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Measuring hfe of "P2N2222A" varies wildly
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TheBay:
Lenovo and Dell use jellybean transistors as thermal sensors on their desktop PC's
SeanB:

--- Quote from: TheBay on November 10, 2021, 06:38:23 pm ---Lenovo and Dell use jellybean transistors as thermal sensors on their desktop PC's

--- End quote ---

Yes, the physics does not change at all for the part being cheap. Going to be accurate to under 5C straight out the box with only the reference voltage of the ADC chip, good enough for a PC, and with a little bit of correction per batch you can get to 1C easily.
Zero999:

--- Quote from: SeanB on November 11, 2021, 06:44:14 am ---
--- Quote from: TheBay on November 10, 2021, 06:38:23 pm ---Lenovo and Dell use jellybean transistors as thermal sensors on their desktop PC's

--- End quote ---

Yes, the physics does not change at all for the part being cheap. Going to be accurate to under 5C straight out the box with only the reference voltage of the ADC chip, good enough for a PC, and with a little bit of correction per batch you can get to 1C easily.

--- End quote ---
It's got nothing to do with the hFE though.
https://www.sensortips.com/featured/get-temperature-sensor-transistor/
David Hess:

--- Quote from: SeanB on November 11, 2021, 06:44:14 am ---Yes, the physics does not change at all for the part being cheap. Going to be accurate to under 5C straight out the box with only the reference voltage of the ADC chip, good enough for a PC, and with a little bit of correction per batch you can get to 1C easily.
--- End quote ---

A good delta Vbe temperature measurement circuit can achieve better than 1C with even a cheap transistor and no calibration.  Accuracy is primarily limited by the accuracy of the current ratio used for measurement, and series resistance including base spreading resistance.  The error contribution from series resistance can be removed with a Vbe temperature measurement using three currents but this is rarely done because keeping the measurement current low has the same effect.
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