A voltage regulating circuit consisting of Zener diode, having break-down voltage of 10 V and maximum power dissipation of 0.4 W, is operated at 15 V. The approximate value of protective resistance in this circuit is _ _ _ _ .

Step-by-step solution

The maximum current through the Zener diode is determined by its maximum power dissipation: I_ Z,max = P_ max V_Z = 0.4 10 = 0.04 A. The protective series resistance must limit the current such that the Zener operates safely. Using Ohm's law across the series resistance and Zener when the Zener carries maximum current: R_s = V_ in - V_Z I_ Z,max = 15 - 10 0.04 = 5 0.04 = 125 Ω