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Pnp transistor help

  1. Sep 7, 2010 #1
    Hi, I'm trying to set up a PNP switch that looks like the attached file. However, the only way I can have the load be completely off is if my 'chip output' is >>5v, only if it's = to Vs.

    I'm new to this, help would be appreciated.

    What should I look for when picking out a transistor so that I can control my load with 0-5V?

    Currently I'm using a 2N5400 and it's not doing the trick...
    EDIT - so i did a little reading and it looks like I can't do what I want to do. It looks like my only option is a relay if I want a high-side switch. Is this correct?

    I didnt want to do the relay because all I really need is 28V @ 300mA. A low-side switch is out of the question due to the specifics of my application.

    Attached Files:

    Last edited: Sep 7, 2010
  2. jcsd
  3. Sep 7, 2010 #2
    What is the chip's output voltage when the transistor is supposed to be off?
  4. Sep 7, 2010 #3
    5V is all I have to play with. I need 5V to control 28V
  5. Sep 7, 2010 #4
    1 Are you saying the emitter of the transistor is at 28 V?

    2 Is it possible to invert your logic, turning on the transistor when the chip's output is high instead of low?
  6. Sep 7, 2010 #5
    1 - yes, the emitter is at 28V. The base is at 5V.

    2 - Unfortunately not, because of the specifics of my application. I can make it work that way, with an NPN transistor instead of a PNP, however because of what my switch looks like I can't "switch ground" in the full application. I can get it to work for one switch, but not for all 3. That's why I want a "high side switch" versus the "low side switch" you're implying.

    In summary, with an NPN I can get my 5V to "control" my 28V no problem. With the PNP, no dice. Looks like I need a relay.
  7. Sep 7, 2010 #6
    Attached is a circuit that should work although some of the values may have to be tweaked. Also I did not optimize the values.

    The NPN is a level shifter that amplifies the output voltage of the chip to the level needed by the PNP. The two circuits show the voltages for both high and low output voltages of the chip.

    I hope this helps.

    Attached Files:

  8. Sep 7, 2010 #7
    This is really cool because I can build it in house, without ordering more hardware. Thanks a lot for the CKT I'm gonna go give this a shot.
  9. Sep 7, 2010 #8


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    Staff: Mentor

    Nice circuit, skeptic. Why did you show an inductor in the load, though? Was it left over from a previous application, or did the OP say that there is a inductor in his load (I could have missed it)?

    The reason I ask is because it looks at first glance like there could be an inductive kick-back (down) issue potentially at turn-off. Did you simulate what the transient looks like?

    EDIT -- Oh, I now see an implied protection diode in the OP figure. Sorry I missed that earlier.
  10. Sep 7, 2010 #9
    I saw the relay in the diagram but the OP never mentioned he was using the circuit with a relay. Originally I just had the resistor but then I wondered if he was planning to use it with a relay, he might be confused with just a resistor. I probably should have added the diode even though the transistor he's using is a high voltage one.

  11. Sep 7, 2010 #10


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    Staff: Mentor

    Ah, got it. I saw the 40V 2907 in your simulation circuit, and missed the higher voltage 5400 in the OP. :blushing:
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