1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Zener diode-finding Resistor value

  1. Mar 9, 2007 #1
    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.
  2. jcsd
  3. Mar 9, 2007 #2


    User Avatar

    Staff: Mentor

    I don't get the 225 Ohm answer either. If the 7.5V zener has 12mA going down through it, and the 1.2k Ohm load resistor next to it has an additional 7.5V / 1.2k Ohm current going through it, then the sum of those currents has to be coming down from the bias resistor, which has 2.5V across it. I get something more like in the 130 Ohm range (not exact). I wonder what we're missing....
  4. Mar 9, 2007 #3
    At least I'm not the only one that is confused why its 225...I'll keep digging...
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook