Variable Bench Power Supply
Variable voltage and current regulated power supply
Safety Warning: This project involves working with mains voltage, which can be lethal. If you're not experienced with high-voltage electronics, seek guidance from someone who is. Always ensure proper isolation and follow electrical safety standards.
Circuit Description
This adjustable bench power supply features variable voltage (1.25V to 30V) and current limiting (0-3A) capabilities. Built around the versatile LM317 adjustable regulator and complemented with current-limiting circuitry, it's an essential tool for electronics enthusiasts, hobbyists, and professionals.
Key Features
- Variable output voltage: 1.25V to 30V
- Adjustable current limiting: 0 to 3A
- Short-circuit protection
- LED indicators for power and current limit
- Digital display for voltage and current (optional)
- Low ripple output (~5mV with filtering)
- Thermal protection
How It Works
The circuit consists of several key stages:
- Power Input and Transformation: Mains AC voltage is stepped down using a transformer, then rectified to DC with a bridge rectifier and smoothed by large filter capacitors.
- Voltage Regulation: The LM317 regulator provides a stable, adjustable output voltage. A potentiometer allows fine adjustment of the output voltage.
- Current Limiting: A current sensing resistor and transistor circuit monitor the output current. When it exceeds the set limit (adjustable via another potentiometer), the circuit reduces the voltage to maintain the current limit.
- Output Filtering: Additional capacitors at the output ensure low ripple and stable voltage under varying loads.
- Display and Indicators: Digital meters show the output voltage and current, while LEDs indicate power status and current limiting mode.
Circuit Schematic (Simplified)
Components List
Step-down transformer230V/120V AC to 24V AC, 3A
Bridge rectifier4A or higher
Electrolytic capacitor4700μF 50V (2 pcs)
LM317 voltage regulatorTO-220 package
Power transistorTIP3055 or 2N3055
Medium power transistorBD139 (2 pcs)
LED indicatorRed and Green with resistors
Resistor240Ω
Potentiometer10kΩ (voltage adjust)
Potentiometer5kΩ (current limit)
Current sensing resistor0.1Ω 5W
Resistor1kΩ (2 pcs)
Electrolytic capacitor10μF (2 pcs)
Ceramic capacitor100nF
PCB or prototyping boardUniversal
HeatsinkFor LM317 and transistor
Case with ventilationMetal enclosure
Binding postsOutput terminals (2 pcs)
Power switchAC rated
Fuse holderWith appropriate fuse
Power cordAC mains
Digital metersVolt and current (optional)
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Installation Instructions
- Design or obtain a suitable PCB layout for the circuit.
- Prepare a metal or plastic case with adequate ventilation. Mount the transformer securely.
- Install the power cord with strain relief, fuse holder, and power switch.
- Mount the bridge rectifier and large filter capacitors, observing correct polarity.
- Mount the LM317 and power transistor on substantial heatsinks with thermal compound.
- Assemble the voltage regulator section with the 10kΩ potentiometer and associated components.
- Assemble the current limiting circuit with the 0.1Ω sensing resistor and 5kΩ potentiometer.
- Mount the output binding posts or banana jacks with appropriate insulation.
- If using digital displays, connect them according to the manufacturer's instructions.
- Wire the LED indicators for power and current limit states.
- Double-check all connections, particularly the mains wiring, for safety.
- Test the power supply without load, adjusting the voltage through its range.
- Test the current limiting with a suitable load (power resistor).
- Mark the front panel with voltage and current scales for the potentiometers.
- Perform final checks and calibration against a known good multimeter.
Usage Tips
- Always set the voltage to the desired level before connecting your circuit.
- For sensitive components, set the current limit to a safe value before powering up.
- Allow adequate ventilation during operation, especially at higher currents.
- Periodically check and tighten all connections, as thermal cycling can loosen them.
- For maximum accuracy, calibrate your power supply's meters annually against a reference.
- Consider adding an output switch to easily disconnect the load without changing settings.