simple clamp circuit resistor calculation

Thread Starter


Joined Jul 2, 2018
Hello guys i want to calculate the resistor R in this circuit.Can anybody help me?

The value of zener diode(VZ)=4.8 volt.

well i try to simplify it

My question is is my solution is right??If not then what will be the value of R?


Joined Jan 29, 2010
hi k,
This looks like a College problem assignment, can you confirm.??

Moderation moved to Homework.
Last edited:


Joined Sep 20, 2005
That resistor is there only to ensure that the leakage current of the zener doesn´t go through the BE junction when the zener is below its rated voltage drop.
A zener´s voltage is usually specified at 5mA, so I would choose a resistor R=0.7V/1mA=140 ohms or higher, but higher resistor will make the zener react at a little bit lower voltage.