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[The_Almighty_Bacon_Lord]

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[David Hess]

Temperature measurement using a PN junction is usually done using the delta Vbe method where the difference in voltage with two different currents which have a fixed ratio is measured.  So what you want is a fixed change in voltage at the two currents.  We do not know the ratio of currents or absolute values used but my guess is that a low valued resistor, about 60 ohms, will yield about a fixed room temperature result.  Adjust the resistance value to get the temperature you want.

I came up with 60 ohms from:

60mV delta-Vbe / decade of current at room temperature.
Typically 1mA is used for the high current and a 10:1 ratio is used.
60mV / 1mA is 60 ohms; I ignored the actual ratio since you will need to adjust the value anyway.

[David Hess]

I understood pretty much none of that (because I'm a beginner), but honestly, thank you so much for the very detailed response!

So you think 60ohm's should work fine to "fake" an NPN transistor being installed in the system for temp monitoring. Would the 60ohm resistor be hooked up between base and emitter, leaving collector disconnected?

The 60 ohm resistor replaces the NPN transistor but you can connect it across the PN junction for the same results.  With the 60 ohm resistor in place, the PN junction will not conduct anyway.

60 ohms is just an estimated value since we do not know the two currents used for the measurement.

[Brumby]

Is one of those 3 legs of the transistor not connected to anything?

I mean, it may be soldered to a pad - but does that pad go anywhere?

[SL4P]

Can I ask why you want to fool the PC ?
Motherboard monitoring usually fulfills a meaningful purpose.

[Wimberleytech]

Here is an article that explains what Hess is describing: http://www.analog.com/media/en/analog-dialogue/volume-33/number-1/articles/measuring-computer-chip-temps-with-speed-and-accuracy.pdf

If you want to replace the 3904 with a resistor, look to see what chip is processing the deltaVBE and see what currents are being applied.  Then calculate the resistor value. The ADM1021A uses 205uA and 12uA thus 0.026*ln(205/12)=74mV ->> 74mV/(205u-12u) = 382 ohms

[David Hess]

"The 60 ohm resistor replaces the NPN transistor but you can connect it across the PN junction for the same results."

But... a resistor has 2 legs, and a transistor has 3... I'm confused

Usually just one of the junctions of the transistor is used and the other is shorted.  Offhand the only company I know of who used all three leads in a slightly different delta Vbe thermometer circuit is Tektronix.

Examining the layout or making measurements should reveal which pair of leads is shorted.  Or just mount the resistor; you have a 2 out of 3 chance to pick the correct pair and connecting it wrong isn't going to damage anything.