Simple 25V /1.5 A Adjustable Power Supply

          Simple & robust           

Introduction

Some time ago, I got a request for a simple 1.2-25 V adjustable power supply.
Well, it is not my design, but this application note of the LM317 of  National Semiconductor is the simplest
adjustable power supply I know. I use two of them in my home lab.

Some features:

• Adjustable from 1.2V - 25 V
• Current output 1.5 Amp
• Short circuit proof
• Thermally protected
   

Other features as adjustable current limit, starting a 0 Volt, higher output voltage
and higher output current are possible too: see the LM317 specification.
 

Circuit description

The circuit is built with very little components.
First a simple DC supply of about 30 V is built with a transformer, a Graetz bridge rectifier
and a large elcap.
The LM317 stabilizes the output voltage Vout at a voltage level set by R1.

25V adjustable power supply schematic:

This is how it works: The LM317 will try to maintain 1.25 V between pin 2 and 3;
to obtain this it will change the voltage on pin 2 (=Vout)
This results in the following formula for the output voltage:

Vout = 1.25Vx(1+(R2/R1)

(I forget about the 100uA current from pin 2 which will induce a
100uA x R2 voltage difference)

Note that the LM317 must be mounted on a heat sink, at low output voltages it may dissipate up to
45 Watts when used as above!
 

→ Download specifications of key components at the  Component specs page

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