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.

Darlington current limiter

Ie of Q3 = 57.3 uA ( From simulations )

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

enter image description here

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). enter image description here

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

  • 3
    \$\begingroup\$ At first glance, R11 is doing most of the limiting, by restricting base current for the darlington pair. Without feedback the values you'll get irl will greatly depend on variation between parts, temperature, ... \$\endgroup\$ Feb 3, 2022 at 8:41
  • \$\begingroup\$ @Unimportant . Agreed . i am having hard time with considerations of beta values . their datasheets value and simulation results are not getting close . \$\endgroup\$
    – Ronnie
    Feb 3, 2022 at 8:51
  • \$\begingroup\$ @Rohan You can see that \$Q_3\$ is operating as a current sink (you can ignore \$R_{13}\$ since the voltage drop across it will be half a millivolt, or so.) Something around \$55\:\mu\text{A}\pm 5\:\mu\text{A}\$. That's going to determine the base drive for the Darlington. \$R_1\$ will pull off some of it (perhaps 10%) of what arrives, after being magnified by \$Q_1\$'s \$\beta\$, to \$Q_2\$'s base. But what do you calculate from your own application of theory and what do you get from LTspice simulation? \$\endgroup\$
    – jonk
    Feb 3, 2022 at 10:04
  • \$\begingroup\$ @Jonk from my own application of theory i get the maximum load current value of 3.3A and minimum 204mA with variable hfe range (50 to 240 ) . from simulations i observe the load current 1.33A . Does it mean the current limit set by this darlington pair can be anything between 204mA to 3.3A when using in practical purpose ? \$\endgroup\$
    – Ronnie
    Feb 3, 2022 at 12:40
  • \$\begingroup\$ @Rohan Hi, You wrote an "answer" which was still asking for advice, so it was moved into the question as an update. As the OP, you would only write an answer if you solved the problem on your own and the topic can be closed. If you ask for more advice in an "answer" then it becomes a question again, and questions are not allowed in the "answer box". However you have made the situation unclear, since you have already selected an answer to "accept" ("green tick"), which indicates your problem is solved - but now you are asking for more advice. Can you clarify? Thanks. \$\endgroup\$
    – SamGibson
    Feb 3, 2022 at 13:12

1 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.

enter image description here

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

enter image description here

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

enter image description here


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