Build an Adjustable Bench Power Supply
An adjustable bench power supply is one of the most useful tools in an electronics workshop. It provides a clean, adjustable DC output for testing, developing and repairing electronic circuits. This project explains how to build a professional laboratory power supply with adjustable voltage, current limiting, digital voltage and current displays, and comprehensive protection features.
Project Overview
Unlike a fixed-voltage power supply, a bench power supply allows both the output voltage and maximum current to be adjusted independently.
Current limiting protects experimental circuits by preventing excessive current during accidental short circuits or wiring mistakes.
A digital voltmeter and ammeter continuously display the output voltage and current, making the supply suitable for laboratory and repair work.
Main Sections
| Section | Function |
|---|---|
| Mains Transformer | Reduces the AC mains voltage. |
| Bridge Rectifier | Converts AC into DC. |
| Filter Capacitors | Smooth the rectified voltage. |
| Voltage Regulator | Provides adjustable output voltage. |
| Current Limiter | Protects the power supply and connected circuits. |
| Digital Meter | Displays output voltage and current. |
Project Difficulty
| Item | Value |
|---|---|
| Difficulty | ⭐⭐⭐ Intermediate |
| Build Time | 4–8 Hours |
| Input Voltage | 110 V / 230 V AC |
| Output Voltage | 0–30 V DC (Typical) |
| Maximum Current | 3–10 A |
Typical Components
| Component | Typical Choice |
|---|---|
| Transformer | 24 VAC Centre-Tapped or 24 VAC Single Secondary |
| Bridge Rectifier | 25–35 A |
| Filter Capacitors | 10,000–22,000 µF |
| Voltage Regulator | LM317, LM338 or Discrete Regulator |
| Power Transistors | 2N3055 or TIP35C (High Current Designs) |
| Digital Voltmeter/Ammeter | LED Panel Meter |
| Heat Sink | Large Aluminium Type |
| Cooling Fan | Optional |
How the Power Supply Works
The transformer reduces the mains voltage to a safer AC level.
The bridge rectifier converts this AC into pulsating DC.
Large electrolytic capacitors smooth the DC, reducing ripple.
The adjustable regulator circuit controls the output voltage while the current limiting circuit prevents excessive current from flowing into the load.
The digital display continuously monitors both voltage and current.
Typical Specifications
| Parameter | Typical Value |
|---|---|
| Output Voltage | 0–30 V DC |
| Output Current | 0–5 A |
| Ripple Voltage | <10 mV |
| Voltage Regulation | Better than ±0.1% |
| Current Limiting | Adjustable |
| Short-Circuit Protection | Yes |
Applications
- Electronics laboratories.
- Arduino and ESP32 projects.
- Amplifier testing.
- Battery charging experiments.
- Microcontroller development.
- Circuit troubleshooting.
- Electronics repair workshops.
- Educational laboratories.
Testing
- Inspect all wiring before applying power.
- Measure the transformer secondary voltage.
- Verify the bridge rectifier output.
- Check the regulated output without a load.
- Adjust the output voltage from minimum to maximum.
- Connect a suitable load resistor.
- Adjust the current limit and verify its operation.
- Confirm the voltage and current readings on the digital display.
- Monitor heat sink temperature during high-current operation.
Troubleshooting
| Problem | Possible Cause |
|---|---|
| No output voltage | Blown fuse, faulty transformer or wiring error. |
| Output cannot be adjusted | Faulty potentiometer or regulator circuit. |
| Output voltage unstable | Insufficient filter capacitance or defective regulator. |
| Heat sink becomes excessively hot | Output current too high or inadequate cooling. |
| Current limit does not work | Incorrect current sensing resistor or faulty control circuit. |
Project Improvements
- Add dual independent outputs.
- Add USB-C and USB-A charging ports.
- Include programmable voltage presets.
- Add over-temperature protection.
- Use an LCD or TFT colour display.
- Add rotary encoder adjustment.
- Include data logging via USB.
- Provide remote control using Wi-Fi or Bluetooth.
Skills Learned
- Linear regulator design.
- Current limiting circuits.
- Power transistor operation.
- Heat sink selection.
- Voltage and current measurement.
- Laboratory power supply construction.
Safety Notes
- Mains voltage is hazardous. Disconnect power before servicing.
- Fit correctly rated mains and output fuses.
- Earth the metal enclosure properly.
- Large filter capacitors can remain charged after the supply is switched off.
- Use insulated output terminals to prevent accidental short circuits.
Key Points
- An adjustable bench power supply is an essential laboratory instrument.
- Independent voltage and current control make it suitable for testing a wide variety of circuits.
- Current limiting protects both the power supply and the equipment under test.
- Digital voltage and current displays improve accuracy and ease of use.
- A well-designed bench power supply is one of the most valuable tools for any electronics enthusiast or professional.