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Illogical Transistors

Discussion in 'General Electronics Discussion' started by Raven Luni, Jun 1, 2013.

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  1. Raven Luni

    Raven Luni

    798
    7
    Oct 15, 2011
    Greetings,

    I seem to have come across a pitfall when using PNP and NPN transistors fro switching. Base emmitter voltage drops pulling things the wrong way etc. I wont post any diagrams as theres an example in my pic programmer project thread.

    But I would like to see what the more experienced here have to say. Should we only use the same type as a rule or are there cases when using both is preferable?
     
  2. Raven Luni

    Raven Luni

    798
    7
    Oct 15, 2011
    Ok now I'm annoyed. WHY, when turning on an NPN transistor (5V at base through 1K resistor), does a voltage appear at the COLLECTOR, when its NOT CONNECTED TO ANYTHING??

    This was supposed to be a simple matter of logic - I mean WTF?!?!?!?!?!?!?!
     
  3. Raven Luni

    Raven Luni

    798
    7
    Oct 15, 2011
    Right - I think I get it - emitter was grounded via a 10k so really what you see through the collector is a potential divider?

    This stuff would put tits on a snake it really would!
     
  4. DuctDuck

    DuctDuck

    52
    0
    Jan 26, 2013
    I can help; I am very familiar with bjt type transistors...especially for those instances using a constant, known supply voltage.

    Try using a voltage divider as it pulls down the base terminal (single Vsupply, right?). When your Vsignal is active (i.e.- +5V applied) a base voltage is present (e.g.- Vb>(Vbe+Ve)) and when no Vsignal is present the base terminal rests at ground.

    Provided the base's Vdivider-input-branch isn't loaded by reflected Re branch, all should be OK. This removes the "current amplifier" nature of the bjt (well not really, but its a get around:)).
     
    Last edited: Jun 1, 2013
  5. Raven Luni

    Raven Luni

    798
    7
    Oct 15, 2011
    Thanks. I gave that a try and there is some improvement but not enough - cant get any lower than around 1V at the collector and thats still potentially enough to turn on a device. I just need to redesign the thing (again)
     
  6. DuctDuck

    DuctDuck

    52
    0
    Jan 26, 2013
    There is something amongst transistor switches known as high-side and low-side loading.

    I'm not sure if you want another transistor circuit suggestion...so, yeah, modify what you currently have (e.g.-double up bjts so Q2 is in cutoff or saturation by Q1, etc).
     
  7. Raven Luni

    Raven Luni

    798
    7
    Oct 15, 2011
    Sounds like it describes the nature of the problem. Its fixed now - changed to a PNP (so essentially loading the other side) which also meant having to invert the input with another transistor.
     
  8. glewis721

    glewis721

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    Apr 30, 2013
    or the transistor might be fryed
     
  9. BobK

    BobK

    7,674
    1,684
    Jan 5, 2010
    Because the base to the collector is a diode, as is the base to the emitter. The base is the anode of each in a NPN transistor. In fact that is where the NPN comes from!

    Bob
     
  10. Raven Luni

    Raven Luni

    798
    7
    Oct 15, 2011
    Aha. That makes more sense :p
     
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