HOW TO MAKE A VARIABLE POWER SUPPLY USING LM317T

In this post we will elaborately discuss how to build a simple LM317 based variable power supply circuit using minimum number of external components.

Contents

1 Introduction

2 Essential Features of a Variable Power Supply Circuit

3 How to Use LM317 for Making a Variable Power Supply

4 How to Connect LM317 in an Adjustable Voltage Power Supply Design

5 Adding a Current Control Feature

6 Enhancing an LM317 Variable Power with High Current Output Facility

Introduction

Whether it’s an electronic noob or an expert professional, a variable power supply unit is required by everybody in the field. It is the basic source of power that may be required for various electronic procedures, right from powering intricate electronic circuits to the robust electromechanical devices like motors, relays etc. A variable power supply unit is a must for every electrical and electronic work bench and it’s available in a variety of shapes and sizes in the market and also in the form of schematics to us. These may be built using discrete components like transistors, resistors etc. or incorporating a single chip for the active functions. No matter what the type may be, a power supply unit should incorporate the following features to become a universal and reliable with its nature:

2 Essential Features of a Variable Power Supply Circuit

It should be fully and continuously variable with its voltage and current outputs. Variable current feature can be taken as an optional feature because it’s not an absolute requirement with a power supply, unless the usage is in the range of critical evaluations. The voltage produced should be perfectly regulated.

With the advent of chips or ICs like LM317, L200, LM338, LM723, configuring variable power supply circuits with the above exceptional qualities has become very easy nowadays.

3 How to Use LM317 for Making a Variable Power Supply

Here we’ll try to understand how to construct a simplest power supply circuit using the IC LM317. This IC is normally available in TO-220 package and has three pin outs. The pin outs are very easy to understand, since it consists of an input, an output and an adjustment pins that just needs to be wired up with the relevant connections. The input pin is applied with a rectified DC input, preferably with the maximum tolerable input, that’s 24 volts as per the specs of the IC. The output is received from the “out” pin of the IC while the voltage setting components are connected around the adjustment pin.

4 How to Connect LM317 in an Adjustable Voltage Power Supply Design

As can be seen the diagram, the assembly needs hardly any components and is in fact a child’s play to get everything in place. Adjusting the pot produces a linearly varying voltage at the output that may be right from 1.25 volts to the maximum level supplied at the input of the Ic. Though the shown design is the simplest one and therefore includes only a voltage control feature, a current control feature can also be included with the IC.

5 Adding a Current Control Feature

The figure above shows, how the IC LM317 can be effectively used for producing variable voltages and currents, as desired by the user. The 5K pot is used for adjusting the voltage, whereas the 1 Ohm current sensing resistor is selected appropriately to acquire the desired current limit.

6 Enhancing an LM317 Variable Power with High Current Output Facility

The IC can be further enhanced for producing currents higher than its rated values. The diagram below shows how the IC 317 can be used for producing more than 3 amps of current.