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2n2222 npn transistor voltage question

  1. Dec 22, 2012 #1
    So im working on a project and Im using a diagram I found on this forum but I want to alter it a little. The transistor on the right is a 2n2222 npn transistor. I want to change the 9v to 24v. Ive been looking up datasheets on it and I think it will do what I want but I wanted to get a second opinion just in case because Im still new. Will this output 24v is my first question? And will the transistor work properly with the increased voltage?

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  2. jcsd
  3. Dec 22, 2012 #2
    2N2222 can work with +24V, so that is safe. The transistor is serve as voltage follow that buffer the amplifier. The question is how much current you are planning to draw from the transistor. You increase from 6 to 24V, power dissipation on the transistor is going to be higher even drawing the same current. There is not enough info to answer that as I don't know your buzzer.
  4. Dec 22, 2012 #3
    sorry i forgot to add that in the first post. the buzzer only draws 5mA (its just one i had laying around).
  5. Dec 22, 2012 #4

    jim hardy

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    What about the 6v to opamp? Will that become 24V also?

    Look at R5-R4 voltage divider - it likely won't turn off with 24 volts to R5.
  6. Dec 22, 2012 #5
    I was only planning on changing the 9vdc to 24. does this mean I need to change up the R5 value?
  7. Dec 22, 2012 #6

    jim hardy

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    Are you at all familiar with circuits? It's okay if you're not, we all start out unfamiliar.

    No, it can't put out more than about the voltage to opamp and R5, which you're leaving at 6 volts.

    Do a quick search on "emitter follower" .
  8. Dec 23, 2012 #7
    Well I dont know very much at all, Im just in my first circuits class at WWU. Ok so i did a quick search for emitter follower and Im going to be honest, Im not sure what I was supposed to take away from my reading. Are you suggesting that I replace everything coming off the second pin of the comparitor with an emitter follower circuit?
  9. Dec 23, 2012 #8
    But why you want to change 9V to 24V?
    Also you already know that Q1 work as a emitter follower.
    So the buzzer voltage will be 0.6V lower the the base voltage.
    And this means that the increase collector voltage from 9V to 24V will not change buzzer voltage.
  10. Dec 23, 2012 #9
    I dont really care about the buzzer so much I just was thinking I could use this circuit as a very basic encoder. I came across an old PLC at a garage sale that Ive been playing with. I want the 24v because that is what the PLC accepts.
  11. Dec 23, 2012 #10

    jim hardy

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    Jony nailed the "emitter follower". That's what the circuit you posted is, an emitter follower.

    key point is - an emitter follower can't give you any voltage gain. You cant turn 6v at base into 24V at emitter with that circuit.

    I encourage experimenting.
    You can get your higher output by:
    1. change 9V to 24V;
    2. replace the buzzer with a short circuit
    3. replace that wire between C of transistor and +9(24?) volts with about 15k ohms.
    4. take output from C of transistor instead of E..

    discuss it with classmates and teacher...
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