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Gross errors with TMP442A

Other Parts Discussed in Thread: TMP422

Hi Guys,

I’m using a TMP442A with a diode connected 2N3904 via about 3 inches of wire.

Most 2N3904 devices give me an accurate reading, but the occasional one reads 30 or even 70 degrees higher than it should.

These devices still respond to temperature changes, but with a gross offset. They measure OK on a DVM i.e. a silicon junction.


Any ideas / help / suggestions would be very welcome.

Thanks & regards,


  • Hi Trevor,

    The first question popped into my mind was, did you verify the good/bad readings on the same board? Did you try swapping the bad 2N3904 with a good one, and then put it back in just to make sure there is no soldering issue?


    Also, is the gross error repeatable?



    Apps-Sensing Products

  • Hi thanks.

    Yes, the 'bad' transistor can be swapped with a good one to get accurate results, then replaced and the error returns.

    I have two 'bad' 2N3904 - one reads 50 in an ambient of 22 and the other consistently reads about 90 in ambient.

    Both respond to being warmed up - the one that reads 50 in ambient will read 60 if warmed up by 10 degrees etc.

  • Hi Trevor,

    I certainly hope the bad 2N3904 is a rare occurrence.


    TMP422 measures temperature based on the ideal diode current-Vbe equation. Due to the nonlinear nature of this equation, a small deviation of Vbe will cause a relatively large error.


    Maybe as a result of process variations, these transistors have a different response at the current excitation levels. It may be worth to evaluate a replacement 2N3904 from a different vendor to see if you have better luck there.



    Apps-Sensing Products