Making A Variable Power Supply With LM317T

An Overview Of LM317T

A variable power supply is useful in a laboratory, where an adjustable regulated DC voltage source is needed to supply the prototyping circuits. In a repair workshop, it is need to recharge any fully discharged battery cells, that the mobile devices were unable to recover the batteries.

My design on LM317T variable DC power supply

A regulated variable DC power supply are widely available online from any on line store especially Ali Express. They are very low cost, around 5 US Dollars. 

However, with existing components stock and in-house design facilities we can make it one with a simple adjustable DC voltage regulated IC such as LM317T.

LM317T sample from an online store

LM317T comes with TO-220 package to withstand with heat dissipation. The back panel must connect to a heat sink to make the heat transfer.

LM317T sample packages from the device datasheet.

It can create a variable voltage output within the range of 1.2 V to 37 V with a current of 1.5 A maximum rating. The variable voltage output is adjusted by two resistor divider network. Most of this IC are manufactured from a big semiconductor fab - the STMicroelectronics. 

This device comes with three pins as list below.

Device pin outs from the datasheet

The ADJUST pin creates a variable DC voltage output via a voltage divider network.

Basic adjustment circuit for LM317T

Resistor R1 and R2 must be a high power type to withstand with heat dissipation. A 1-W or a 2-W type is recommended for the design.

Variable output voltage relations.

Schematic And PCB Design

I have test the circuit in the simulator and a real hardware prototyping on the breadboard. It was done in my own workshop many years ago.

The circuit worked well, so I decide to draw the schematic and PCB in an EDA software.

Schematic Diagram

Resistor R1 is a fixed high power resistor, while RV1 is also a high power variable resistor. A low power resistors of both type could be burned whenever coping with a heavy load, or a higher voltage output.

Copper Side

PCB Top Silk

The complete design of this project could be download here.