Basic circuit operation of voltage regulator

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Discussion Overview

The discussion revolves around the operation of a voltage regulator circuit, specifically focusing on the roles of components such as R1, R2, Q1, and Q2. Participants explore the mechanisms of current sensing and diversion within the circuit, as well as the interactions between these components in regulating voltage and protecting against overloads.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Homework-related

Main Points Raised

  • One participant describes R1 as a sensing resistor that activates Q1 when it detects a voltage drop, allowing current to pass through the regulator.
  • Another participant questions the sequence of events regarding R2's role in sensing voltage when Q1 is off, seeking clarification on the mechanism of current diversion.
  • A different participant reiterates the function of R1 and explains that when the voltage across R2 increases, it activates Q2, which then creates a low resistance path to protect Q1 from overload.
  • One participant asks for further explanation on how the current is diverted into the regulator, indicating a need for clarity on the underlying mechanism.
  • Another participant proposes that a forward biased zener diode within the regulator diverts excess current, suggesting that its low resistance minimizes the impact of large current changes on voltage.
  • A participant clarifies the cause and effect relationship in the circuit, stating that excess current through Q1 leads to R2 activating Q2, which then shorts R1 and alters the current handling of the regulator.

Areas of Agreement / Disagreement

Participants express varying degrees of understanding and interpretation of the circuit's operation, with some points of agreement on the roles of R1 and Q1, while other aspects, particularly regarding the sequence of events and mechanisms, remain contested and unclear.

Contextual Notes

There are unresolved questions regarding the exact mechanisms of current diversion and the interactions between components, as well as dependencies on specific circuit configurations and assumptions about component behavior.

Who May Find This Useful

Readers interested in voltage regulation circuits, component interactions in electronic designs, or those seeking clarification on circuit operation may find this discussion beneficial.

CNC101
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Homework Statement



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Homework Equations

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.
 
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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.
CNC101 said:
...when there is excess current at Q1 it switches off and error resistor R2 senses a volt drop...

How can R2 sense anything if Q1 is off?
When Q2 is turned on, what happens to the voltage across R1?
 
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:
Well... mostly. I think I'll be a little picky here.

CNC101 said:
...a low resistance path and diverting current into the regulator...

Could you explain the mechanism that causes the current diversion into the regulator?
 
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.
 
The first sentence in your original post was an the right track.
CNC101 said:
R1 is a sensing resistor and when Q1 senses a voltage drop at its base, it switches on and passes current around the regulator.

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.)
 
Well I know a lot more now than when I started, Thanks!
 

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