Mobile install -- 12v relay that shuts off when car off..

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W5XXL
Posts: 20
Joined: Fri Jan 27, 2012 2:05 am

Mobile install -- 12v relay that shuts off when car off..

Post by W5XXL » Tue Jun 26, 2012 5:40 pm

Hi there folks,

Is there such a thing as a relay I can put inline between the radio and my car battery that will turn the radio on when the car starts and shut it off when I turn the car off? I know this is not a reccomended practice but I am curious. :twisted:
Thanks! Please email W5XXL@aol.com

Wes

k4kk
Posts: 207
Joined: Tue Dec 30, 2003 12:20 am
Location: The great state of Tennessee

Post by k4kk » Tue Jun 26, 2012 11:32 pm

Any 12VDC relay with contacts rated for the current required by the radio (almost all of them) should work if you can find a good source for it that is powered only when the car is "ON." Some cigarette lighter/accessory sockets are powered down when the car is "OFF" and should work well to key the relay if so. If not, there are others but you will have to find the source. Many recent radio models have a menu driven time-out feature that will also shut them down. The default setting is often "never" but can be adjusted.

k9yli
Posts: 226
Joined: Fri Aug 12, 2005 11:35 pm
Location: northwest wisconsin

Post by k9yli » Thu Jun 28, 2012 2:06 pm

a auto horn relay has nice big high current contacts but the coil is designed for momentary use..

I have purchased relays from MARLIN P JONES (MPJA,COM)
couple of bucks each.

coil must be designed for continuous use.

At least GM cars had an 'accessory' location on the fuxe box.
just find a hot voltage location that is live only in "accessory" mode..
Justlike the BC radio accessory is hot in both accessory and run..

The relay wont draw much current so wire size to the 'hot spot' shoud not be a consideration..

I have helped out relays ,and their current use, by putting a variable resistor in series with the coil.. Then reduce the voltage to the coil until it fails to pick.

then increas the voltage to half the volts between there and 13.8
measure the resistance, calculate the wattage size,, put in a fixed resistor.
Now the relay will run cool but still be reliable in pick up and hold with out chatter.
Too low a voltage and as load changes and auto voltage varies, the relay can chatter up and down.
Use a double pole relay and wire the contacts in parallel for high current and less point burning..

k9yli
Posts: 226
Joined: Fri Aug 12, 2005 11:35 pm
Location: northwest wisconsin

Post by k9yli » Thu Jun 28, 2012 2:09 pm

I forgot.. This method, a relay tied to the key was critical whern I ran a

50 watt tube type motorola.... leave it on over night and dead battery.
With my Kenwoods and Icom i have left radios on for a couple of days with no effect on the battery..

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