How to power up a 478 motherboard off 12V

Discussion in 'The Projects Forum' started by Batista230, Jan 7, 2010.

  1. Batista230

    Thread Starter Member

    Feb 11, 2009
    Hello everone

    Not sure im on the right thread hear but basically im moving into a caravan soon!
    And I only have 12V DC and gas
    And my motherbords for PCs have 478 pin pentium 4 processors
    Which has a 20 and a 4 pin power supply

    I have been told that the 4 pin is 12V but im not sure
    Basically I need to find out the voltage for every pin so I can wire it from a 12v power supply with resistors

    Or if poss do some wiring inside the power box/ transformer that would allow a 12V power supply in; bypassing the AC/DC transformer so I could leave most of the circuity in tact
    But I would not know wear to stat on this one

    BTW I have the CD-rom hardrive and A-drive to power as well
    I have been told but i'm unsure its 12V and 5V
    and then there is the monitor????

  2. thyristor

    Active Member

    Dec 27, 2009
    Wouldn't it be easier to buy a small inverter (600W or so) and run the PC from that? It would be marginally less efficient by about an additional 10% but a lot less hassle.

    You don't need a pure sine wave inverter, a modified (quasi) sinewave version will do and you can probably pick that up for less than $100.
  3. SgtWookie


    Jul 17, 2007
    A basic 250W ATX form factor supply has more than +12v output.
    +5v, +3.3v, -5v, -12v are there as well.

    The easiest solution is to use a 12VDC-120VAC inverter. You're going to need one for the monitor anyway. Keep in mind that inverters aren't 100% efficient. You won't be able to use it unless your engine is running, or you'll quickly wind up with a dead battery.

    If you want to be able to operate the computer without the engine running, you will need a separate battery; preferably deep-cycle.
  4. S_lannan

    Active Member

    Jun 20, 2007
    if you are running off batteries i'd ditch the power hungry p4 and go for one of those mini itx motherboards with intel atom processors.

    Heres a site that sells 12vdc fed computer power supplies suitable for them
  5. BMorse

    AAC Fanatic!

    Sep 26, 2009
  6. Tahmid

    Active Member

    Jul 2, 2008
    Or you can get a Celeron 420/430. They are rated at 35W.
  7. Batista230

    Thread Starter Member

    Feb 11, 2009
    Im not keen on one of those power transformers that you can buy from Halfords as they bur too much electrisety

    I have looked at the side of a 24 pin power supply box from a pentium 3 PC

    and wire colors are as follows

    Orn +3.5 V
    pur +5 Vsb
    red + 5 V
    Wht - 5 V
    yel + 12 V
    blu - 12 V
    gry pg signal
    blk earth

    I hope the wiring colors are standard for all PCs across the world as my 20 and 4 pin power supply dose not have a color code
    And I dont have laptops and dont understand how to repair them and dont like there slowness; I think most of them are 19V so there would need to be some wiring done there

    I dont understand what the Vsb stands for
    and what is a pg signal
    I have only ever wired up 12V cars and plant so I dont know the diference between +12V and -12V
    Do you wire it the other way round??? earth to load to positive
    And all the earth wires are black can you wire dirrerant voltages all back to the same voltage of earth i.e. 12V

  8. Wendy


    Mar 24, 2008
    You will have to create all those voltages, which is going to be inefficient no matter how its done. Either you're going to have to make some very fancy circuitry, which I doubt you're up to, or get a second car battery and charge it separately to create -12VDC. You need two batteries, or some custom designed regulators that aren't simple to make.

    A 12VDC/120VAC inverter is the way to go. Computers and their power supplies aren't as simple as you think they are, they need all those odd voltages, and the power supply unit is meant to provide them, all of them. Modern electronics is pretty efficient (not perfect), so with a little research you can do pretty well.

    If this is completely unacceptable, that then you'll have to go with a laptop. The reason laptops are slower as a class it they don't have the power supplies and associated support circuitry. You can get fast laptops, it just costs money.
    Last edited: Jan 9, 2010
  9. SgtWookie


    Jul 17, 2007
    Trying to power a standard PC that has a 250W power supply from a 12V battery will take 21A even if the power conversion is 100% efficient.

    No conversion is 100% efficient.

    If you had a 210AH battery, you would have perhaps 4 hours of use before the battery was well over 50% discharged, which is a deep discharge. You might get 250 charge/discharge cycles out of such a battery before it was dead.

    This is not considering the additional power that the monitor will require.
  10. Wendy


    Mar 24, 2008
    Maybe a marine battery, which is designed for deep discharge, is in order?
  11. SgtWookie


    Jul 17, 2007
    Yes, I mentioned a deep-cycle in my 1st reply. However, if he's going to have it inside the caravan (trailer in UK English) it'll have to be a sealed lead-acid type; a marine battery is vented and therefore not suitable for use indoors.
  12. Melchior

    New Member

    Feb 2, 2008
    A Common myth is that a Desktop PC automatically uses 350 Watts of power.

    Not true!

    Even a (Horribly power hungry) Pentium 4 could run off a efficient DC-DC ATX power supply.

    Problem is I've never seen these bigger than 250W.

    An automotive DC ATX PSU is what you are looking for:
  13. 3ldon

    Active Member

    Jan 9, 2010

    Few computers consume 250 watts.
    If you insist on using the existing board, measure its power consumption on each dc bus, then pick a supply rated +20%, and make sure to get a few hundred joules worth of 18 volt MOVS to protect it.
    There is a full line of "picoPSU-120" and they were designed for car-puters.
    there are other sellers as well
  14. Batista230

    Thread Starter Member

    Feb 11, 2009
    We have no shortage of 12V batteries
    I just thought electricity going through a 12 V DC - 240 V AC transformer and then into a 240 V AC - multiply DC was very very inefficient
    and have a direct 12V to resistors would save a lot of power

    So what about the monitors
    Can they be easily changed to 12V