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Run PC off 12VDC, efficiently

Discussion in 'Electronic Design' started by George R. Gonzalez, May 4, 2004.

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  1. I know just enough about PC power supplies to be dangerous...

    I'd like to run my PC in the back of my van, as efficiently as possible, so
    using a 12V to 120VAC inverter is out.

    My idea is, and please correct me if you see some major goof, is to do this:

    (1) Snip out the MOSFET or transistor that is switching the rectified mains
    voltage into the ferrite transformer.

    (2) Install a hefty MOSFET to do the same switching, only from the 12V car
    battery into the 5V secondary of the ferrite transformer.

    (3) snip off the 120V wires on the transformer and hook them to the 120VAC
    input wires (to power the voltage doubler, which powers the regulator chip).


    The idea is to turn this circuit semi-inside-out, so we're using the 5v
    winding as a primary and source for 5V, then the transformer works
    backwardfs, powering the 12V and 120VAC lines.

    The transformer should be running cooler, as it isnt loaded with the major
    load (5V at many amps).

    Sounds a bit crazy, but maybe has a chance of working?
  2. Ian Stirling

    Ian Stirling Guest

    You can purchase these.
    The most efficient way, probably 200 or 300% efficient is to swap the PC
    for a laptop.
  3. Tim Wescott

    Tim Wescott Guest

    You mean there's truth to that other thread titled "330% Efficient"?
    Can we hook the output shaft of the laptop to the -- oh, right.
  4. mike

    mike Guest

    If you're using a CRT monitor, the loss of efficiency in that
    is probably worse than the computer.

    I recall a thread about a commercial PC power supply replacement with
    12V input. Google is your friend.

    Return address is VALID.
    Bunch of stuff For Sale and Wanted at the link below.
    Toshiba & Compaq LiIon Batteries, Test Equipment
    Honda CB-125S $800 in PDX
    Yaesu FTV901R Transverter, 30pS pulser
    Tektronix Concept Books, spot welding head...
  5. Or even better, use a 12v only embedded PC. Cheaper too.
  6. Ian Stirling

    Ian Stirling Guest

    For some apps, a laptop with a broken screen from ebay can be a
    really, really good match.
  7. N. Thornton

    N. Thornton Guest

    I'm doubtful about your original scheme, seems to me there are a
    number of pitfalls, such as: starting up, the fact that the 120ac wont
    be converted to dc to run the SMPS chip, current ratings, but probably
    the biggest of all is this:

    Auto 12v supplies carry transients of upto 60v.

    And there is also the fact that such 12v sources are really 10-15 v.

    Regards, NT
  8. Yeah, me to.

    However, there are sites on the web with details how to convert a PC
    power supply to run from an auto 12v source instead of 120vac. So
    it can be done.

    | Sylvan Butler | Not speaking for Hewlett-Packard | |
    | Watch out for my e-mail address. Thank UCE. >>>> change ^ to @ <<<< |
    It may be better to live under robber barons than under omnipotent moral
    busybodies. The robber baron's cruelty may sometimes sleep, his
    cupidity may at some point be satiated; but those who torment us for our
    own good will torment us without end, for they do so with the approval
    of their consciences. -- C. S. Lewis
  9. Ian Stirling

    Ian Stirling Guest

    There are also sites where you can purchase a power supply that will
    run off 12V, for not much money.
  10. There are several approaches to 12V-PC running.

    You need to consider if it is running in a car re voltage
    o Cars can spike very high even in normal operation
    o Particularly so on intermittent bad ground (battery = capacitor)

    Two basic 12V PC PSUs exist
    o Traditional ATX SMPS -- switch the 12V up to HV AC & back down
    ---- pro -- they can do 350W, anything you want
    ---- con -- they are often (very) expensive
    o Laptop brick fed DC-to-DC convertor board -- convert 14-24V to 12/5/3.3V
    ---- pro -- they are cheaper, compact
    ---- con -- some can't power an ATX board, despite highish rating

    The DC-to-DC convertor boards...
    o Good efficiency 75-80-85%
    ---- you might get a bit better with pricey true industrial solutions
    ---- some PC104 solutions might give higher reliability if critical
    o 55, 60, 70, 80, 110, 150, 170, 190W ratings
    ---- subject to the particular laptop brick you use
    o Fanless, at least for the lower wattage items
    ---- some of the high wattage laptop bricks do have a fan
    o Some can power ATX boards
    ---- however a P4-Prescott might be pushing it
    ---- if you need P4 in a car/12V-battery, the P-M is available

    If it's a car application, remember shock figures are often for 1ms and
    tight temperature environment re hard-drives (even 2.5"), so shockmount.

    You can go down to <5W PC/104 boards, if needed for say 12V battery
    life or use Mini-ITX cheapies, or forthcoming Nano-ITX which move the
    consumer market closer to the embedded PC style at a lower price point.
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