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 guide you through the process of creating a variable voltage power supply using a 7805 IC, which functions as a fixed voltage regulator.

Voltage regulators on the market are typically categorized into two series with three-pin configurations: the 78XX series and the 79XX series. The 78XX series of voltage regulators is designed for positive voltage supplies. Therefore, if you require a +5V power supply, you can employ a 7805 voltage regulator. Conversely, the 79XX series is intended for negative supply applications, and if you need a -5V power source, the 7905 regulator is the suitable choice.

Voltage regulators, exemplified by the 7805, serve the purpose of delivering a consistent voltage at the output terminal, regardless of variations in the input voltage, provided it remains above the required voltage level.

But what if you only require a single Linear Voltage Regulator IC to offer a spectrum of output voltages? This idea can certainly be put into practice. I will now illustrate the process of crafting a Variable Voltage Power Supply employing a Fixed Voltage Regulator.

The LM317 represents a fundamental Variable Voltage Regulator IC capable of generating voltages spanning from 1.5 to 37 volts. Nevertheless, a similar outcome can be achieved using a fixed voltage regulator like the 7805, albeit within a different voltage range. This can be achieved by incorporating two resistors into the circuit.

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
• Mini Breadboard
• Connecting Wires

Circuit Design 

Initially, the primary winding of the 230V to 12V Step Down Transformer is linked to the AC mains supply, while the secondary winding is connected to a bridge rectifier. The output of this bridge rectifier undergoes filtration via a capacitor before being directed to the 7805 Voltage Regulator IC.

A 0.22F capacitor is placed between the INPUT and COMM (GND of 7805), while a 0.1F capacitor is linked between OUTPUT and COMM.

Now, let’s dive into the intriguing part. Positioned between COM and OUT is a 470 ohm resistor. To facilitate the switching of output resistors, we employ a rotary switch, which connects to the following resistors: 100, 220, 330, and 470.

Principle

This project involves the creation of a Variable Voltage Power Supply using a Fixed Voltage Regulator. The fundamental principle underlying this project is quite straightforward.

Connect two resistors, R1 and R2, as illustrated in the image below. Place one resistor between the output and GND Pin, and position the other resistor between the GND Pin and the ground of the 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.

V_OUT = V_REG + R₂ * (V_REG/R₁ + I_S)

where V_REG = 5V (for 7805) and

I_S = 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?

Suppose we have a resistor connected between the COM terminal and the regulator’s output terminal with a value of 470 ohms. This implies that the current passing through it is 10.6 mA (calculated as V = 5V and V = IR).

Additionally, there’s a minor standby current of 2.5 mA between the rotary switch and ground, resulting in a total available current of approximately 13.1 mA.

Let’s consider that we need an output voltage ranging from 5 to 12 volts from the circuit. Initially, we have a minimum of 5V directly from the regulator output. If we require 12V, we have the 5V available between COM and output, and the remaining 7V can be obtained by selecting the appropriate 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).