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Thread: Powering CarPC/LCD from Cig Lighter

  1. #1
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    Powering CarPC/LCD from Cig Lighter

    I'm using a laptop carpc setup, but my question involves powering multiple devices, so I thought it best to post in the power supplies forum.

    I will be using a DC/DC adapter to power the laptop. The adapter is rated at 15V/5A = 75w. The screen I'll be using is either a Xenarc 700TS or 700TSV. I'm not sure how much power it requires. I'll be using the cigarette lighter adapter for both devices.

    Ideally, I'd like to purchase one of those cig. lighter splitters and run both the laptop and the LCD off of the same lighter input. However, I want to make sure the input can handle the amount of current that will be required to power both devices.

    Has anyone done this?

  2. #2
    Neither darque nor pervert DarquePervert's Avatar
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    Has it been done? Yes.
    Is it recommended? No.

    Where's Toaster-Boy and his drawing for Maheriano?
    Have you looked in the FAQ yet?
    How about the Wiki?



    Under normal circumstances, a signature would go here.

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    I assume that's the drawing you're talking about..

    However, I'm not looking to mount the laptop under any seats - I'll be mounting it in the trunk. I just purchased a DC-DC adapter with the exact specs as the original AC adapter that came with the laptop. Is it still unsafe to use the cigarette lighter in this case?

    It sounds like you recommend running a wire from the battery to the trunk. This seems unnecessary in my case, since I don't need power for anything else back there.. but maybe it's safer? So then, do I cut off the connector end off the laptop DC adapter and splice it to the battery wire and a ground source in the trunk?

    What about the screen? Can I just run the screen off the cigarette lighter adapter? The Xenarc I'm buying requires no more than 1A, and it has a built-in voltage regulator.. or should I splice the power wire to something else that has power with ignition?

    Sorry for all the questions - trying to avoid making redundant posts. Thanks.

  4. #4
    Maximum Bitrate binary.h4x's Avatar
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    I would run a wire from the battery to the trunk and add a cig lighter adapter to it, that way you don't have to cut up your DC-DC adapter.

    The monitor will be fine up front on the cig lighter adapter.
    2007 Honda Fit Sport 1.5L SOHC-VTEC

  5. #5
    Neither darque nor pervert DarquePervert's Avatar
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    Quote Originally Posted by binary.h4x View Post
    The monitor will be fine up front on the cig lighter adapter.
    Aesthetically, it would be better to wire it up directly.

    Where you been binary.h4x?
    Have you looked in the FAQ yet?
    How about the Wiki?



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    Quote Originally Posted by binary.h4x View Post
    I would run a wire from the battery to the trunk and add a cig lighter adapter to it, that way you don't have to cut up your DC-DC adapter.

    The monitor will be fine up front on the cig lighter adapter.
    That's a good idea binary, I didn't think of that.

    Quote Originally Posted by DarquePervert View Post
    Aesthetically, it would be better to wire it up directly.

    Where you been binary.h4x?
    Darque, I plan on "hiding" the monitor/lighter connector underneath the dash, there's extra room. My car has a secondary factory cig. lighter, so it's all good.

    The disadvantage I see now is that the laptop will be drawing power from the battery even after the car has been shut off.

    Can I use a relay to cut power to the trunk battery cable when the ignition's off?
    If so, what kind of relay do I need, and how do I connect it?
    What gauge wire do I need to run from the battery?
    What should the inline fuse be rated at? The laptop is 15V x 5A = 75w.

    Thanks!

  7. #7
    Maximum Bitrate binary.h4x's Avatar
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    Relays: http://www.the12volt.com/relays/relays.asp
    Wire Size: http://www.the12volt.com/info/recwirsz.asp

    The fuse should be rated at something between your device (5A) and the max current your wire will handle.
    2007 Honda Fit Sport 1.5L SOHC-VTEC

  8. #8
    Neither darque nor pervert DarquePervert's Avatar
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    Hiding the cig lighter connection is a good idea. Makes it easy to connect/disconnect if you need to remove the screen.

    Perhaps you should consider the laptop power control module or even a regulater with statup/shutdown controller like the Carnetix P1900 or P2140.

    Sure, it's an additional expense, but it's feature packed and you're supporting the hobby.

    You could also use hibernation mode on the laptop when you turn off the ignition so that there's no power draw from the laptop.
    Have you looked in the FAQ yet?
    How about the Wiki?



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  9. #9
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    Yes, I do plan on using the hibernation to put laptop to sleep.

    Should I opt for the SPDT relay with a built-in diode? It says the diode's purpose is to protect circuits that may be sensitive to voltage spikes. Although the laptop's DC/DC adapter should theoretically protect against this, it doesn't hurt to give it a little help does it? Or is it overkill?

    Where's the best place to mount the relay? And is this the correct setup?

    - Run wire from ACC to terminal 85 (where's the best place to splice into the ACC line?)
    - Run ground to terminal 86
    - Run low-gauge wire from DC adapter (+) to terminal 87
    - Run low-gauge wire from DC adapter (-) to terminal 30
    - Terminal 87a is not used

    If this is the case, you guys now understand why I preferred to just connect directly to the switched cig. lighter (I know I know it's dangerous)!

  10. #10
    Maximum Bitrate binary.h4x's Avatar
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    You could connect it to the switched cig. Worst case scenario, the fuse blows.
    2007 Honda Fit Sport 1.5L SOHC-VTEC

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