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

[larry moe 'n curly]

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).

 

[Rick S.]

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?

 

[larry moe 'n curly]

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).

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?

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?

 

[larry moe 'n curly]

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).

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?

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.

 

[larry moe 'n curly]

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).

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?

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.

 

[Joel Kolstad]

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

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.

 

[Franc Zabkar]

Rick S. wrote:

I'm sure it doesn't because it connects through an adjustment for the
PSU's output voltage, ...

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

 

[larry moe 'n curly]

On 8 Jan 2006 15:27:24 -0800, "larry moe 'n curly"
<[email protected]> 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.

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.

 

[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.

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