using tip31`s to increase current from voltage reg`s ?

Discussion in 'General Electronics Chat' started by turbo1600, Feb 4, 2016.

  1. turbo1600

    Thread Starter New Member

    Jan 12, 2016
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    i have a lot of TIP31 transistors , can i use them to increase the current handling capacity of LM78xx regulators ? i need the voltage reg to handle 3 to 4 amps. will this also work for the LM317 adj ?

    any body provide diagram on how to do this. ? ( i have the voltage reg side sorted out , just need the current increased.)

    thanks, mk
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Page 8 of the National Semiconductor datasheet for the LM317 shows how to do it.
     
  3. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    To boost the current a PNP transistor is used.
    The TIP31 is a NPN transistor of only 3amps.

    Here is how the schematic looks like:

    LM78XX_with_pass_transistor.png

    I also have attached the TIP31 datasheet.

    Bertus
     
  4. dl324

    Distinguished Member

    Mar 30, 2015
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    Welcome to AAC!
    To use power NPN transistors, you need this circuit (from a NatSemi Voltage Regulator Handbook):
    upload_2016-2-4_12-22-38.png
    Q1 is a general purpose PNP, Q2 is the power transistor.
     
    Last edited: Feb 4, 2016
    absf likes this.
  5. hp1729

    Well-Known Member

    Nov 23, 2015
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    Questions on figure 7.4. How do you select resistor values?
     
  6. hp1729

    Well-Known Member

    Nov 23, 2015
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    Super! I have used such designs often but never actually stopped to think about how they work. Attached is a few drawings. Current ranges have been brought down to something I can breadboard but the principle is the same.
    "A" shows the ever popular PNP used for a positive regulator.
    "B" shows how that PNP is controlled by the inerds of the LM78L09 I used.
    "C" shows the suggested use of the NPN.
    Testing the NPN version under different loads my voltage varied about 500 mV either way. I confess a lack of proper design in resistor selection.. I assumed they set the point where the transistors started conducting.
     
  7. Lestraveled

    Well-Known Member

    May 19, 2014
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  8. crutschow

    Expert

    Mar 14, 2008
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    So here's my LTspice simulation of a LM317 circuit boosted with a 2N2907 and two TIP31's, showing the output current and voltage versus the load resistance.
    The maximum current for a regulated output voltage is about 4.5A with a 10V supply.
    This will vary some with different supply voltaes.
    The LM317 is configured for a nominal output voltage of 5V.

    Note that with such a high output current the short-circuit dissipation can be quite high, depending upon the source voltage.

    LM317 Boosted.PNG
     
  9. hp1729

    Well-Known Member

    Nov 23, 2015
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    Thanks. How did you come to a value of 100 ohms for R6?
    Do you have a 660 ohm resistor? Maybe 665 1%, or a 620 with a 100 ohm trimpot?
    Or a 1K pot trimmed for 5.0 V out? :)
     
    Last edited: Feb 5, 2016
  10. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    @hp1729 , why do you answer for @turbo1600 ?
    Are you related to him?

    Bertus
     
  11. crutschow

    Expert

    Mar 14, 2008
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    R6 is just a bias leakage resistor to shunt away any high temperature leakage current.
    It's value is somewhat arbitrary and can vary over a wide range. It never has more than about 1V across it.
    Where does the value of 660Ω come from?
    R6 has nothing to do with the 5V output which is determined by R1 and R2
     
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