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What does the missing thermistor do? (PC power supply)

Discussion in 'Electronic Basics' started by larry moe 'n curly, Jan 8, 2006.

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  1. I have some 350W Fortron-Sparkle model FSP-350BU PSUs with empty spots
    where a thermistor and fixed resistor would normally be connected in
    series, and I'd like to know their purpose. I think that they connect
    this way:

    http://static.flickr.com/39/83308555_59e5e557a6_o.jpg

    Capacitor C10 and pin 18 are on the small vertical circuit board
    (beneath the removed fan controller board in the photo below) housing
    the Fairchild KA3511DS PSU controller chip, and according to the
    datasheet at

    www.ortodoxism.ro/datasheets/fairchild/KA3511.pdf

    pin 18 is the Extra Protection (EP) input, which, when its voltage goes
    high enough, is supposed to shut down the PSU. In other words, this
    seems to rule out the thermistor being used as a overheat sensor
    because the pin 18 is low anyway in normal operation.

    Here are where the missing resistor and thermistor are located (circled
    in red):

    http://static.flickr.com/39/83310554_91d773cf62.jpg

    This thermistor is not for fan speed control -- that's handled by
    another thermistor (next to the screw on the heatsink, higher up in the
    picture).
     
  2. Franc Zabkar

    Franc Zabkar Guest

    I suspect the PT (extra protection) input may be a current sensing
    input. I don't understand the purpose of the thermistor, though,
    unless it is there to derate the PSU at higher temperatures.

    - Franc Zabkar
     
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