Zener Diode-Find Input Resistance

Thread Starter


Joined Mar 8, 2007
Need some help finding the value of the input Resister...

Design a 7.5V zener regulator circuit using a 7.5 zener specified at 12 mA. The zener has an incremental resistance of 30 ohms and knee current of .5 mA. The regulator operates from a 10 V supply and has a 1.2 k-ohm load. What is the value of R.

I figured the value of Vznot to be 7.14 from the following equation


The answer is 225 ohms, but I don't understand how that can be...

I know you have to subtract the diode voltage from the 10 v supply, then divide by the total current, but I can't see to find the correct value of the total current.

Any help would be appreciated.