NPN to PNP circuit help

Discussion in 'General Electronics Chat' started by Jared Turner, Nov 30, 2014.

  1. Jared Turner

    Thread Starter New Member

    Nov 30, 2014
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    Hi

    Was wanting to convert three outputs from NPN to PNP. They are all 12vdc and have a common positive rail and negative output but I need them to have a common negative rail and positive output. They need to be at least 3 amp outputs so was looking at a BD650 To-126 PNP transistor but since I do not have any electronic background I am not sure what else I need in the way of resistors or how to wire it up. I don't think it is that hard but any help would be appreciated.

    Thanks
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    To get 3A, you will need a PNP Darlington transistor. It will be dissipating substantial power, so will have to be a TO220 on a heatsink.

    It would be much better to use a PFET. Lower disspation, so less heatsinking required.
     
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  3. Jared Turner

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    Nov 30, 2014
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    Sorry excuses my ignorance but what PFET would you recommend? and is the circuit diagram the same?
     
  4. GopherT

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    Hard to know, we haven't seen your circuit yet.
     
  5. Papabravo

    Expert

    Feb 24, 2006
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    I doubt the circuit wiring would be the same for a PNP transistor and any conceivable FET.
     
  6. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Pretty clearly several very experienced members think it is hard and with good reason.

    We (I) do not understand what you want to achieve so it is encumbent upon you to provide as much information as possible to help your cause.

    What is the purpose of this device?

    Do you have an existing unit to modify or do you intend to build from fresh using an existing circuit diagram, but changing the polarities?

    What do you mean by a 3amp output?
     
  7. MikeML

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    Here is the circuit reconfigured for a PFET (pmos). DigiKey sells this one, which is through-hole (not surface mount).
     
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  8. Jared Turner

    Thread Starter New Member

    Nov 30, 2014
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    Thank you very much Mike awesome work :)
     
  9. Jared Turner

    Thread Starter New Member

    Nov 30, 2014
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    Sorry a couple of more questions
    1: What wattage are the resistors?
    2: having a bit of trouble getting a 3.8 ohm for R4 what would it do if it was a 10 ohm?

    Thanks
     
  10. MikeML

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    1/4W for R2 and R3.

    You realize that R4 is your 3A load, whatever it is.... You know, R = e/i = 12/3 ~= 4Ω
     
  11. Jared Turner

    Thread Starter New Member

    Nov 30, 2014
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    You realize that R4 is your 3A load, whatever it is.... You know, R = e/i = 12/3 ~= 4Ω

    No but it makes more sense now :)
     
  12. MikeML

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    You started this thread asking for a PNP driver circuit that would drive a grounded load and source 3A @ 12V, which is close to 3.8Ω. To simulate (test) the operation of the circuit, I had to insert a 3.8Ω resistor to act like your unspecified load. The 3.8Ω resistor in the schematic is not part of the driver circuit; it is what the driver circuit drives... That is why I used a different symbol for the load resistor, but I guess I was too subtle...
     
  13. Jared Turner

    Thread Starter New Member

    Nov 30, 2014
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    Built and works awesomely even after I put the PFET around the wrong way :p Have no heat sinks on the PFET's but put the circuit board into a fan enclosure so should be good.
     
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