PNP Darlington pair as a current limiter

Question

I am trying to analyse what is controlling the load current in the below Darlington configuration.

1. Is it the base current of transistor Q3 which is controlling current through R14 / emitter current of Q2?
2. If it is, then is the maximum permissible load current is calculated by Ic(Q3) * Q1 beta * Q2 beta ?
3. From light load to maximum load, will all three transistor stay in the active region? For light loads Q2 would get into saturation I suppose.

---

Ie of Q3 = 57.3 uA ( From simulations )

Theoretically 2N5401 ( Q2 and Q1) has min. hfe of 50 to max. 240 from the datasheet.

So load current value can vary from 57.3uA x 60 x 60 = 206 mA (min) to 57.3 uA x 240 x 240 = 3.3A (max)

From simulation results , getting the maximum load current value of 1.303 A (max).

does this mean load current limit set by darlington pair is 3.3A (max) while using this circuit for practical use ?

Answer 1

EDIT: important note : Be sure that all specifications are ok.

Vce max = -150 V , not ok (must be + 25% over) ...

Ic max = -300 mA , not ok ( * 2) ...

Ptot = 630 mW, not ok ...

On the external characteristic, draw the hyperbolas Vout * Iout = P (for P = 1 W, 3 W, 10 W, ... ).

So one can choose the "power" necessary for the "power" BJT (is your Q2). And so choose carefully the other also.

As @Unimportant pointed, R11 do the job.

Note only the very dispersive curves (beta of Q1,Q2,Q3 change from 50 -> 250, and Temp for 0, 25, and 50 °C).

Here is what one can see ... when load R14 varies from 1k -> 0.

I added one resistor at emitter Darlington. Now, one can see clearly the effect of feedback ... and Temperature.

Edit: And here, the external characteristic (Vout versus Iout) of the "system". Best view with Power Hyperbola's.