The I-V characteristics in the breakdown region of a Zener diode is almost vertical. That is, the current I_Z can rapidly increase at constant V_Z. To prevent damage due to excessive heating, the Zener current should not exceed the rated maximum current, I_ZM. Hence, a current-limiting resistor Rs is connected in series with the diode

I_Z and the power dissipated in the Zener diode will be large for I_L = 0 (no-load condition) or when IL is less than the rated maximum (when Rs is small and RL is large). The currentlimiting resistor R_S is so chosen that the Zener current does not exceed the rated maximum reverse current, I_ZM when there is no load or when the load is very high.

The rated maximum power of a Zener diode is

P_ZM = I_ZM = V_Z

At no-load condition, the current through R_s Z = I_ZM and the voltage drop across it is V – V_Z, where V is the unregulated source voltage. The diode current will be maximum when V is maxi-mum at V_max and I = I_ZM. Then, the minimum value of the series resistance should be

R_s, _min = \(\cfrac{V_{max}−V_Z}{I_{ZM}}\)

A Zener diode is operated in the breakdown region. There is a minimum Zener current, I_Z (min), that places the desired operating point in the breakdown region. There is a maximum Zener current, I_ZM, at which the power dissipation drives the junction temperature to the maximum allowed. Beyond that current the diode can be damaged. Hence, the supply voltage must be greater than Vz and the current-limiting resistor must limit the diode current to less than the rated maximum, I_ZM.