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NPN as PNP — Parallax Forums

NPN as PNP

ProcessingData...ProcessingData... Posts: 208
edited 2011-05-28 22:20 in General Discussion
Hello!
Does anyone know if I can use a NPN transistor as if it were a PNP?

I have searched my electronics components throughly, and found to my surprise that I own not a single PNP Transistor! (At least, not that I can find....)

I understand that an NPN can invert a Positive signal, and a PNP can invert a negative signal, but can you change up where you connect the Base, Collector and Emitter to inverse th normal function of either one?

(I would much prefer to use an NPN that I have, than to drive to Radioshack and pick up a Pack of 15 2N3096 PNP Transistors for $3.00. :tongue:

I am trying to build this circuit:
http://www.parallax.com/Portals/0/Downloads/docs/prod/prop/SerialtoPropeller.pdf

Comments

  • LeonLeon Posts: 7,620
    edited 2011-05-27 12:38
    You can't use an NPN in a circuit to replace a PNP without some extensive redesigning.

    Just buy some PNPs. I keep several on hand, of various types.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2011-05-27 12:41
    No, you really have to use a PNP where one is called for. In that circuit, the PNP switches in a positive voltage when TxD is low to overcome the negative voltage bias from DTR. An NPN in the same circuit would always be conducting, since the base would always be more than 0.6V above DTR's negative voltage.

    -Phil
  • ercoerco Posts: 20,260
    edited 2011-05-27 12:56
    "Reversing the polarity" could solve any problem in the TV shows Lost in Space, Superman, and the Brady Bunch, but sadly it rarely works in real life.

    What Leon & PhiPi hath decreed, I will not put asunder. They are positively negative about using NPNs as PNPs.
  • ProcessingData...ProcessingData... Posts: 208
    edited 2011-05-27 13:18
    Okay. Thanks!
  • localrogerlocalroger Posts: 3,452
    edited 2011-05-27 16:11
    Trust me, you would much rather drive to Radio Shack and spend $3 than do what you would have to do to adapt all circuits to a PNP-free world.
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2011-05-28 09:47
    i didn't know "Lost is Space" taught electronics.
  • kwinnkwinn Posts: 8,697
    edited 2011-05-28 09:55
    Loopy Byteloose wrote: »
    i didn't know "Lost is Space" taught electronics.

    Virtually every sci-fi show taught electronics, computers, and science in general. Unfortunately almost always 100% wrong!
  • ercoerco Posts: 20,260
    edited 2011-05-28 14:38
    Plenty to learn; there are flashing lights and blinking binary computer displays everywhere! http://www.youtube.com/watch?v=-mF6dCD8n2E&NR=1
  • davidsaundersdavidsaunders Posts: 1,559
    edited 2011-05-28 22:20
    If you really wish to slow things down you could pair two NPNs so there collectors are common acting as a base, and the two bases would act as the collector and emitter, though do not expect its response curve to match what you need.
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