Variable Power Supply With Protection and Replaceable Module

A variable DC bench power supply is one of the most important tools for electronics students, hobbyists, and laboratories. Instead of purchasing expensive commercial units, a reliable and adjustable power supply can be designed and built using commonly available electronic components.

In this project, a linear adjustable DC power supply is implemented using an LM338T adjustable voltage regulator and TIP3055 power transistors. The power supply provides adjustable output voltage and current, along with relay-based output protection. A modular design approach is used, making the system suitable for educational laboratory use and easier maintenance.

LM338T adjustable voltage regulator

TIP3055 power transistor × 2

Step-down transformer (230 V AC to suitable AC output)

Bridge rectifier (KBPC5010)

Filter capacitor (6800 µF)

Potentiometers:

1. 10 kΩ × 2 – coarse adjustment (voltage & current)
2. 1 kΩ × 2 – fine adjustment (voltage & current)

Resistor (220 Ω)

Current sense resistor

Relay (for output protection)

DC cooling fan

PCB

The step-down transformer lowers the 230 V AC mains supply to a safe AC voltage suitable for the power supply. This AC output is converted to DC using a KBPC5010 bridge rectifier.

A 6800 µF filter capacitor is used to smooth the rectified DC voltage and reduce ripple, providing a stable DC input for the voltage regulation stage.

The LM338T is configured as an adjustable linear voltage regulator. Output voltage control is achieved using a coarse and fine adjustment method.

A 10 kΩ potentiometer provides coarse voltage adjustment, while a 1 kΩ potentiometer allows fine tuning. This arrangement enables both quick voltage setting and precise control.

Although the LM338T can supply higher current, TIP3055 power transistors are used as series pass elements to further increase the current handling capability of the power supply. The LM338T controls the base of the TIP3055 transistors, which deliver most of the load current.

A low-value current sense resistor monitors the output current. When the current exceeds the preset limit, the base drive to the TIP3055 transistors is reduced, limiting the output current and protecting the circuit.

A relay is incorporated at the output stage to provide additional protection.

During normal operation, the relay remains energized and allows power to reach the output terminals. In the event of overcurrent, overheating, or fault conditions, the relay disconnects the load, protecting the transformer, voltage regulator, and power transistors from damage.

All components were securely mounted on the base of the enclosure. The LM338T voltage regulator and TIP3055 power transistors were mounted directly to the enclosure chassis, which acts as a passive heat-dissipating surface.

A DC cooling fan was installed inside the enclosure to improve air circulation and assist in heat removal. Proper spacing and ventilation openings were maintained to ensure safe thermal performance during normal laboratory operation.

The power supply was tested under different load conditions. The output voltage was smoothly adjustable using the coarse and fine controls, and the current limiting function operated reliably.

The relay-based protection system responded correctly during fault conditions, and the completed unit functioned as a stable bench power supply.

This project demonstrates a simple and effective method of building an adjustable DC bench power supply using an LM338T voltage regulator and TIP3055 power transistors. The inclusion of current limiting, relay-based protection, and assisted air cooling improves reliability, making the system suitable for educational and laboratory applications.