What's the purpose of a transistor in that circuit?

Discussion in 'General Electronics Chat' started by electronewb, May 16, 2012.

  1. electronewb

    Thread Starter Member

    Apr 24, 2012
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  2. BSomer

    Member

    Dec 28, 2011
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    That circuit is just a really basic visualization of the operation of a transistor as a switch. The thing to remember is that the thing you are trying to turn on may draw more current or have a different voltage than the switch you are using. For instance if you are using a micro controller to turn on a motor, you need to use something to switch on the voltage to the motor because the micro controller outputs cannot handle the current that most typical motors require.
     
  3. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    If you disconnect the base of the transistor from +v in that diagram, the LED would not light. As BSomer said, it's just a basic way of showing how adjusting current on the base of the transistor can turn the LED on and off. For example, if you turn on the switch, but don't connect the base to +v, it won't work. You can see how only current on the base can control the LED.
     
  4. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    That would be a better example if the 680 Ohm were connected directly to the 9V supply instead of being switched.

    That way you would see that switching the base controls the Collector current.
     
  5. electronewb

    Thread Starter Member

    Apr 24, 2012
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    Awesome!! That makes sense now I know the purpose of a transistor!! Thanks guys
     
  6. electronewb

    Thread Starter Member

    Apr 24, 2012
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    So basically a transistor can amplify voltage? What would the output voltage of the transistor with 9Volt input?
     
  7. P-MONKE

    Member

    Mar 14, 2012
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    At least a gazillion, I reckon. ;)

    Seriously though, it can't magically 'produce' more voltage than is available in the circuit. They are used to either amplify voltages that are much smaller than the supply to a higher value, as in an audio amplifier, or as switches to turn stuff on and off, like in the circuit you posted.

    Here's a good place to start reading: Introduction to bipolar junction transistors
     
  8. electronewb

    Thread Starter Member

    Apr 24, 2012
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    What's the most common type of transistors? Something I could experiment with

    I have some that I've gathered


    c338 pn91
    2N3904 331
    2N3906 331
    IRFZ44
    IRF840
     
  9. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    I would say the most well-known transistor would be an NPN, and a PNP is right up there. The 2N3904 is NPN, the '3906 is basically the PNP version of the '3904. The IRFZ44 and IRF840 are MOSFETs. When in comes to transistors, you really have to match them up to your particular project. There is no "one size fits all". You'll have to look at the max voltage and current that your transistor can switch and make sure it's quite a bit higher than what you're actually planning on switching. If you're working with higher frequencies, you'll have to look at the switching speed of your transistor as well.

    The list you have above is good to start with, but I wouldn't stop there. I would recommend collecting as many as you can. The 2N3055 is another one you probably ought to have.

    Hope this helps :)

    Regards,
    Der Strom
     
  10. electronewb

    Thread Starter Member

    Apr 24, 2012
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    Cool Thanks!!!
     
  11. dataman19

    Member

    Dec 26, 2009
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    incidentally, the transistor can be controlled separately by breaking the lead from the transistor base to the 9V supply, then applying a logic input. When the logic is high, the LED will light, when low it won't...
    ..
    Transistor circuits similar to these are use in logic monitors (old school before the computerized logic monitors).
    ..
    Dave
    Phoenix, AZ
     
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