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Basic circuit operation of voltage regulator

  1. Feb 23, 2016 #1
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
    I need to know if my understanding of this circuit is correct in terms of R1, R2, Q1, Q2.

    R1 is a sensing resistor and when Q1 senses a voltage drop at its base, it switches on and passes current around the regulator. I think when there is excess current at Q1 it switches off and error resistor R2 senses a volt drop to switch on Q2 at the base to divert current away from Q1. I think I'm close, who knows.
  2. jcsd
  3. Feb 24, 2016 #2


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    I agree with your first sentence.

    The second sentence is a bit confusing in that cause and effect seem out of order.
    How can R2 sense anything if Q1 is off?
    When Q2 is turned on, what happens to the voltage across R1?
  4. Feb 24, 2016 #3
    R1 is a sensing resistor and when Q1 senses a voltage drop at its base, it switches on and passes current around the regulator. When the voltage across R2 (error resistor) increases to a value just below Q1 max emitter-collector current limit, Q2 switches on and shunts R1 by creating a low resistance path and diverting current into the regulator and thus protecting the "pass transistor" Q1 from overload current.
    Last edited: Feb 24, 2016
  5. Feb 24, 2016 #4


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    Well... mostly. I think I'll be a little picky here.

    Could you explain the mechanism that causes the current diversion into the regulator?
  6. Feb 25, 2016 #5
    Well I think inside the regulator is a forward biased zener diode which diverts excess current to its ground or "ADJ" in the diagram. Its own low resistance path means large changes of current have a small affect on its voltage and therefore protecting the load from excess current.
  7. Feb 25, 2016 #6


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    The first sentence in your original post was an the right track.
    Here is why I said cause and effect were out of order in post #2.
    When there is excess current thru Q1, the voltage drop across R2 turns on Q2. Q2 shorts R1 depriving Q1 of base drive. This forces the regulator to handle most of the load current, and the regulator either goes into current limit or shuts down due to over heating.

    BTW: The regulator output stage is very much like the R1, R2, Q1, Q2 circuit. The difference is that the base of Q1 is also driven by the difference between the ADJ input and an internal reference voltage (often around 1.25V). If you're interested, there is a regulator schematic on pg.12 at http://www.ti.com/lit/ds/symlink/lm117.pdf. The output stage (Q1 in your ckt.) is Q25,26 on the right edge of the diagram, and the overcurrent sense resistor is R26 (R2 in your ckt.)
  8. Feb 26, 2016 #7
    Well I know a lot more now than when I started, Thanks!
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