Linear Power Supply Design
MANIMOHAN T. · MIRANDA C.M.C.C. · METHSARANI H.E.N. · Supervisor: Dr. Thayaparan Subramaniam · May 2023
Overview
This project implements a fully discrete transistor-based adjustable linear power supply with over-current protection for module EN2111 — Electronic Circuit Design. The supply maintains a stable output voltage from 8 V to 14 V across varying input line voltages (16–20 V) and varying load conditions, while limiting the maximum output current to 100 mA to protect connected circuits. Key figures of merit — line regulation, load regulation, and current limiting — were all measured on a breadboard and tabulated.
Specifications
| Input Voltage Range | 16 V – 20 V DC (typical 18 V) |
| Output Voltage Range | 8 V – 14 V DC (adjustable via trimmer) |
| Achieved Output Range | 6.8325 V – 14.1534 V |
| Current Limit | 100 mA (over-current protection) |
| Achieved Max Current | ≈ 103 mA |
| Mid Output Voltage | 11 V (target) |
| PCB Size | 46 mm × 31 mm (EasyEDA) |
| Simulation Tool | Multisim |
Circuit Design & Calculations
The circuit is a series-pass linear regulator with feedback. A Zener diode (4.7 V) provides the voltage reference. Two BC109 transistors form the error amplifier that compares the sampled output (via voltage divider R1/R2 + trimmer VR) against the Zener reference. The TIP31C pass transistor drives the load. A small sensing resistor r = 6.8 Ω detects over-current and switches in T3 to limit current.
Components
| Component | Qty | Role |
|---|---|---|
| TIP31C (NPN Power Transistor) | 1 | Pass transistor — handles full load current; P_max > 1.2 W |
| BC109 (NPN General Purpose) | 2 | Error amplifier transistors in feedback path |
| 4.7 V Zener Diode (C4V7) | 1 | Voltage reference for feedback divider |
| Resistor 5 kΩ | 1 | R2 in voltage divider feedback |
| Resistor 1.5 kΩ | 1 | R1 (fixed) in voltage divider |
| Resistor 6.8 Ω | 1 | Emitter resistor (r) for current sensing / limiting |
| Resistor 650 Ω | 1 | Base current limiting resistor (R) for TIP31C |
| 10 kΩ Trimmer | 1 | Variable resistor VR — adjusts output voltage |
Measured Results
Current Limiting (Output voltage = maximum, input = 18 V)
| Load Resistor (Ω) | Load Current (mA) |
|---|---|
| 6 | 103.00 |
| 25 | 102.00 |
| 56 | 99.60 |
| 71 | 99.40 |
| 240 | 40.20 |
| 500 | 21.05 |
| 750 | 13.62 |
| 1000 | 10.23 |
| 2000 | 4.02 |
| 3000 | 2.49 |
| 4000 | 2.01 |
| 5000 | 1.84 |
Current clamps at ≈103 mA for low-resistance loads (highlighted), then drops naturally at high resistance.
Line Regulation (Output set to 8 V, load fixed)
| Input Voltage (V) | Output Voltage (V) |
|---|---|
| 16.0 | 8.00 |
| 16.5 | 8.06 |
| 17.0 | 8.12 |
| 17.5 | 8.19 |
| 18.0 | 8.24 |
| 18.5 | 8.30 |
| 19.0 | 8.35 |
| 19.5 | 8.41 |
| 20.0 | 8.47 |
Output varies by only 0.47 V across a 4 V input swing — good line regulation.
Load Regulation (Output set to 10 V, input fixed)
| Load Resistor (Ω) | Output Voltage (V) |
|---|---|
| 6 | 0.98 |
| 25 | 3.54 |
| 56 | 6.60 |
| 71 | 8.00 |
| 240 | 9.95 |
| 500 | 9.98 |
| 750 | 9.99 |
| 1000 | 10.01 |
| 2000 | 10.02 |
| 3000 | 10.02 |
| 4000 | 10.03 |
| 5000 | 10.04 |
Output stays within ±0.05 V for loads ≥ 240 Ω — excellent regulation in the normal operating range. Below 71 Ω the current limiter engages and voltage drops (by design).
