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PC power supplies

Discussion in 'Electronic Design' started by Vesku, Oct 2, 2004.

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  1. Vesku

    Vesku Guest

    I bought couple of used PC power supplies to get 12 V supply. Table on
    supply says 8 A with +12 V. But when connect to mains, voltage only 10 V
    without load. When connecting to load that should take 2..5 A with 12-14
    V supply, voltage at +12V goes down to about 6 V and current probably
    only one amp or so (I did not measure)

    Manufacturer Zeck, model ZKS-620T, I didn't find spec fron web.

    Should there be some load on +5V as well to have +12 functioning
    properly and if yes how much ?

    Is it possible to connect two or three separate PC power supplies +5V
    outputt in series to get +10 or +15 V ?

    br Vesku
  2. The standard PC supply, does not regulate on the 12v. Instead the 5v rail
    controls the pulses fed to the transformer, and the load on the 12v rail
    is assumed to be moderately 'proportional' to the 5v load, and the voltage
    is therefore held within reasonable limits. With no load on the 5v rail,
    the supply will be feeding the smallest pulses i can manage into the
    transformer. Hence the behaviour you are seeing.
    Most require a reasonable load (perhaps an amp or more) on the 5v rail
    before the behaviour can be guaranteed. Often there is a tendency to
    oscillate with less load.
    On many supplies, it is possible to rewire them internally, so that the
    feedback connection is taken from the 12v rail (with a suitable resistive
    devider), instead of the 5v rail, which then gives a regulated 12v output.
    Yes you can generally 'series connect' a couple of 5v outputs.

    Best Wishes

    D MARSHALL Guest

    put a 5 watt 100ohm resistor on the 5v+
    to ground, i do that with all mine and work fine.

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