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I found surfing internet an overvoltage protection circuit where I need to choose the value of R1 and D1 to set the value of the trip voltage to protect the load circuit.

How should I correctly choose the value of R1 and D1 for a trip voltaje of 5 ~ 5.2?

Is it correct to use D1 zener BZX84C5V1 of 5.1V?

enter image description here

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This circuit cuts out when the input voltage exceeds the Zener voltage. Is that what you really want? Or do you just want to limit the voltage?

At any rate I tried this with Q1 base = 1K and R1=2.2K and it works as I described above. 5.1V zener will begin to conduct at its threshold, turning on Q2 and turning off Q1.

You could do this with a much simpler circuit that needs only an NPN pass transistor, the zener diode and a single resistor. This will set the max voltage to 5.1V but will not shut off. This may be more what you want.

More here: https://www.electronics-notes.com/articles/analogue_circuits/power-supply-electronics/over-voltage-protection.php


Here's an alternative. It works by monitoring the output, and will limit the voltage to 5.25V rather than cutting it off. Because it uses a FET there will be less drop than the PNP. The Vin stimulus will show how it works.

schematic

simulate this circuit – Schematic created using CircuitLab

(Yes, I realize I just re-invented the LDO. A really, really crappy one.)

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  • \$\begingroup\$ I am not sure if it is safe, appear it is very basic how described \$\endgroup\$
    – asterix
    Aug 11 '19 at 22:37

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