Zener diode-finding Resistor value

[jlucas134]

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

Vz=Vznot-Iz*rz

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.

 

[berkeman]

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...

 

[jlucas134]

At least I'm not the only one that is confused why its 225...I'll keep digging...