Build a Regulated Linear Power Supply
Nearly every electronic project requires a clean and reliable power supply. A linear power supply converts AC mains voltage into a stable DC output using a transformer, bridge rectifier, filter capacitors and voltage regulator. Although less efficient than modern switching supplies, linear power supplies are extremely quiet electrically and are widely used in audio equipment, laboratory instruments and precision electronic circuits.
Project Overview
A linear power supply performs four main functions:
- Reduces the mains voltage using a transformer.
- Converts AC into DC using a bridge rectifier.
- Smooths the DC using large electrolytic capacitors.
- Regulates the voltage to produce a stable output.
The result is a clean, low-noise DC supply suitable for sensitive electronic equipment.
Main Sections
| Stage | Function |
|---|---|
| Mains Transformer | Steps the AC voltage down to the required level. |
| Bridge Rectifier | Converts AC to pulsating DC. |
| Filter Capacitors | Reduce ripple voltage. |
| Voltage Regulator | Maintains a constant output voltage. |
| Protection Circuit | Provides fuse and short-circuit protection. |
Project Difficulty
| Item | Value |
|---|---|
| Difficulty | ⭐⭐ Beginner–Intermediate |
| Build Time | 2–4 Hours |
| Input Voltage | 110 V or 230 V AC |
| Output Voltage | 5–35 V DC |
| Output Current | 1–10 A (design dependent) |
Typical Components
| Component | Typical Choice |
|---|---|
| Transformer | 12 V, 18 V or 24 V AC |
| Bridge Rectifier | 4–35 A |
| Filter Capacitors | 2,200–22,000 µF |
| Voltage Regulator | LM317, 7805, 7812, 7824 or LM338 |
| Heat Sink | Aluminium, sized for output power |
| Fuse | Primary and secondary protection |
| LED Indicator | Power ON indication |
How the Circuit Works
The transformer isolates the circuit from the mains and reduces the AC voltage.
A bridge rectifier converts the alternating voltage into pulsating DC.
Large electrolytic capacitors charge to the peak voltage and smooth the output by reducing ripple.
Finally, the voltage regulator removes remaining variations and provides a stable DC output for the connected circuit.
Advantages
- Very low electrical noise.
- Excellent voltage regulation.
- Simple construction.
- Reliable operation.
- Ideal for audio equipment.
- Easy to repair.
Disadvantages
- Lower efficiency than switching power supplies.
- Larger and heavier because of the transformer.
- Requires heat sinks for the voltage regulator.
- Produces more heat at high output currents.
Applications
- Bench power supplies.
- Audio amplifiers.
- Preamplifiers.
- Microcontroller projects.
- Laboratory instruments.
- Radio equipment.
- Battery chargers.
- DIY electronic circuits.
Testing
- Inspect all wiring carefully.
- Check transformer secondary voltage before connecting the rectifier.
- Verify bridge rectifier polarity.
- Measure the DC voltage after the filter capacitors.
- Measure the regulated output voltage.
- Connect a suitable load resistor.
- Verify voltage stability under load.
- Check regulator temperature during operation.
Troubleshooting
| Problem | Possible Cause |
|---|---|
| No output voltage | Blown fuse, faulty transformer or wiring error. |
| High ripple voltage | Defective or undersized filter capacitor. |
| Output voltage too low | Incorrect transformer or overloaded regulator. |
| Regulator overheats | Insufficient heat sink or excessive load current. |
| Fuse blows immediately | Short circuit or reversed bridge rectifier. |
Project Improvements
- Add adjustable output voltage using an LM317.
- Add adjustable current limiting.
- Include digital voltage and current meters.
- Add short-circuit protection.
- Install a cooling fan for high-current versions.
- Add multiple regulated outputs (5 V, 12 V and 24 V).
- Include USB-C and USB-A outputs.
- Add reverse-polarity protection.
Skills Learned
- Transformer selection.
- Bridge rectifier operation.
- Filter capacitor sizing.
- Voltage regulation.
- Heat sink selection.
- Power supply testing.
Safety Notes
- Mains AC voltage is dangerous and can cause serious injury or death.
- Always disconnect the power before making wiring changes.
- Use correctly rated fuses on the primary and secondary sides.
- Ensure the metal enclosure is properly earthed.
- Large filter capacitors can remain charged after power is switched off. Discharge them safely before servicing.
Key Points
- Linear power supplies provide clean, stable DC with very low electrical noise.
- They are ideal for audio and precision electronic circuits.
- Proper transformer and capacitor selection are essential for reliable operation.
- Voltage regulators require adequate heat sinking.
- A good linear power supply is the foundation of many successful electronic projects.