Power Supplies

# Variable Power Supply Circuits

Have you ever tried your hand at constructing a variable-rate power supply? This guide will walk you through the process of building a variable power supply circuit. While we’ve explored various power supply circuits before, the distinctive feature of this one is its ability to adjust both the output voltage and current.

### Outline

• Variable supply that can be varied from 1.2V to 30V at a current of 1 Amphere
• Output Video
• Circuit Diagram
• 0-28V, 6-8A Power Supply Circuit Diagram using LM317 and 2N3055
• Circuit Components
• Circuit Design
• LM317 Voltage Regulator
• 0-28V, 6-8A Power Supply Circuit Applications
• Note
• Variable Power Supply Circuit from Fixed Voltage Regulator
• Circuit Diagram
• Working
• How to Calculate the Resistance Value for the Different Voltage?

## Circuit Diagram

In electronics projects, prototypes, and for hobbyists, having a reliable power supply is essential. Typically, we rely on batteries to provide stable power for lower voltage requirements.

However, instead of using batteries with their limited lifespan, we can utilize a variable DC power supply in this project.

This is a robust, reliable, and user-friendly variable DC power source. Here’s how the circuit operates:

An AC supply is first reduced to 24V at 2A using a transformer. Then, a bridge rectifier is employed to convert this voltage into DC.

The resulting pulsating DC is smoothed out with a capacitor, producing a clean DC signal that is subsequently fed into the LM317 variable voltage regulator IC.

To adjust the output voltage, two variable resistors are used: one with a value of 1K for fine adjustments and another with a value of 10K for larger voltage changes.

The ADJ pin of the LM317 receives a fraction of the output voltage as feedback, allowing the output voltage to be modified based on the POT settings.

A capacitor is positioned at the voltage regulator’s output to ensure that the output voltage remains stable without any spikes.

You can modify the output voltage of this variable DC power supply within the range of 1.2V to 30V, with a maximum current rating of 1A. This circuit serves as a dependable DC power source and can even act as a substitute for batteries.

Due to the heat generated during its operation, it’s crucial to attach a heat sink to the LM317 voltage regulator IC for proper cooling.

### Note

Because the above circuit only employs a 15 volt transformer as an input, it can only be changed up to 15 volts. To increase the voltage up to 30 volts, a 30 volt input should be used.

## 0-28V, 6-8A Power Supply Circuit Diagram using LM317 and 2N3055

By making some adjustments, you can modify this circuit to deliver a current of up to 20 amps. To achieve this, ensure that you use an appropriately rated transformer and install a substantial heat sink equipped with a fan. The requirement for a large heat sink is essential because the 2N3055 transistors generate significant heat when operating at full capacity.

## Circuit Components

• 30V, 6A Step down Transformer
• Fuse F1 – 1 Amp
• Fuse F2 – 10 Amp
• Resistor R1 (2.5 watt) – 2.2k ohm
• Resistor R2 – 240 ohm
• Resistor R3, R4 (10 watt) – 0.1 ohm
• Resistor R7 –
• 6.8k ohm
• Resistor R8 – 10k ohm
• Resistor R9 (0.5 watt) – 47 ohm
• Resistor R10 – 8.2K
• Capacitors C1, C7, C9 – 47nF
• Electrolytic capacitor C2 – 4700uF/50v
• C3, C5 – 10uF/50v
• C4, C6 – 100nF
• C8 – 330uF/50v
• C10 – 1uF/16v
• Diode D5 – 1n4148 or 1n4448 or 1n4151
• D6 – 1N4001
• D10 – 1N5401
• D11 – LED red
• D7, D8, D9 – 1N4001
• Pot RV1 – 5k
• Pot RV2 – 47 ohm or 220 ohm, 1 watt
• Pot RV3 – 10k trimmer

## Circuit Design

While the LM317 voltage regulator provides protection against overheating and overload in the circuit, additional safeguards are in place through Fuses F1 and F2. Capacitor C1 handles a rectified voltage of approximately 42.30V (30 volts * SQR2 = 30V * 1.41 = 42.30), which is why all capacitors in the circuit need to be rated at 50 volts.

Potentiometer RV1 facilitates the adjustment of the output voltage within a range of 0 to 28 volts, taking into account that the LM317 voltage regulator has a minimum output voltage of 1.2V. To attain a 0V output, three diodes, namely D7, D8, and D9, are employed. For increased current handling, the circuit relies on the use of 2N3055 transistors.

The maximum allowable output current can be adjusted using Potentiometer RV2. For example, with a 100 ohm/1 watt potentiometer, the current is limited to 3 Amps at 47 ohms and 1 Amp at 100 ohms.

## LM317 Voltage Regulator

The LM317 is a 3-pin adjustable voltage regulator. It offers an output voltage range spanning from 1.2V to 37V with a current capacity of 1.5 amps. Operating this IC is straightforward, requiring only the use of two resistors to provide a variable power supply.

In comparison to fixed voltage regulators, the LM317 boasts internal current limiting, thermal shutdown, as well as improved line and load regulation. These characteristics make it a widely utilized component in a diverse array of applications.

## 0-28V, 6-8A Power Supply Circuit Applications

• DC supply is used in a variety of power amplifiers and oscillators.
• This circuit is utilised as an RPS
• (Regulated Power Supply) in appliances to give DC power to various electronic circuits.

### Note

This circuit is studied theoretically and may require some changes to implement it in practical.

## Variable Power Supply Circuit from Fixed Voltage Regulator

The fixed voltage regulator is used to provide a constant voltage at the output terminal regardless of the input voltage. The circuit for generating variable voltage power supply using fixed voltage regulators is shown below.

## Working

• When converting AC to DC, a bridge rectifier is employed.
• The voltage is then applied to the voltage regulator 7805.
• The resistance attached to the 7805’s common pin can be changed to change the regulator’s output.

## How to Calculate the Resistance Value for the Different Voltage?

Let’s consider a scenario where the resistor between the COM terminal and the output terminal of the regulator has a value of 470 ohms (referred to as R1). According to Ohm’s law (V = IR), this implies that the current flowing between the COM and output terminals is 10.6 mA. Additionally, there’s a minor standby current of 2.5 mA between the rotary switch and ground.

Consequently, the total available current is approximately 13.1 mA. Let’s assume that we need an output voltage ranging from 5 to 12 volts from the circuit. Initially, we have a minimum of 5V right at the regulator output. If we require 12V, we already have 5V between the COM and output terminals, and we need to select the remaining 7V using an appropriate resistor value.

Here R =?

V = 7V

I =13.1mA

Therefore V =I*R

R = 543ohm

As a result, we must combine a 543 ohm resistor with a 470 ohm resistor to get the desired output of 12V. While finding such a resistor value on the market is difficult, we can use a resistor with a similar value, such as 560ohm.

Now, if we want a different voltage between 5 and 12 volts, we’ll need to change the resistor value.
Let’s say we need 6 volts.

V =6V

I = 10.6mA

R = 6V/10.6mA

R = 566ohm

However, because the resistor R1 is already connected in the circuit at 470 ohm, the value of the resistor for 6V will be around 100 ohm (566-470=96). Different resistance values will be determined in the same way for different voltages.

Variable resistors can be used in the circuit to get varied voltage levels despite the different values of resistors.

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