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I am trying to understand how the circuit in the image works. I have managed to understand how the 40mA constant current source works, but I cannot understand how it performs the switching between transistors Q3 and Q4. Could someone give me a clue or an explanation of how the calculations for this switching circuit are? Q5 works in cut-off/saturation, depending on its state Q3 or Q4 must conduct. Thank you very much.

LTspice simulation based on the block diagram [here] https://i.sstatic.net/M6LQJ1lp.jpg of the manual: https://es.scribd.com/document/674848740/4C-Device-Manual-Sibas

enter image description here

I think I've solved it, here is the solution I found: In R4 we have 21.1V that the zener sets, if Q5 does not conduct, in R6 we have 24-21.1-0.6=2.3V. We must calculate R4 and R6 so that the current flows through D4 and is enough for the zener to maintain 3.9V. In this way, Q4 conducts, so that it conducts the constant current source must provide at least 21.7V, polarizing with 0.6V base-emitter. On the other hand, in the base of Q3 there is 21.7V, the same voltage as in the emitter, so it does not conduct.

When Q5 conducts in R9 we have the base voltage (5V) - 0.6 = 4.4V, if we now calculate a value for R9 such that the current flows through D3 the voltage at the node between R4 and D3 will now be 20.5V, so Q3 will conduct and the constant current source will decrease the voltage to 21.1V so Q4 stops conducting. I hope I have explained myself correctly and thank you all very much.

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    \$\begingroup\$ Please post the block diagram print (and any other relevant information) in your question and preserve the closed link for the PDF as the original source. Questions should be self contained and it is worse if the reference is not open. \$\endgroup\$
    – devnull
    Commented Dec 2 at 11:27
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    \$\begingroup\$ Tornillo - Hi, I was just working on what devnull has mentioned above. (a) I think that this is the English language equivalent to the document linked in the question: scribd.com/document/674848740/4C-Device-Manual-Sibas Do you agree? If so, let's add that link to the question as well, since although it's difficult to go through the ad-supported scribd interface, more people here are likely to read English (I think). (b) I extracted the diagram from the English language link above - see here. It's small and low res, unfortunately :( \$\endgroup\$
    – SamGibson ♦
    Commented Dec 2 at 11:33

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Better than thinking about voltage across R4 is look at voltage across R6 vs. voltage across D2:

When Q5 is conducting the voltage across R6 is higher than voltage across D2. That makes the Q3 carries all 10mA supply current, because Q3 base is lower than Q4 base.

When Q5 is nonconducting the base of Q4 is lower so now Q4 carries all 10mA current.

Another words the Q3 & Q4 form a comparator of R6 & D2 voltages.

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