Computer PSU wattage vs car subwoofer?

Discussion in 'General Electronics Chat' started by DMahalko, Jan 4, 2016.

  1. DMahalko

    Thread Starter Active Member

    Oct 5, 2008
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    I want to use a computer PSU as a temporary indoor power supply for a car amp / subwoofer.

    It seems pretty well ironed out how the technology should be selected. The power supply specifically needs to have a single huge 12v rail, that can be accessed by collecting all +12v yellow wires and all black grounds into two common + / - connections.

    Fortunately it appears this single rail design has now become a commonplace specification for computer power supplies. For example NewEgg currently says there are over 400 PSU's with a single 12v rail.

    However the next question is how closely or loosely the PSU wattage rating should match the car amplifier wattage. I know if the instantaneous power draw is too high in a single spike, the PSU will treat it as a short circuit, crowbar itself, and shut down / reset. Oversizing the PSU may be necessary to deal with sudden high power speaker thumps that are within the normal operating limits of the amplifier..

    Also a PSU is normally designed to supply more than just +12v, and +5v takes up a large amount of the PSU's wattage capacity. If the PSU wattage rating is the sum total of those voltage values, then a 500 watt PSU will be insufficient for a 500 watt car amplifier, and it may need to be perhaps 50% larger or 750 watts..

    Does anyone know about these details?

    (I would prefer to just use the PSU directly with a car amplifier, without having to fool with 0.5F capacitors, capacitor charge limiting, and bleeder resistors.)
     
  2. wayneh

    Expert

    Sep 9, 2010
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    3,027
    I don't have much advice but I know one thing: The PSU has to be sized to match the actual wattage in use, not the max rating of the amp. Obviously performance would be limited by the weakest link, but getting 300W would produce a heckuva lot of sound. If you actually run at, say, 50W, then you don't need to design for 500.
     
  3. Papabravo

    Expert

    Feb 24, 2006
    10,136
    1,786
    A computer PSU will favor the +5V rail for most of its power capability. The +12V rail is unlikely to do what you want it to, and it won't even be close.
     
  4. DMahalko

    Thread Starter Active Member

    Oct 5, 2008
    174
    14
    Well one issue is that the amp likely already contains large filter capacitors for power buffering, and simply turning on a small wattage power supply may draw so much to charge those capacitors that the power supply may treat it as a short circuit, even if in actual operation the speakers don't run at anywhere near their max rated power level.

    I do not know if car amplifiers are capable of "soft start" to not surge in the current at powerup.

    Also another side topic is that there are usually two power connections, one for the battery, and a second tiny control wire to actually turn on the amplifier so it doesn't drain the battery with the ignition turned off. It seems possible that a brief delay between connecting main and control power of just a few seconds with a 555 timer and relay, may help get an undersized computer power supply through the inrush current startup sequence. A big PSU may however just power through the inrush without need for a stepped powerup delay.


    In this particular case it's an 800 watt amp driving two 400w RMS 12-inch woofers in stereo channels. I expect I am probably going to have to go with a minimum 1000w power supply, though which fortunately can be had nearly dirt cheap as some off-brand:

    http://www.newegg.com/Product/Product.aspx?Item=N82E16817517012

    12v @ 83A = 996 watts (.. in theory anyway, since a car amp is really built to expect 13.6vDC,.or higher when charging.)
     
  5. SLK001

    Well-Known Member

    Nov 29, 2011
    810
    224
    The capacitors in a computer power supply don't provide the filtering that you will probably need. Most of the large caps are for filtering the rectified line voltage. You might have to add extra on the outputs. The 12V that the PS outputs can be changed to 13.6V with a simple voltage divider change.
     
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