existing relay?
 

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Discussion Starter · #3 ·
No idea yet, may even add a relay if I can figure out how to turn it off when power is supplied and turn it on when there is no power supplied.
 

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You should readily be able find a relay in the autoparts store. Look for one that has at least 5 terminals. More is acceptable. The 5 terminals will be:

+12V
Ground
NC
Common
NO

Most relays have a diagram on the top showing what terminal is what. A multimeter will assist you.

Wire the +12V on the relay to the switched ignition (fuse this). Ground the ground terminal. Run a (fused) wire directly from your battery to the common (not to be confused with ground!) Run a wire from the NC to whatever you want to power. Ground your device you are connecting.

Voila!
 

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Discussion Starter · #5 ·
senglehart,
Thanks for the help, but don't think this is what I am looking for. Probably my question is not clearly stated. Or I am completely misunderstanding the answer.

I have installed relays before and the wiring is not a problem. Example when power is detected on one leg of the relay it switchs power from the dedictated 12v supply to the device terminal. I installed a relay for my aux reverse lights that detects power to the normal reverse lights that then supplies power from a dedicated 12v line to the aux lights.

This instance is a reverse of a normal relay. I want power to be supplied to a device when I turn off the ignition and when I turn on the ignition power is switched OFF to the device.

What are the definition/functions of NO and NC terminals as you describe?

Thanks again,
Brad
 

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Sorry.

NC = normally closed contact when the coil is de-energized, such as when the relay is at "rest." Sitting on a shelf, switched +12V power to the coil removed, etc.

NO = normally open contact when the coil is de-energized, such as when the relay is at "rest." Sitting on a shelf, switched +12V power to the coil removed, etc.

With your device wired to the NC, it will only be powered when no voltage is applied to the coil. When you turn your ignition ON, the relay energizes, the NC contact opens, and your device is switched OFF.
 

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Here's a pic of a sample relay. It's got 3 sets of form C contacts.

Terminals A and B are the coil. Wire this to your vehicle's ignition switched +12V and ground. Turn your key on, relay is energized. Turn your key off, relay de-energizes.

7 is the common (connect directly to your battery)
1 is the NC contact (connect to the +12V input on the device your are powering)
4 is the NO contact

Ground your device (hook a wire to the negative input)
 

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The above is a mechanical switch/ button relay

Or you could get a bosch 5 pin relay

terminal 30 12+
terminal 86 switched 12+
terminal 85 ground
terminals 87+87 OR 87/87a

Depending on the switched relay, 87/87 will be in contact together with relay UNswitch, OR no power at 86. Switched they will contact 30 and be powered.

You want a 87/87a where UNswitched 87a is in contact with 30 when 86 is not powered. But when you switch power it, it will UNswitch 87a and switch on 87..

Lol, is that clear enough,.. what do you want to do, make a security switch?

Let me know if I can clear it up some more
 

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Discussion Starter · #9 ·
Guys, thanks for the help and very understandable instructions. I never realized that a relay worked in "both directions". Perfect solution.

Mudchucker - this is for the power/battery charger for a clip on Bluetooth phone rearview mirror. Currently I pulled a line off the cigar lighter into the overhead console and connected to the short pigtail power connector for the mirror. The "problem" is that when it is in the charge mode (when the ignition is on) it has two alternating/flashing red and blue lights at the bottom of the mirror. I can live with it, but the wife about wet herself when she drove to the store sunday morning. Kept thinking she had a cop behind her. LOL. When the mirror is running on its internal battery, there is just a small single blue light indicating the Bluetooth is connected to the cell phone.

For those of you that haven't seen these mirrors, they are an economical alternative to the aftermarket UConnect system. Have found only one drawback/flaw. They are currently designed for the European/Australian market and the FM modulator for using the truck stereo speakers only tunes to even number frequencies (89.4) and US FM radios tune on odd number frequencies (89.3, 89.5). Here's a link about the product.

http://www.geeks.com/details.asp?invtid=VTB-88&cat=CON I got mine on ebay for about $25 less from a private seller.

Thanks again,
Brad
 
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