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What's the missing thermistor for? (Fortron-Sparkle PC PSU)

Discussion in 'Electronic Repair' started by larry moe 'n curly, Jan 7, 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
    URL=http://www.ortodoxism.ro/datasheets/fairchild/KA3511.pdf]Data sheet
    for KA3511DS[/URL], pin 18 is the Extra Protection (EP) input, which is
    supposed to shut down the PSU if its voltage goes high. In other
    words, this seems to rule out the thermistor being used as an overheat
    sensor because 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

    The missing thermistor has nothing to do with controlling the speed of
    the fan, and there's another thermistor for that purpose anyway (on the
    heatsink, just above the screw).
     
  2. Rick S.

    Rick S. Guest

    Just a guess, but some FSP supplies have dual fans (front and
    rear, or bottom and rear). Maybe the "missing" thermistor and
    resistor and for a second fan?
     
  3. This thermistor has absolutely nothing to do with a second fan, and the
    PC board has no place to connect one (and I'm counting even the
    unlabelled holes). BTW, Antecs also have a second thermistor (not
    missing) that has nothing to do with fan control, and I can't figure
    out what it does either (shorting across it does nothing.

    Does anybody have any idea?
     
  4. I'm sure it doesn't because it connects through an adjustment for the
    PSU's output voltage, and the circuit board has no place to connect a
    second fan (and I'm counting even the unmarked empty holes). The small
    circuit board on the right side of the photo is the fan controller.

    Antecs also have a second thermistor (installed) that's unrelated to
    fan control, but nothing happened when I connected a variable resistor
    across it and took the resistance way down.
     
  5. I'm sure it doesn't because it connects through an adjustment for the
    PSU's output voltage, and the circuit board has no place to connect a
    second fan (and I'm counting even the unmarked empty holes). The small
    circuit board on the right side of the photo is the fan controller.

    Antecs also have a second thermistor (installed) that's unrelated to
    fan control, but nothing happened when I connected a variable resistor
    across it and took the resistance way down.
     
  6. Joel Kolstad

    Joel Kolstad Guest

    It looks like they're using that diode as their comparator for whether or not
    the output voltage is too high. Since the point at which the diode starts to
    conduct quite well (the knee) is a small but non-negligible function of
    temperature, I'd guess the missing thermistor is there to compensate the
    trigger point based on the diode's changing Vt.
     
  7. Franc Zabkar

    Franc Zabkar Guest

    Are you sure? The application circuit in the datasheet shows that the
    pot connects to a current transformer, suggesting that the PT input is
    used for current sensing.

    - Franc Zabkar
     
  8. I noticed that, but apparently in Fortron-Sparkle PSUs it's a voltage
    adjustment because other people have tweaked the voltages by turning
    that pot.
     
  9. Franc Zabkar

    Franc Zabkar Guest

    I can't see how that would be possible. AFAICS, any output voltage
    adjustment would need to be applied to the error amp at pin 4. Pin 18
    is just an extra OVP input.

    - Franc Zabkar
     
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