Zener Diode-Find Input Resistance

Discussion in 'Homework Help' started by jlucas134, Mar 8, 2007.

  1. jlucas134

    Thread Starter New Member

    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.