help selecting General purpose PNP transistor

Discussion in 'The Projects Forum' started by markosillypig, Jan 4, 2013.

  1. markosillypig

    Thread Starter Active Member

    Jul 21, 2008
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  2. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    2N3906 should work fine.

    I think I bought the parts for that thermostat once, but I never built it. I wonder if I still have the TL431.

    ETA: The 2N3906 does not have the ECB output; it is EBC, so you will have to adjust the PCB layout.
     
    Last edited: Jan 4, 2013
  3. markosillypig

    Thread Starter Active Member

    Jul 21, 2008
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    sorry to be a pain but could some one tel me why the TL431 and not a fixed volt reg
     
  4. wayneh

    Expert

    Sep 9, 2010
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    It'll be more accurate, I think, than a 7805 for instance. But that works, too. You could use a 7812 for the power supply and the reference voltage.

    I've seen mixed opinions on this, but I believe the LM35 needs a load on its output to stabilize it. I believe the datasheets recommend a 200Ω resistor.
     
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  5. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    I think the reference voltage must be adjustable, which can be done with an LM78xx regulator, but is easier and more accurate with the TL431.
     
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  6. DickCappels

    Moderator

    Aug 21, 2008
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    The TL431 is set up as a 1.62 volt fixed reference. They probably used it instead of a fixed regulator because it is very stable (50 ppm/°C) and can also be very accurate (1%).
     
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