A Zener diode of Zener voltage 5 V is used to regulate an unregulated dc input of 25 V. A 400 Ω resistor is connected in series, and the Zener current is four times the load current. The load current I(L) and load resistance R(L) are:
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Question

A Zener diode of Zener voltage 5 V is used to regulate an unregulated dc input of 25 V. A 400 Ω resistor is connected in series, and the Zener current is four times the load current. The load current I(L) and load resistance R(L) are:

Options

G1 Physics · Accepted Answer

The voltage drop across the series resistor is V(R) = 25-5 = 20 V. Series current: I = V(R)/R = 20/400 = 0.05 A = 50 mA. Given Zener current I(Z) = 4I(L). At the node, I = I(Z) + I(L) = 4I(L) + I(L) = 5I(L). Therefore I(L) = 50/5 = 10 mA. The load voltage is 5 V, so R(L) = 5/(10×10⁻³) = 500 Ω. Correct option: B.