The given zener regulator circuit causes some trouble for the students:
The specification of zener such as IZT, IZK and IZM are not clearly undertood by the students. Take a look at the symbols: I stands for current, Z stands for Zener, T stands for Test, K stands for Knee, and M stands for Maximum.
So if IZT is 21mA, it means that the zener under test of 21mA has VZ=12V.
IZK of 0.25mA means that this current is needed to keep the zener diode to maintain at 12V.
IZM of 76mA means that the zener will be burnt out if the current is exceeding that value.
Therefore using IZK and IZM will be able to calculate the range of RL with the two permissible values to keep the zener as a regulator or regulating at VZ.
Firstly, the supply current in R1 should be calculated: VR1=28-12=16V and Is=16V/120=133.33mA.
Secondly, calculate the portion of the current that can go to the load RL: 133.33mA-76mA (using IZM)=57.33mA @ 12V (VZ). The RL (keeping the zener from burning out by at least drawing this amount of current) can be calculated to be 12V/57.33mA=209.3 ohms. (at most, ie, max)
Lastly, calculate the portion of the current goes to the load RL: 133.33mA-0.25mA (IZK)=133.08mA @ 12V (VZ). The RL can be calculated to be 12V/133.08mA=90.17 ohms. This is the smallest (minimum) resistance that can be used by the load without losing the zener’s regulation, ie, maintaining the voltage VZ unchanged.