Is there a Transistor Switch where it is a closed circuit when off?

Discussion in 'General Electronics Chat' started by x1222, Dec 19, 2012.

  1. x1222

    Thread Starter Member

    Oct 22, 2011
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    For example, A NPN BJT switch is open when the base collector voltage is too low, but is there an transistor where the switch is closed when the transistor is off and open when voltage is high? Thanks.
     
  2. tshuck

    Well-Known Member

    Oct 18, 2012
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    PNP?


    -paddding-
     
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  3. nomurphy

    AAC Fanatic!

    Aug 8, 2005
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    It is possible to use a PNP as a voltage follower, where the xstr is on when the base is pulled low, and off when pushed high (must be equal to voltage on collector).
     
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  4. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    I agree with NoMurphy, just be careful if you are controlling the PNP with a microcontroller. The output pins of the uController might not reach a high enough voltage to turn the PNP off completely. Most microcontrollers output will be about 4.2 to 4.5 volts (roughtly) where the PNP will just be turning on. If that is the case, post a followup if needed.
     
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  5. tindel

    Active Member

    Sep 16, 2012
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    Yes, google JFET
     
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  6. WBahn

    Moderator

    Mar 31, 2012
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    You might look at depletion mode MOSFETs. Depending on exactly what you are doing, a PFET might also work.
     
  7. vk6zgo

    Active Member

    Jul 21, 2012
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    Use two transistors!
     
  8. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    NJFETs also are on when Vgs=0.
     
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