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Thread: ignition switched 12v

  1. #1
    Newbie
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    Mar 2009
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    ignition switched 12v

    I'm looking to power my laptop (via dc-dc adapter)and screen from a ignition switched 12v feed. Now I know I could splice into the cig lighter feed but mine seems to stay on when the car is off and keys out, so this has the potential to drain my battery if the laptop doesnt close down as it should. I'll be turning the puter/screen on via a momentary button on my bezel each time I get in the car - but I need to make sure that when I get out the car, that the system isn't still draining the battery.

    As I'm ditching the oem h/u can I just use the switched 12v that feeds the h/u currently? Or will a 7" screen and laptop be too much for that feed


    any help/pointers would be lovely


    cheers guys

    Lewis

  2. #2
    Variable Bitrate dMand's Avatar
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    Oct 2007
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    Hi Lewis! What is the voltage requirement for the laptop? and the DC-DC adapter?

  3. #3
    FLAC
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    Sep 2004
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    Falls Church, VA
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    The device should have a dedicated 12v+ source, a ground and an ign+ wire. The 12v is ALWAYS connected to a constant 12v source (battery). The ground to ground and the ign+ is connected to a switched 12v source that turns on with the key. the ign+ is a low amperage draw wire and can be tapped in almost any point where you find the proper voltage. You can use the point behind the factory radio and I would advise it. Its fused and easy to get to.

    DO NOT cut the wire to where you are unable to repair your splice later down the road. I would recommend carefully removing the insulation around the wire in the harness (1 inch removed is fine) and soldering your new wire to the exposed. Wrap it with electrical tape and you're onto step 2.
    Take my advice: Do not try to build a system that includes EVERY feature. Start with the basics, build it to a bug free state, and THEN add on.

  4. #4
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    What I've done it used a relay that when detects the switched ignition through it's coil(pins 1&2) it closes the connection for the 12v constant(3&4) from the battery. This way I don't have to worry about the laptop draining the car battery if for some reason it doesn't shut down properly. I'll be turning it on via a momentary switch that will be in my bezel I've yet to make.....! I have it turning it's self off, firstly by the shut control program putting it into hibernation 1 sec after ign is off and it is running on laptop batteries....then if it hangs for some reason, I have XP forcing a shutdown(whilst ignoring programs that are hanging) once the laptop battery reaches 85% of full charge. This will also stop any deep cycles on the laptop battery

    I think that will work fine...............

    well it does in my head

  5. #5
    FLAC
    Join Date
    Sep 2004
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    Falls Church, VA
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    I would go back to the drawing board and simplify your design. This is way more complex then it needs to be... doesnt the power supply do the switching and delayed turn off for ya?
    Take my advice: Do not try to build a system that includes EVERY feature. Start with the basics, build it to a bug free state, and THEN add on.

  6. #6
    Newbie
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    Mar 2009
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    eh....that's exactly what I want to happen - when I turn off the ignition the power is cut to the laptop(thus not putting my car battery at risk).....laptop now on it's batteries......1 sec later shut control puts the laptop into hibernation cleanly.....next day I get in the car.....ignition on....push momentary button, laptop comes out of hibernation, shut control starts any programs I set it too

    IF for some reason my laptop doesn't go into hibernation quickly, XP will force a shut down once my laptop battery get to a preset % left....this save my battery from damaging deep cycles

    I've tested this and it has been working fine


    am I missing something?




    It is very simple.....I'm using a bog standard atomotive relay. my power pack it a 12v DC-DC adapter, set to match the input required by the laptop....well .3amp under

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