Modded ATX psu

Discussion in 'The Projects Forum' started by davelectronic, Sep 28, 2011.

  1. davelectronic

    Thread Starter New Member

    Dec 13, 2010
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    I know its been done so many times before, what i plan on doing is using an atx power supply to drive a 200 watt linear RF amplifier, the supply is 370 watts total output.

    Ive modded these supplys before, but not with a load as large as 200 watts, i plan the usual approach, a power resistor to the 5+ volt rail, normally 10 watts 10 ohms does the trick, on a couple of supplys ive had to go to 5 ohms at 10 watts, to get + 12 volts output.

    Can or has any one else loaded an atx power supply this heavy, i must state the power consumption will be on about a 50% duty cycle.

    Any ideas as to thoughts appreciated.
    Dave. :)
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Hmm, well - that's going to be a rather unbalanced supply, since the regulation is primarily from the 5v side. If you could re-wire the regulation scheme to go from the 12v supply rather than the 5v supply, you'd eliminate the need for the load on the 5v side, and achieve better regulation on 12v.

    Although ATX form factor supplies all have the same connectors and the basic same size box, the selection of internal components can vary considerably. However, you can start figuring it out by documenting the part numbers of all of the ICs that are on the board, researching them, and looking at application notes. It shouldn't take too long to figure out where it's getting the feedback to regulate the voltage output.

    Are you converting an ATXplus12 supply? Hope so; they're geared to output more at 12v.
     
  3. davelectronic

    Thread Starter New Member

    Dec 13, 2010
    4
    0
    Thanks for your insight, yes its a 12+ volts supply.
    Tested the supply today, and as you said best line and load regulation is on the 5+ volt rail, the 12+ volt rail works, but there is a drop of 0.23 volts with a 50 watt resistive load. Unloaded is 12.13+ volts, loaded was 11.90+ volts. So even heavier loads will probably show a bigger drop still, or see it as an overload, and just shut down.

    I will do as you suggest, look for the feed back, see if i can find some answers.
    Any way thanks again for your help.
    Dave. :)
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You're welcome. :) Look for parts with '431' in the number; they are frequently used as references. Sometimes they're TO-92 packages, sometimes they're DIPs. Also look for ICs with "SG" and/or "UC" prefixes; those are usually PWM controller ICs that will switch your MOSFETs on the primary side.
     
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