Design regulator for 16 V from 25 V

Requirement: Output 16 V dc from a 25 V input. Choose a simple Zener shunt regulator for small loads, and give the LM317 alternative for larger loads.

Design 1: Zener shunt regulator

 25 V o----[ Rs ]-----+-------+----o Vo = 16 V
                      |       |
                    (Dz 16V) [RL]
                      |       |
                     GND     GND

Assumptions: load current IL(max) = 50 mA, minimum Zener current Iz(min) = 10 mA (above the knee), Zener Vz = 16 V (standard BZX/1N-series 16 V, 1.3 W).

1. Current in the series resistor: Is = IL(max) + Iz(min) = 50 + 10 = 60 mA
2. Rs = (Vin − Vz)/Is = (25 − 16)/60 mA = 150 Ω
3. Power in Rs: Is² Rs = (0.06)² × 150 = 0.54 W → use a 1 W resistor.
4. Maximum Zener current (no load): Iz(max) = (25 − 16)/150 = 60 mA, and Pz = 16 × 60 mA = 0.96 W → use a 1.3 W Zener (or 2 W for safety).
5. Add C = 100 µF across the output to reduce ripple.

Design 2: LM317 (for current up to 1.5 A)

Vout = 1.25 (1 + R2/R1): with R1 = 240 Ω, R2 = 240 (16/1.25 − 1) = 2.83 kΩ (use 2.7 kΩ + 130 Ω, or a 2.2 kΩ fixed with 1 kΩ trimmer). Vin = 25 V is above Vout + 3 V = 19 V, so regulation is maintained. The LM317 dissipates (25 − 16) IL, so for 0.5 A it needs about 4.5 W and a heat sink.

Answer: Rs = 150 Ω and a 16 V, 1.3 W Zener for up to 50 mA, or LM317 with R1 = 240 Ω, R2 = 2.83 kΩ.