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There is a resistor R5 in the 3A load box below. It seems it add some positive feedback to Q1, but I'm not sure. What's the exact reason to add it? The circuit comes from here.

enter image description here

diverger
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  • You might want to link to the page/appnote/etc where you got the diagram. It probably contains some explanation that just needs a little clarification. –  Mar 09 '16 at 11:57
  • I've add the original link. – diverger Mar 09 '16 at 12:12

1 Answers1

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Q1 and Q2 seem to form a bi-stable circuit. It's purpose is unclear since you haven't supplied any information as to what the 4th line going into the top of the second amp is, but it seems to have something to do with switching modes depending on the supply voltage.

R5 adds positive DC feedback, called hysteresis to the Q1-Q2 system. That will make is "snap" between states. It will also create a small "backlash" band. The threshold going down will be a little lower than going up. This prevents the circuit from oscillating if held right at a single threshold.

Olin Lathrop
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  • I've add the origional link. – diverger Mar 09 '16 at 12:11
  • But, it seems if Vin always greater than 0, then it will always increase the base voltage of Q1. Where is the *hysteresis*? – diverger Mar 09 '16 at 12:41
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    No, that isn't true. As the input voltage ramps up, Q1 is off and Q2 keeps the right end of R5 close to ground. This doesn't change until there's enough current through D1 to turn on Q1, at which time Q2 begins to turn off. R5 provides the positive feedback to make that switching happen more quickly. – Dave Tweed Mar 09 '16 at 13:34
  • Yes, you are right. – diverger Mar 09 '16 at 14:14