Voltage Regulators Circuit Diagrams

# Variable Voltage Power Supply from Fixed Voltage Regulator

Outline

• Introduction
• Circuit Diagram of Variable Voltage Power Supply From Fixed Voltage Regulator
• Components Required
• Circuit Design
• Principle
• How to Calculate the Value of Resistance for the Different Voltages?
• Important Notes

## Introduction

In this project, I’ll teach you how to make a variable voltage power supply using a 7805 IC as a fixed voltage regulator.

The two series of three pin voltage regulators available on the market are 78XX and 79XX. The 78XX series of voltage regulators is for positive voltage supplies, thus if you need a +5V supply, you can use a 7805 voltage regulator. While the 79XX series is for negative supply, the 7905 regulator is utilised if you need a -5V supply.

Voltage regulators, such as the 7805, are used to provide a constant voltage at the output terminal that is independent of the input voltage (as long as it is greater than the required voltage).

What if you only need a single Linear Voltage Regulator IC to provide a range of output voltages? This concept can be put into action. I’ll now demonstrate how to create a Variable Voltage Power Supply using a Fixed Voltage Regulator.

The LM317 is a basic Variable Voltage Regulator IC that can produce voltages between 1.5 and 37 volts. However, you may get the same result using a fixed voltage regulator like the 7805. (although not the same range). This can be accomplished by combining two resistor.

## Circuit Diagram of Variable Voltage Power Supply From Fixed Voltage Regulator

### Components Required

• 230V to 12V Step Down Transformer
• Bridge Rectifier
• 7805 Regulator IC
• 1000μF Capacitor
• 0.22μF Capacitor
• 0.1μF Capacitor
• 470Ω Resistor x 2
• 100Ω Resistor
• 220Ω Resistor
• 330Ω Resistor
• Connecting Wires

## Circuit Design

The 230V to 12V Step Down Transformer’s primary is initially connected to the AC mains supply, while the secondary is connected to a bridge rectifier. The bridge rectifier’s output is filtered by a capacitor before being fed to the 7805 Voltage Regulator IC.

Between INPUT and COMM (GND of 7805), a 0.22F capacitor is connected, and between OUTPUT and COMM, a 0.1F capacitor is attached.

Now we’ll get to the fun part. Between the COM and the OUT lies a 470 ohm resistor. To switch between output resistors, a rotary switch is utilised, with the following resistors connected to it: 100, 220, 330, and 470.

## Principle

A simple project where a Variable Voltage Power Supply from Fixed Voltage Regulator is implemented here. The main principle behind the working of this project is very simple.

Connect two resistors R1 and R2 as shown in the image below: one between the output and GND Pin and the other between GND Pin and GND of power supply.

The amount of current flowing through R2 is a combination of current through R1 and the standby current of 7805. Depending on the output voltage requirement, we can calculate the value of this resistor and finally the output voltage can be calculated as follows.

VOUT = VREG + R2 * (VREG/R1 + IS

where VREG = 5V (for 7805) and

IS = standby current of 7805 (≈2.5mA).

Based on the above calculations, you can get anywhere between 5V and 12V using a 7805 Regulator from a 12V Supply.

The following images show the range of outputs obtained with a 7805 Voltage Regulator IC.

## How to Calculate the Value of Resistance for the Different Voltages?

Assume that the resistor connected between the com terminal and the regulator’s output terminal has a value of 470. This indicates that the current 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

Hence, we have  to attach resistor of 543 Ω with 470 Ω so to obtain the wanted output i.e. 12V. While it is difficult for us to get such a value of the resistor in the market so we can use the nearby value of the resistor i.e. 560 Ω.

Now if we wish to have some other voltage from 5V to 12V then we have to attach some other value of the resistor.

Suppose we need 6V, then

V =6V
I = 10.6mA
R = 6V/10.6mA
R =  566 Ω

But the resistor R1 is already on 470Ω which is already connected in the circuit, hence for 6V value of the resistor will be 100 Ω approximately (566-470=96).

In the same manner for different voltages different value of resistance will calculated. In spite of the different values of resistors available, a variable resistor can be used in the circuit to get different values of voltage.

## Important Notes

• When switching from one resistor to other, the load will get 12V.
• So, before switching, make sure that the load is disconnected (or you can completely switch off the power supply, make a switch and then switch on the power supply).

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