How does this overcurrent indication circuit work?
Output voltage



Figure 1. R38 turns on Q11. Current flows to VDD.

Note that the voltage out will droop as current increase due to R36.

Current limit



Figure 2. When the voltage drop across R36 + D11 increases to about 0.7 V Q12 starts to turn on.

Q12 turning on will steal the bias from Q11: the base of Q11 will be pulled high and it will start to turn off. This is the current limiter.

Over-current indication



Figure 3. As Q11 turns off the voltage across its emitter-collector will increase.

When Q11's VEC exceeds about 0.7 V Q14 will start to turn on and pass current through D13, etc., to indicate over-current.

The circuit seems to be intended to interface with 5 V logic.


D13 &amp; D10 prevent interference between the two circuits.
R39, R40 and D14 form a 5 V voltage limiter for the logic interface.