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Bench Power Supply from PC PS

Discussion in 'Electronic Basics' started by Grostle News, Jun 14, 2008.

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  1. Grostle News

    Grostle News Guest

    Ongoing threads about Power Supplies has inspired me to copy and post

    The following is from the FAQ, electronics repair group, I think....

    4.16) Making a bench power supply from a PC power supply

    The power supply from a long obsolete PC can be the basis for a low cost
    unit useful for a variety of design and troubleshooting applications.
    The typical 200 W PC power supply will provide +5 V at 20 A, +12 at 8 A,
    and low current -5 V and -12 V outputs.

    However, these are not that well filtered - at least not where low noise
    analog circuits are concerned. They are fine for digital and power
    circuits as is. For analog work, additional post regulation (e.g.,
    LM317s) and filtering may be needed.

    * Typical (but not always) color codes for PC power supplies:
    Red: +5, Yellow: +12, Black: Gnd (Probably case as well).
    White: -5, Blue: -12, Orange: Power_good (output).
    (Some newer supplies may have a +3.3 output as well which may be

    * PC power supplies (as well as most other switchers) need a minimum
    load on +5 and possibly on +12 as well. An amp (e.g., 5 ohms on +5)
    should be enough.

    I use an old dual beam auto headlight. It adds a touch of class as
    well to an otherwise totally boring setup :). You can also use auto
    tail light bulbs or suitable power resistors or old disk drives you
    don't really care about (you know, those boat anchors).

    * There are no sense lines. There is a 'Power_Good' line which is an
    output from the power supply to the mainboard and can be ignored unless
    you want to connect it to an indicator to let you know all the outputs
    are within specs (it may need a pullup and I don't know its drive

    * Pinout for the standard PC and clone connector (some companies like
    Compaq do NOT use this type of connector, however.). Black (Gnd) wires
    together for the P8 and P9 connectors when installed to mainboard.
    J8: Pin 1 = Power_Good J9: Pin 1 = Gnd
    Pin 2 = +5 Pin 2 = Gnd
    Pin 3 = +12 Pin 3 = -5
    Pin 4 = -12 Pin 4 = +5
    Pin 5 = Gnd Pin 5 = +5
    Pin 6 = Gnd Pin 6 = +5
    Note: for an XT only, J8-Pin 1 is Gnd, J8-Pin 2 is no connect.

    * The peripheral connectors are: Pin 1: +12, Pin 2 and 3: Gnd, Pin 4 =
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