Building a Variable Power Supply with 2N3055 Transistor

If you’ve been tinkering with electronics for a while, you know that a good power supply is essential. Most circuits need a stable and adjustable voltage source to test, troubleshoot, and operate properly. Instead of buying a bench power supply, you can build one yourself using a handful of components.

One of the most popular designs is based on the 2N3055 transistor, a rugged power transistor that has been around for decades. In this article, we’ll walk through the design, explain how the circuit works, and highlight the purpose of each stage. By the end, you’ll see how this simple project can give you a reliable bench supply for your workshop.

Why Use a 2N3055 Power Supply?

The 2N3055 is a well-known NPN power transistor with high current and power handling capacity. It has been used in amplifiers, power regulators, and chargers for decades. Its durability and availability make it perfect for building a variable power supply that outputs 1.2V up to about 24V at 2–3 amps.

Instead of depending only on linear voltage regulators like the LM317, this design uses the 2N3055 as the main pass element. It takes the raw DC after rectification and, under the control of an op-amp and supporting transistors, delivers a stable and adjustable output voltage.

Circuit Overview

Looking at the schematic, you’ll notice four main stages:

1. Transformer and Rectification
2. Filtering with Large Capacitors
3. Voltage Regulation and Control using Op-Amp (μA741)
4. Pass Transistors (BD137 + 2N3055) Delivering Current to the Load

Let’s break them down step by step.

Step 1: AC to DC Conversion

The circuit begins with a 220V AC input applied to a transformer that steps it down to 24V AC at 3 amps.

This AC is converted into DC by a bridge rectifier (PR). At this point, the DC is pulsating and not smooth enough for electronics use. That’s where the 10,000 µF / 40V capacitor comes in. This large electrolytic capacitor smooths the DC, reducing ripple.

At this stage, you have about 30–32V DC (unloaded) available for regulation.

Step 2: Reference and Control Stage

The control stage is what turns the raw DC into an adjustable, stable voltage.

• A 12V zener diode (1W) provides a reference voltage. This is fed into the input of an op-amp μA741, which is configured here as a comparator and regulator.
• A pair of 1N4148 diodes helps set a stable voltage drop for feedback.
• The BC327 transistor provides biasing and current control for the regulation loop.

By comparing the feedback from the output voltage with the reference setpoint, the op-amp adjusts its output. This forms the heart of the regulation.

Step 3: Driving the Power Transistors

The op-amp itself cannot supply amps of current. That’s where the BD137 transistor comes into play as an intermediate driver, followed by the 2N3055 power transistor, which handles the heavy current flowing to the output.

Here’s how it works:

• The op-amp output drives the base of BD137.
• BD137 then drives the base of the 2N3055.
• The 2N3055 acts as a series pass element, delivering a controlled amount of current from input to output.

A 33 ohm resistor is connected in the circuit for current stability and protection.

Finally, extra smoothing is done with a 220 µF capacitor, ensuring the output voltage is steady during load changes.

Step 4: Voltage Adjustment

The feedback network contains a 4.7k potentiometer (variable resistor). This allows you to adjust the output voltage. By changing the potential divider ratio feeding the op-amp, you can vary the output smoothly from near 1.2V up to close to the input DC (around 24V, depending on load).

Key Components Explained

• 2N3055: Handles the output current, the main pass transistor in series regulation.
• BD137: Works as a driver to relieve the op-amp from sourcing too much current.
• μA741 (Op-amp): Central regulator that compares feedback against the reference voltage.
• BC327: Helps provide biasing and stability.
• Zener diode (12V): Provides a constant voltage reference.
• Electrolytic capacitors (10,000 µF and 220 µF): Smooth DC and filter output.
• Potentiometer (4.7kΩ): User-adjustable control of the output voltage.
• Diodes 1N4148: Provide voltage drop for stability in the feedback loop.

Performance and Output

• Input: 220V AC stepped down to 24V AC, rectified to ~30V DC.
• Output: Variable from ~1.2V to 24V DC.
• Current: Up to 2–3A, depending on transformer and heatsinking of 2N3055.

This makes it very handy for powering different circuits during testing, since you can dial up the exact voltage you need.

Applications

This power supply design is a versatile tool for:

• General electronics prototyping and breadboarding
• Repair work where different voltages are required
• Testing DC motors, LEDs, and small modules
• Educational labs for students learning about regulated supplies
• DIY audio amplifier projects that require stable DC rails

Safety Considerations

Whenever working directly with mains power (220V AC here), be extremely cautious.

• Always use a proper fuse on the transformer’s primary side.
• The 2N3055 must be mounted on a heatsink; otherwise, it will overheat quickly.
• Make sure the electrolytic capacitors are rated well above the rectified DC (here, at least 40V).
• Build the circuit on a good PCB and avoid loose wiring.
• When testing, use a multimeter first to check voltages before attaching sensitive circuits.

Advantages of This Design

• Higher current capacity than just using a linear IC like LM317.
• Smooth voltage adjustment from 1.2V to 24V.
• Easy to build with widely available components.
• Reliable because the 2N3055 is robust and can survive abuse if properly cooled.

Possible Improvements

You can make this design even better with a few tweaks:

• Add a current-limiting circuit with a shunt resistor to protect against overload.
• Include a digital voltmeter/ammeter module on the output for easy monitoring.
• Use a better op-amp (like LM301 or TL081) for improved stability compared to the older 741.
• Add small ceramic bypass capacitors close to the transistor leads for better noise performance.

Conclusion

The 2N3055 variable power supply is a classic electronics project. It’s straightforward, uses easily available components, and delivers a robust, adjustable power source for your workbench. By combining an op-amp regulator, a zener reference, and a powerful pass transistor, you get a stable voltage up to 24V at several amps.

If you’re serious about electronics as a hobby or profession, building your lab power supply is an excellent way to learn about regulation, feedback, and power transistor operation. It gives you both the theory and the practical experience you need while providing a tool you’ll keep coming back to.