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Using a PC PowerSupply as a bench power supply.

Discussion in 'Electronic Design' started by sid, Apr 24, 2008.

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  1. sid

    sid Guest

    I have the need to for a cheep 5/12v power supply and have a lot of
    out dated PC(s) that I can rob the supply from, I just had some
    questions about how to set it up.

    There are some articles on on-line that discuss the subject but don't
    really go into specifics like, if I need to draw current from the 5v
    circuit to get the PS to regulate correctly, specifically how much
    current do I need to draw ? Are we talking milli-Amps or Amps ?
    Also, the power good signal. I understand that that the power-good
    line must be held low, but these articles state just short it to one
    of the ground wires. Back when I had circuits we use a 470ohm
    resister to hold a logic signal down ? Right ?

    How do I use the power reset ? Can I just place a momentary contact
    switch in the PS case and use it as On/Off ?


  2. Tim

    Tim Guest

    Check here;

    - Tim -
  3. sid

    sid Guest

  4. w_tom

    w_tom Guest

    Not stated if these are AT power supplies (IBM PC or AT vintage) or
    ATX power supplies. Older supplies had a power switch attached to the
    supply. Newer ATX supplies are controlled by a power supply
    controller and front panel switch.

    This assumes ATX type supply. Power supply controller ordered the
    PSU on via the green wire. Then gray wire (Power OK) would tell the
    power supply controller that power supply was working. You don't care
    about the gray wire signal (unless you put an LED on it to see that
    message). But you need a non-momentary switch to connect green wire
    to any black wire (to reduce green wire voltage from above 2.0 volts
    to below 0.8 volts).

    Some power supplies need a minimum load. Others do not. Which
    ones? Only answer is provided by numerical spec sheets that came with
    the power supply. Since so many marginal supplies (missing required
    functions) are dumped into the market, then very unlikely anyone has
    that tech specs. In short, nobody can answer your question. So, if a
    minimum load is needed, well Radio Shack sells some 50 ohm 10 watt
    resistors. Put a resistor one of the red and yellow wires. That
    should guarantee a minimum load.
  5. sid

    sid Guest

    The article discussed minimum loads, but it I don't recall where it
    stated AT or ATX.
    You stated the gray (power good) wire could be ignored, but I thought
    that the board needed to put a signal back on this wire to turn the PS
    on for the rest of the system ?
    Is this not right ?
  6. sid

    sid Guest

    I guess what's confusing is you are talking Pins and the article
    denotes colors.
    The only place in the article that I found that permanently connects
    any two wires (and not using a resistor) is a call-out at the bottom
    of PIC 10 "Solder 3.3v Orange wires to Brown sense wire".

    I would think that the "Brown sense wire" is probably your Pin14 and
    should be soldered to one of the black ground wires ?
  7. w_tom

    w_tom Guest

    Power supply controller ordered the PSU on via the green wire. Then
    gray wire (Power OK) would tell the power supply controller that power
    supply was working. You don't care about the gray wire signal. But
    is using a power supply controller, then the controller does care
    about the gray wire voltage.

    Posted were wires defined by color. Color was defined in ATX specs;
    not in AT power supply specs. That posting assumed ATX wire colors.
    And then some ATX type supplies don't use industry standard wire
    colors. If in doubt, this graphic helps identify what each wire is
    by connector position:

    Every voltage has sense connection. Usually sensing is performed
    inside the power supply. However, some supplies may locate the sense
    connection via motherboard so as to compensate for wire voltage
    losses. IOW might use one 3.3 volt supply lines, instead, as a 3.3
    voltage monitoring wire. If that sense is implemented and not
    connected to other 3.3 volts (orange) wires, then the sense will
    instead monitor 3.3 volts via a diode inside power supply. This will
    cause the 3.3. volts to be maybe 0.5 volts higher than what the power
    supply believes it to be. IOW if a power supply needs a sense
    connection between "a brown and orange wire", then connecting all
    three 3.3 volt wires together (at twenty pin connector - see chart in
    previous citation) will cause the power supply to drop maybe 0.5 volts
    to be closer to 3.3 volts.

    Most supplies don't use a sense wire. Instead the sense wire is used
    as another source for 3.3 volt current as shown in that chart.

    Best is to connect all same voltage wires together at the load - to
    eliminate any unexpected variation - such as using one 3.3 volt wire
    to monitor 3.3 voltage.
  8. sid

    sid Guest

    Thanks for all the input !
    One last question, no where did I see the discussion of how much
    current can safely be drawn from each circuit.
    Is there no hard-set rules on this or just a fucntion of the size of
    your PS.

  9. w_tom

    w_tom Guest

    Power supply should list a maximum current for each voltage. A
    number that decreases when other voltages provide higher currents.
    How much for each voltage when all lines are loaded? Charts for this
    are found in the ATX power supply standards. Easier is to just use a
    larger wattage supply. Smarter is to look at those charts anyway to
    learn from the experience.

    How much power on each circuit - each wire? Bottleneck is defined
    by Molex connector specs. Typically 6 amps per wire - less if
    concerned for contact resistance, temperature, or environmental
    degradation - which would be irrelevant as a bench power supply.
  10. sid

    sid Guest

    If I look at the pinout at
    it shows "PIN 9 5VSB Purple +5 VDC Standby Voltage (max 10mA)",
    but if I look at
    page 56 it shows the rated current on +5VSB at 2A ?

    Am I reading this correctly ?

  11. sid

    sid Guest

    I went out to our warehouse last evening and dug the PS out of two old
    Compaq EVO (Convertible Mini/Desk top) units.
    For the first time I found units that have specs on the outside of the
    unit. Most of the small form factors that we have been using on
    desktops for a couple of years don't have specs on the PS. (Sorry if
    I was annoying anyone)

    The units are rated at 250W with several of the legs being redundant.
    +5v 11A
    +3.3v 17A
    +12v 5A
    -12v 0.15A
    +12v 7.5A
    +5.05v 1.7A Aux <- ( I am assuming this is the stand by power)
    +3.3v 2.2A Aux

    From reading this I have two 12v legs with 5A and 7.5A. Can I use
    those together to drive one circuit 12.5A ? Or should I keep them
    separate ? The only reason that I ask before about the current on the
    5v stand-by was because the one diagram show "max 10ma", and I wasn't
    sure if that was enough to drive an Led.
    Now that Compaq has been acquired by HP, I wouldn't even know where to
    look for specs on a unit like this.

  12. sid

    sid Guest

    My bad, the 12v legs are not the same. one is 12v at 5A and another
    12.8v at 7.5A.
  13. w_tom

    w_tom Guest

    Nobody can really answer that question without better understanding
    circuits for each voltage source. Typically, when two power supplies
    drive a common load, one supply with slightly higher voltage provides
    all current until the load wants too much current from that one
    source. Then that source goes into current foldback limiting; dumping
    the entire load on the other supply. Second supply also goes into
    current foldback limiting. Both supplies combined cannot provide
    enough current until power is recycled to reset that current foldback
    limiting condition (a safety feature).

    Some of the tricks to better make two power supplies work in
    parallel were to use bipolar transistors rather then Fets; put a small
    resistance power resistor on the output of each supply (voltage
    decreases slightly as current increases), or an interconnecting signal
    wire between two supplies so that one power supply voltage regulator
    will track the other.
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