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Newbie transistor question

Discussion in 'General Electronics Discussion' started by Thesecret20111, Dec 2, 2014.

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  1. Thesecret20111

    Thesecret20111

    18
    3
    Apr 11, 2014
    Hello guys!
    I've got this question thats been bugging me for a while now, however I've put off asking it as it probably just shows my low level of electronics knowledge.
    Anyways my question is: With a transistor, how is it possible to make it switch on and off by simply connecting the base to +v?
    I've always thought that current can only flow in a closed circuit, where there's a return path to the negative terminal?

    I'd appreciate any explanation of this.

    Thanks!
     
  2. davenn

    davenn Moderator

    13,671
    1,892
    Sep 5, 2009
    Yes, there has to be a return path. if you have a positive voltage from say a battery going to the base of an NPN transistor via a current limiting resistor,
    then the -V of the battery must also be connected ... it will go to the emitter

    so a small current flowing from base to emitter will open the collector emitter path for a larger current via that path

    untitled.png

    cheers
    Dave
     
  3. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,270
    Nov 28, 2011
    Hi there and welcome to Electronics Point :)

    You're right, current can only flow in a closed circuit.

    When you connect the base of an NPN transitor to the positive supply (through a resistor, to limit the current), current flows in a circuit from the positive side of the power source, through that resistor, into the base of the transistor, out the emitter and onto the 0V rail, and back to the power source.

    So in a common emitter circuit, i.e. an NPN with its emitter connected to the 0V rail, you feed current into the base (and it flows out the emitter) to make it conduct. It conducts current into its collector and out the emitter. So this second current path goes from the positive side of the power source, through the "load" (the thing that's connected between the positive supply and the collector - e.g. an LED (with current limiting resistor), the coil of a relay, etc), in the collector, through the transistor, out the emitter, through the 0V rail and back to the negative side of the power source.

    The collector current is controlled by the base current, and is a lot higher than the base current, so the circuit has gain, i.e. it's an amplifier.

    There are two other configurations that transistors (or to give them their full name, bipolar junction transistors, BJTs) can be used in: common collector (also called emitter follower), which is used often in buffering applications, and common base, which is used in certain special cases.

    This explanation uses "conventional" current, which flows from positive to negative. Current is actually the flow of electrons, which is in the opposite direction (but exactly the same path). Electron flow is important for explaining the internal operation of semiconductors, but for general explanations, conventional current is more widely used, because it fits with component symbols - current flows in the direction of the arrow in a diode, and in the emitter of a transistor.
     
    Thesecret20111 likes this.
  4. Thesecret20111

    Thesecret20111

    18
    3
    Apr 11, 2014
    Thanks so much for the quick replies!!!

    This completely makes sense now!
    I obviously need to start some experimenting with physical transistors to visualise how this would work in an actual circuit. - I'm excited :)

    There's just one more small question that I forgot to included in my original post, that is: How much current is the recommended amount for the base or does it change between transistors?
     
    Arouse1973 likes this.
  5. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,270
    Nov 28, 2011
    It depends how much collector current will be flowing, and whether the transistor is used as a "saturated switch". If you're turning the collector current ON and OFF, then the transistor IS being used as a saturated switch. In this case, have a look at these resources:

    1. https://www.electronicspoint.com/resources/using-a-bipolar-transistor-to-turn-a-load-on-and-off.30/
    2. https://www.electronicspoint.com/resources/saturation-in-transistors-bjts-why-and-how.28/
    3. https://www.electronicspoint.com/resources/how-a-bjt-transistor-works-base-current-version.37/
     
    Thesecret20111 likes this.
  6. Thesecret20111

    Thesecret20111

    18
    3
    Apr 11, 2014
    Thanks again for all the replies! Time for me to get an electronics kit! :)
     
    Arouse1973 and KrisBlueNZ like this.
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