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How does this PNP circuit work? (Overvoltage protection?)

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MaximilianMM:
Hi,

I came across this circuit, while reverse-engineering a device. It looks like a overvoltage protection circuit but has a different arrangement.
This circuit drives an optocoupler for serial communication, the input ranges from 15V-24V (idle), depending on the used actuator.

What is the purpose of this circuit and how does it work, I am pretty sure it is supposed to be a overvoltage protection for the Optocoupler, right? If so, why this weird arrangement?

Thanks in advance.

Edit: I don't know the exact specifications for the used Zener diode (The Mfr. No isn’t correct).

xavier60:
When the voltage is high enough, about 7V, current flows out of Q5's Base through R10 and D4A to ground.
Q5 partly turns on, passing current through the opto's LED.
As the current approaches 0.5mA the voltage across R7 approaches 0.6V, partly turning on Q4 which prevents further increase in  Q5's Base current. It becomes a constant current source for the LED.
The value or R10 is unusually high.

MaximilianMM:
Thanks for the good explanation, you helped my allot.

So, the Zener voltage determines at which voltage (I+) the LED turns on? This circuit than also works like a comparator, right?
How does this circuit cope with high voltage? Could such a circuit be designed to withstand 230V?

I would like to design a circuit, which could withstand such voltages in the worst case.

xavier60:

--- Quote from: MaximilianMM on January 04, 2022, 11:55:03 am ---Thanks for the good explanation, you helped my allot.

So, the Zener voltage determines at which voltage (I+) the LED turns on? This circuit than also works like a comparator, right?
How does this circuit cope with high voltage? Could such a circuit be designed to withstand 230V?

I would like to design a circuit, which could withstand such voltages in the worst case.

--- End quote ---

Yes, the Zener voltage rating determines the threshold voltage. For predictable operation, low Zener current should not be allowed to drive Q5.
To withstand 230V, Q5 needs to have high voltage spec like 400V or more. The circuit should be redone to use NPN transistors so that there is a large selection or transistors to choose from.
What's the intended detection voltage? If it's much lower than the 230V expected maximum, the design become difficult trying to avoid excessive Zener current.

MaximilianMM:
The threshold voltage is between 5-10V. (The exact value isn't really important)

The circuit doesn’t need to work for 230V, it would just be nice to design it in a way, so it doesn’t get destroyed if such high voltages occur.

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