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using PC psu as power source

Discussion in 'Electronic Basics' started by Andy C, May 18, 2007.

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  1. Andy C

    Andy C Guest

    hi there,
    i need a 12v regulated power source to run approx 10 CCTV cameras (total
    should be 3 or 4amps max) which use a dedicated 'CCTV' PC... is there any
    reason i can't use a 12volt output from the PC powersupply (inside the case)
    for this ?
    thanks !
  2. Pete D

    Pete D Guest

    Lots of Pc power supplies need to have some load on the 5V line too and
    the oututs are quite noisy
  3. Eeyore

    Eeyore Guest

    A PC's power supply is designed to provide much of its power on the 5V supply.
    If it's totally unconnected, there may be operational problems. Also, such power
    supplies aren't always happy with such a light loading. You'd need to

  4. Marra

    Marra Guest

    Thats easy to fix, put a resistor across the output and some caps to
    get rid of noise.
  5. Circa Fri, 18 May 2007 19:47:10 GMT recorded as
    <2pn3i.26833$> looks like "Andy C"
    I get the impression from your question that you intend to use the 12V from
    a functioning computer. If so, forget it. The power supplies are designed
    to run a motherboard and a few peripherals, and won't have enough reserve
    power to be used for this kind of external load. Others have touched on
    the load requirements if the PS is to be run independently.

    Stand-alone 12V 5A supplies are plentiful and inexpensive, and far less
  6. zack

    zack Guest

    50 to 100 ohms across the 5v works fine
  7. Eeyore

    Eeyore Guest

    A 50mA load ?

  8. Andy C

    Andy C Guest

    yes, that is exactly what i had planned - we have a dedicated PC to run the
    software for the CCTV cameras and i was hoping to run the cameras from one
    of the 12v supplies from this functioning pc...
    how so ? the computer PSU boasts 500watts - wouldn't 4amps @ 12volts be a
    puny external load ?
  9. Eeyore

    Eeyore Guest

    Most of these ratings are fairly ficticious.

  10. Guest

    Hi Andy
    I'm a totaly newbie here, but I would find out what uses 12V on this
    computer, and if it is things like floppy and CD drives (I'm too lazy
    to look it up :) and you are not using these and there is 50W of 12V
    power to spare, it might work. But your computer might run a little
    hotter (well PS, anyway) so I would be looking for devolution in this
    system, ie, having a separate PS for the cameras. Just my idle
    thoughts.... jack
  11. Guest

    How big, how many watts, how hot?
    How's he going to run his computer with this setup?

  12. Jasen

    Jasen Guest

    If the cameras need a negative supply that could be a problem.

    Check that with the camera running there's no voltage between the
    battery negative and the ground of the video output.

  13. Jasen

    Jasen Guest

    Recently there's more power provided on the 12V. Much of it is
    downcoverted to below 2V on the motherboard to run the CPU.

    there's not much in a modern PC that runs at 5v
    this one is going to be running a PC at the same time as the cameras
    so that shouldn't be a problem.

  14. Circa Sat, 19 May 2007 00:57:01 GMT recorded as
    <xXr3i.37104$> looks like "Andy C"
    That rating is for the entire supply. Most of that rating will be for the
    5V and 3.3V rails. You have to look at the specs for the individual
    outputs. Suppose the 12V output is rated at 15A. That seems like plenty,
    but have you spec'd and measured what the MB and peripherals are drawing?
    Suppose that total is typically 10A. Now you risk topping out the PS, and
    causing a voltage droop under full load.

    Furthermore, you will have no isolation on the 12V rail between the
    computer peripherals and the cameras you want to use. Noise and
    instability from any one device will affect all other devices on the rail.
    So you are faced with doing a complete load analysis on the computer and
    the cameras in order to find out whether or not this idea is feasible.

    All that instead of spending $25 on a 12V 5A dedicated switching supply.
    It just doesn't make sense, unless you are more interested in experimental
    engineering than creating a workable, stable system. What if something
    unpredictable happens with the bus you have routed external to the
    computer, and you wipe out your hard drive? To me, the cost-risk-benefit
    analysis is weighted very much in opposition to the shared bus idea.
  15. Circa Sat, 19 May 2007 00:59:50 GMT recorded as
    <a_r3i.37117$> looks like "Andy C"
    I'm not sure what the point of posting those links is, but what strikes me
    from reading the "Discussion:" is that there are a hell of a lot of people
    out there playing with electricity who don't know jack shit about it. You
    did well to post your question here, and I hope you take the advice you've
    been given. I see from other posts that my estimate of $25 for a separate
    supply was very much on the high side. That makes the decision even more
    of a slam-dunk, IMO.
  16. Andy C

    Andy C Guest

    yes, that makes good sense, thanks everyone - i will buy an independent
    supply for the cameras -
    i was just thinking it would be convenient if it was possible as they would
    power up and down with the PC, the cables are already routed to the PC and
    at face value the '500watt' PC PSU seemed ample-
    thanks again,
  17. Circa Sat, 19 May 2007 21:50:44 GMT recorded as
    <UiK3i.27213$> looks like "Andy C"
    Consider getting a switched power strip, and plug the computer, monitor and
    camera power supply into it. Make turning the strip on and off part of the
    routine of starting and stopping the computer. Good luck! :)
  18. zack

    zack Guest

    works for me for a 200 watt supply
    for no load testing.
    i power my ex modom, a hub and some
    fans, has run for years now.
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