LM317 Adjustable Voltage Regulator Circuit Working Explanation

The adjustable voltage regulator can be adjusted to provide any d.c. output voltage that is within its two specified limits.

The most popular three-terminal integrated circuit adjustable voltage regulator is the LM317 which has an input terminal, output terminal, and adjustment terminal.

An external voltage divider is used to change the output d.c. voltage of the regulator.

It can also be used to make a programmable output regulator circuit or by connecting a fixed resistor between the adjustment and output terminals to use as a precision current regulator circuit.

LM317 Pin configuration

Pin Number Pin Name Description
1 Adjust This pins adjusts the output voltage
2 Output Voltage (Vout) The regulated output voltage set by the adjust pin can be obtained from this pin
3 Input Voltage (Vin) The input voltage which has to be regulated is given to this pin

Features of LM317

  • Adjustable 3-terminal positive voltage regulator
  • Output voltage can be set to range from 1.25V to 37V
  • Output current is 1.5A
  • Maximum Input to output voltage difference is 40V, recommended 15V
  • Maximum output current when voltage difference is 15V is 2.2A
  • Operating junction temperature is 125°C
  • Available in To-220, SOT223, TO263 Package

LM317 Adjustable voltage regulator Circuit diagram

Schematic of standard lm317 adjustable voltage regulator circuit shown below.

Components used

  • U1 - LM317
  • R1 - 240Ω
  • R2 - 5k POT
  • C1 - 0.1uF
  • C2 - 1uF/50V

Working principle of LM317 Adjustable voltage regulator circuit

The working principle of LM317 adjustable voltage regulator circuit is simple.

The operating voltage of LM317 is 3V to 40V DC and can supply 1.5 Amp of load current over an adjustable output range of 1.25V to 37V.

In operation, the IC lm317 develops and maintains a nominal 1.25 V reference (Vref) between its output and adjustment terminals.

This reference voltage is converted to a programming current (IPROG) by resistor R1, and this constant current flows through potentiometer R2 to the ground.

By changing R2, a wide range of output voltage can be obtained, and a suitable heatsink should be placed to dissipate excess heat from the IC.

The output voltage Vout is dependent on values R1 and R2, according to the following formula:

Vout = 1.25 (R2/R1 + 1)

A disc or tantalum input bypass capacitor C1 is recommended to reduce the sensitivity to the input line impedance.

And, an output capacitance C2 in the form of a tantalum or aluminum electrolytic capacitor on the output swamps, this effect and ensures stability.

LM317 Adjustable voltage regulator - Demo & Testing

I simply assembled the circuit on a breadboard and connected it to the 18V DC supply.

Testing LM317 adjustable voltage regulator circuit
Testing the LM317 adjustable voltage regulator in a breadboard.

Here, the 5K POT(potentiometer) is used to adjust the output voltage, and a digital multimeter measures it.

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