Power Supplies

# Variable Power Supply Circuits

Have you ever attempted to create a variable-rate power supply? This tutorial will show you how to create a variable power supply circuit. We’ve seen a number of power supply circuits up to this point, but the key advantage of this one is that it can alter 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

A DC Variable For electronics projects, prototypes, and hobbyists, power supply is critical. We usually use batteries as a reliable supply for lower voltages.

A variable DC power supply, which is employed in this project, can be used instead of batteries, which have a limited lifespan.

It’s a variable DC power source that’s tough, dependable, and simple to use. The circuit’s operation is as follows.

The AC supply is stepped down to 24V at 2A using a transformer. To convert this voltage to DC, a bridge rectifier is utilised.

This pulsating DC is filtered with a capacitor to produce a clean DC, which is then fed to the LM317 variable voltage regulator IC.

Two variable resistors with values of 1K and 10K are used to alter the output voltage. For substantial changes in voltage, 10K POT is employed, whereas 1K POT is used for tiny adjustments.

The ADJ pin of the LM317 receives a little amount of the output voltage as feedback, and the output voltage is modified, depending on the POT settings.

A capacitor is used at the output of the voltage regulator so that the output voltage doesn’t have any spikes.

The output voltage of this variable DC power supply can be adjusted from 1.2V to 30V at a current of 1A. This circuit can be used as a reliable DC source or as a battery replacement.

Because the voltage regulator IC LM317 gets hotter during operation, it’s critical to connect it to a heat sink.

### 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

With a little tweaking, this circuit can provide a current of 20 amps (use proper rating transformer and a huge heat sink with fan). This circuit requires a huge heat sink since 2N3055 transistors generate a lot of heat when they’re fully loaded.

## 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

Although the LM317 voltage regulator safeguards the circuit from overheating and overload, the power supply circuit is protected by the Fuses F1 and F2. Capacitor C1 has a rectified voltage of roughly 42.30V (30 volt *SQR2 = 30v *1.41 = 42.30).

As a result, we must use all of the capacitors in the circuit that are rated at 50 volts. Pot RV1 allows us to adjust the output voltage from 0 to 28 volts. The LM317 voltage regulator’s minimum output voltage is 1.2V.

Three diodes, D7, D8, and D9, are used to achieve 0V at the output. To get greater current, 2N3055 transistors are employed.

The maximum current allowed at the output is set with Pot RV2. The output will be 100 ohm/1 watt if you use a 100 ohm/1 watt potentiometer current is limited 3 Amps at 47 ohms and 1 Amp at 100 ohm.

## LM317 Voltage Regulator

The 3 pin series adjustable voltage regulator is the LM317. The output voltage of this regulator ranges from 1.2V to 37V at 1.5 amps. This IC is simple to operate, and the variable supply is provided by simply two resistors.

When compared to fixed voltage regulators, it has internal current limiting, thermal shut down, and more line and load regulation. Because of all of these characteristics, these ICs are commonly employed in a wide range 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?

Assume that the resistor between the com terminal and the regulator’s output terminal has a value of 470 ohm (R1). This indicates that the current between com and output is 10.6 mA (since V = 5V and V=IR). There is a small amount of standby current of 2.5 mA between the rotary switch and ground.

As a result, approximately 13.1 mA of total current is available. Assume that we require 5 to 12 volts from the circuit. We received 5V minimum right from the regulator output. If 12V is required, 5V is accessible between com and output, and the remaining 7V must be selected using the proper 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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