I do not know how to find the value of the series resistor(Rs) because there are many variable. It is hard to decide which current(knee/maximum) or voltage should I select . Can some one teach me?? Question: Given that the zener voltage Vz = 8.2V, with the condition that the zener current 75mA ≤ Iz ≤ 1A and the load resistance RL = 9Ω. Calculate resistance Rs so that load voltage VL is regulated to 8.2V while the input voltage Vin varies by ±10% from its nominal value of 12V. Also check that the zener current Iz is larger than 75mA at lowest input voltage Vin .