In the circuit shown below, maximum zener diode current will be _________ mA.

Solution

Physics sir · 4 steps

Zener current is largest when the supply is largest, because the load always takes the same 6 mA.

1. Fixed load current. While the zener is in breakdown, it holds the load at 60 V. So I_L = (60 V)/(10 kΩ) = 6 mA, whatever the supply is.
2. Pick the worst case. The series resistor drops whatever the supply gives beyond 60 V. The largest drop, and so the largest I, comes at the top of the range, 120 V.

   Iₘₐₓ = (120 - 60)/(4 kΩ) = (60 V)/(4 kΩ) = 15 mA

3. Split at the junction. The current I divides between the zener and the load, so I_Z = I - I_L = 15 - 6 = 9 mA.
4. So. The maximum zener current is 9 mA.

The common slip: 15. Forgetting to subtract the load current gives the full series current of 15 mA, not the zener current.

The idea to keep. In a zener regulator the load current stays fixed at V_Z/R_L. Any extra current from the supply goes into the zener, so its maximum comes at the maximum input voltage.

Answer from the NTA final answer key. Solution by PrepWiser.