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pnp switch problem

Discussion in 'General Electronics Discussion' started by KUMARA SHP, Feb 1, 2015.

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  1. KUMARA SHP

    KUMARA SHP

    122
    4
    Aug 1, 2014
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    here is my design. it is use to power up 12v common anode display. It takes 300mA when fully lit. I need to confirm that this design is good for that.
    what do you recommend?
    this one or another circuit?
     
  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,270
    Nov 28, 2011
    That idea will work, but Q2 will need more than 1 mA base current.

    The 2N4403 (the Fairchild one, anyway) is specified with a minimum hFE (current gain) of 100 at 150 mA collector current, and only 20 at 500 mA collector current.

    2N4403 hFE specifications.png

    The typical characteristics graph shows that typically, the gain at IC = 350 mA is about 70% of the gain at IC = 150 mA.
    2N4403 hFE typical graph.png

    So you should be able to see that the 2N4403's current gain could get pretty low as IC gets up into the hundreds of milliamps. This is a big advantage of MOSFETs and it's why I would use something like this one instead of a 2N4403, especially if there are only a few in the circuit.

    If you really want to use the 2N4403, have a look at the VCE(sat) specification for 500 mA collector current. They specify a base current of 50 mA! That corresponds to an R2 value of about 82Ω! And the 50 mA current is wasted. But that will work.
     
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