source and sink related to transistors

Discussion in 'General Electronics Chat' started by falade47, Mar 17, 2017.

  1. falade47

    Thread Starter Member

    Jan 24, 2017
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    Pls i need. A brief explanation on sourcing and sinking of current..am new to electronics andd i read an article that the pnp transistor turns on when a voltage is not applied to the base..does that mean that my led will turn on when aa base current or voltage is not applied..

    Thanks in advance
     
  2. ian field

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    Oct 27, 2012
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    You might be thinking of the difference between bipolar and FETs.

    Bipolar transistors regardless of polarity require appropriate base current to flow to get collector current.

    Depletion mode JFETs are conducting with no bias - a reverse bias on the gate pinches off the channel and halts conduction.

    Enhancement mode JFETS would be a bit impractical because it isn't a good idea to forward bias the gate junction.

    MOSFETs can be either enhancement or depletion - power MOSFETs are usually enhancement, depletion mode is probably the most likely in low power RF devices.
     
  3. Kermit2

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    Feb 5, 2010
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    I always used the middle letter to tell me.

    PNP middle letter is N. NEGATIVE in relation to emitter for purposes of forward bias (turn on transistor)


    NPN middle letter is P. POSITIVE in relation to emitter.

    For the schematic symbol
    P oint iN P
    Not Point N
     
  4. hp1729

    Well-Known Member

    Nov 23, 2015
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    Design 1016 sink and source.PNG
     
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  5. dl324

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    Mar 30, 2015
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    Please post a schematic so we can better understand what you're talking about.

    BJTs don't normally conduct with no voltage applied to the base.

    By voltage not applied to the base, does that include connecting the base to ground through a resistor?
     
  6. falade47

    Thread Starter Member

    Jan 24, 2017
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    No that wasnt stated..
     
  7. falade47

    Thread Starter Member

    Jan 24, 2017
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    Yh...thank you..i could rellate it now..can pnp used in place of npn?
     
  8. hp1729

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    In what design? With appropriate changes in the design, yes.
     
  9. ian field

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    If you redesign the circuit.......................
     
  10. ScottWang

    Moderator

    Aug 23, 2012
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    What you want is here, the values of resistor are depend on the Vcc, Input voltage and the current of load.
    HighLowLogic_SourceandSink_ScottWang-02.gif
     
  11. falade47

    Thread Starter Member

    Jan 24, 2017
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    Thanks very much...sorry to ask...I tried designing a H bridge with just pnp traansistors but it diddnt work out..i'll uploadd the schematics
     
  12. ErnieM

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    Apr 24, 2011
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    "Source" and "sink" in this context would mean the direction of (conventional) current into or out of a transistor.

    To eliminate some confusion about what transistor is used for now, here is an H bridge made of switches:

    [​IMG]

    One could say that S1 and S3 are sourcing current, as current flows out of them to the motor. Similarly S2 and S4 are sinking current.

    In normal operation these are turned on in pairs, S1 and S4, or S3 and S2. That drives the motor in one direction or another.
     
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  13. falade47

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    Jan 24, 2017
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    Can I design my H bridge to use just npn transistors
     
  14. ErnieM

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    Sure you can, but either you are limited by the drive voltage you can achieve or you need an even higher voltage to drive the top two transistors.
     
  15. falade47

    Thread Starter Member

    Jan 24, 2017
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    Could you throw more light on that..thanks​
     
  16. dl324

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    Mar 30, 2015
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    Draw an H-Bridge with only NPN transistors and see what it will take to turn on the top transistors.

    There's a reason why H-Bridges normally use complementary transistors...
     
  17. falade47

    Thread Starter Member

    Jan 24, 2017
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    Ye
    Yeah...saatisfied..thanks
     
  18. ian field

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    At one time; all NPN half-bridge audio amplifiers were popular - using 2 output stages as full bridge is effectively a BTL amplifier. When there were plenty of old 8 track players in the salvage sources, I made BTL amplifiers out of a stereo unit.

    Most SMPSUs - whether full or half-bridge; use all NPN.

    Most early audio amplifiers and all SMPSUs use a driver transformer with a separate secondary for each transistor - design is not trivial for pulse drive.

    Modern audio amplifiers use a combination of Darlington and Sziklai pairs to arrange for all the high power transistors to be NPN, they require temperature compensated automatic bias. I've never seen the arrangement used in a SMPSU. There's probably a difficulty with base carrier storage time delaying turn off - for a brief instant; both transistors conduct at once across the supply and punch through.
     
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