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

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:

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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):

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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).

Reply to
larry moe 'n curly
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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?

Reply to
Rick S.

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.

Reply to
larry moe 'n curly

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.

Reply to
Joel Kolstad

On 8 Jan 2006 15:27:24 -0800, "larry moe 'n curly" put finger to keyboard and composed:

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

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Reply to
Franc Zabkar

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.

Reply to
larry moe 'n curly

On 9 Jan 2006 20:41:05 -0800, "larry moe 'n curly" put finger to keyboard and composed:

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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Reply to
Franc Zabkar

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