how PNP Transistors and batteries work

Discussion in 'General Electronics Chat' started by jordanwb, Jun 25, 2008.

  1. jordanwb

    Thread Starter Active Member

    Jun 22, 2008
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    Hello again. I have two questions.

    1: I understand the operation of NPN transistors, but how do PNP work? Is it if power is flowing away from the base of the transistor, it allows power to flow from the collector to the emitter? I found this page, I understood the part about PNPs being oppisite of NPN transistors, then nothing made any sense after that.

    2: This question is a bit of an addition to the dude's question in this thread. If a light is connected to a battery and the battery can produce 1 amp of power at 6 volts, and the light draws 0.5 amps of power; does 0.5 amps of current flow out of the battery or does 1 amp flow out and 0.5 amps flow back into the battery?
     
    Last edited: Jun 25, 2008
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    They're nearly exactly the same as NPN's, except complete opposites. Confusing initally, to be sure.

    When I was a teen, I was given a Norelco kit for building circuits. They were primarily PNP transistor circuits. I had a hard time accepting NPN transistors when I was later introduced to them. They seemed so upside-downish and improper from what I'd learned.

    Only 0.5A flows out of the battery. There is no "flowing out, and flowing in". In DC circuits, there is no reverse electron flow. If there were, it would be an AC circuit.
     
    Last edited: Jun 25, 2008
  3. beenthere

    Retired Moderator

    Apr 20, 2004
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    As another illustration, a PNP transistor can be substituted for an NPN in a simple circuit. Reverse the battery connections, and the circuit operation will be the same.
     
  4. jordanwb

    Thread Starter Active Member

    Jun 22, 2008
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    So for PNP if the base is connected to ground, it will allow power to travel from emitter to collector?
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    If the base is connected to ground then an extremely high current will flow in the base-emitter diode which will blow up the transistor. The base current must be limited by a series resistor. The base to emitter voltage of a turned on transistor is only about 0.7V.
     
  6. jordanwb

    Thread Starter Active Member

    Jun 22, 2008
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    Um what? (I added this so the message is at least 10 letters)
     
  7. beenthere

    Retired Moderator

    Apr 20, 2004
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    Any discussion of actual cases really needs a posted schematic to let everybody get on the same playing field.
     
  8. jordanwb

    Thread Starter Active Member

    Jun 22, 2008
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    So since there's no actual case there's no need for a schematic, right? Right. Moving on. So basically a PNP transistor lets current flow from the emitter to the collector if current is flowing away from the base to ground.
     
  9. Norfindel

    Active Member

    Mar 6, 2008
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    The emitter and base form a junction that works like a diode, if you have the emitter directly connected to the supply (+), and the base to ground (-), then you need a resistor there to limit the current, or it would be the same than putting a diode across the power supply.
    With a small current flowing thru that diode, then a larger current will flow from emitter (+) to colector (-). To remember it, think about the base-emitter diode forward-biased, and the collector with the opposite polarity than the emitter. Works for me :)
     
  10. jordanwb

    Thread Starter Active Member

    Jun 22, 2008
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    Yeah I learned that the hard way. Okay I understand how PNP transistors work now.
     
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