Making switching NPN with PNP

Discussion in 'General Electronics Chat' started by crazybuoy, Nov 17, 2010.

  1. crazybuoy

    Thread Starter Active Member

    Sep 19, 2010
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    There is a small circuit schematic attached where I am using voltage regulator 7812. I input 24VDC and 12VDC comes out from OUT pin. I want to make switch off & on NPN transistor with 12VDC buy using PNP transistor before NPN. And I will use PC printer port to provide the 1 & 0 pulse of 5VDC then how I should wire the circuit I want to just get the idea. Or is there any other way to do the same?
     
    Last edited: Nov 17, 2010
  2. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    You can't connect the base of a PNP directly to the printer port as its emitter is at 12V.

    One would need to use an extra NPN transistor for the voltage level translation, as shown.

    [​IMG]
     
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  3. crazybuoy

    Thread Starter Active Member

    Sep 19, 2010
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    That is really great. I want to use this stepper motor. It has four connections of coils and one common wire. And this bock of schematic will be used for each wire of coil. Please tell me the possible specification & values of components.
     
  4. crazybuoy

    Thread Starter Active Member

    Sep 19, 2010
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    Now I have edit the schematic and also set some values of components to energize the stepper motor's coil. Such the same components will be used with other three wires of motor as well. What do you think about my specification of components?
     
  5. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    Is your goal to drive the stepper motor via signal from the parallel port?

    The stepper motor you have chosen has a low winding resistance. It is meant to be used with series resistor to limit the idle(holding) current at 24V because the static current can go to about 2A and the motor will burnt.

    With the common of the stepper motor connected to +24V, you only need a single NPN transistor(darlington) to drive each phase of the motor coil. I don't understand why you would need to have a separate +12V voltage too.

    Anyway I'm not experienced in stepper motor drive circuit so I would invite others to comment.

    [​IMG]
     
  6. crazybuoy

    Thread Starter Active Member

    Sep 19, 2010
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    I have tried the following attached circuit in the same structure having fixing the components on a paper card. The real schematic is also attached. I am testing the switching of TIP42 by attaching + probe of voltmeter with collector of TIP142 and – probe of voltmeter with – of power supply. It showing always 24 volt on both switch on & off. I think at switch off, it should show zero volt at voltmeter. Please tell me where I am wiring wrong in paper card. And also tell me the method of testing switching process.
     
  7. Audioguru

    New Member

    Dec 20, 2007
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    896
    The A733 PNP transistor is probably burned out because the 1k resistor in series with its collector and the base of the TIP142 darlington transistor is missing.
     
  8. debjit625

    Well-Known Member

    Apr 17, 2010
    790
    186
    Use H-bridge circuit,they are best for driving stepper motors.Google for it you will find a lots of them

    Good Luck
     
  9. crazybuoy

    Thread Starter Active Member

    Sep 19, 2010
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    0
    I have try again it by changing the values of resistors in the following way as changed in attached circuit. I am testing the switching of TIP42 by attaching + probe of voltmeter with collector of TIP142 and – probe of voltmeter with emitter of TIP142. At 0 pulse from parallel port, voltmeter show 24VDC and at 1 pulse it show 12VDC. I want to make it show closer to zero volt at 1 pulse. So how to edit it to do so?
     
  10. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    I made a real schematic for you.

    If the collector and emitter of a transistor is connected backwards then the transistor will not work properly. The datasheets show which pin is which.

    Connect the (-) lead of the voltmeter to ground (0V).
    1) Connect the (+) lead of the voltmeter to the collector of the 2N3904 transistor. It should be near 0V when the input is 1 and should be +12V when the input is 0.
    2) Connect the (+) lead of the voltmeter to the collector of the 2SA733 transistor. It should be near +12V when the input is 1 and should be 0V when the input is 0.
    3) Connect the (+) lead of the voltmeter to the base of the TIP142 darlington. It should be about +1.0V when the input is 1 and should be 0V when the input is 0.
    4) The output at the collector of the TIP142 darlington should be +0.6V (it will be about +0.9V when the load draws 4A) when the input is 1 and should be +24V when the input is 0.

    These are continuous DC voltages. A voltmeter cannot measure the voltage of pulses.
     
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