Adjustable Voltage Regulators For Electronics Mini Projects

How to use Adjustable Voltage Regulator IC LM317?

In our previous post, we have seen the 7800 series regulator ICs which will provide fixed output voltage.
In market some regulator ICs are available whose output voltage is within certain voltage range (ie, with minor adjustment we can alter the output voltage).
The most popular adjustable voltage reference ICs available are LM317, TL783 and LM337T.
In this post let us see how to use the Adjustable Voltage Regulator ICs? What are the constraints? What are the limitations etc.

Adjustable Voltage Regulator LM317: An Introduction

The LM317 is an adjustable three terminal positive voltage regulator IC.
It can supply output voltage from 1.2V to 37V with 1.5A load.
To adjust the output voltage, it requires an external resistor and a potentiometer.

Remember the following two points about the LM317 regulator.

  • In operation, the LM317 will maintain a nominal voltage of 1.25V reference (Vref) between its output and adjustment terminals.
  • The regulated output voltage is given by:

Vout = Vin \left ( 1+\frac{R_{2}}{R_{1}} \right )+ Iadj .R_{2}

The typical Iadjust value is 50μA. 

Vout = Vin \left ( 1+\frac{R_{2}}{R_{1}} \right )+ 50u .R_{2}

  • The power dissipation of the LM317 is calculated by the following formula:

Power Dissipation = (Vin - Vout)\times Iout

The maximum power should not exceed 20watts. The LM317 IC is manufactured by TI (Texas Instrument) and OnSemi.

LM317 Adjustable Voltage Regulators Operation:


  • The LM317 is designed to accept an unregulated input voltage of up to 37 V and can output a maximum current of 1.5 A
  • By providing reference voltage, the regulator IC's output voltage can be altered.
    The reference voltage is applied by means of voltage divider circuit(R1 and R2).
  • Increasing R2 increases the reference voltage consequently increases the output voltage.
  • Capacitor  should be included at the input side, if the regulator is far from the power source.
  • It should be around 0.1 μF or so.
  • Capacitor is included in the output side to eliminate voltage spikes at the output. It should be around 0.1 μF or larger.
  • If the output voltage is more than 25V it is recommended to place diode across input and output terminals as shown in the below figure.
  • It will prevent the discharge of output capacitor thru IC when the input short circuited.


LM317 Adjustable Regulator IC Pin Identification:


Pin-1: Adjust
Pin-2: Output
Pin-3: Input

The TL783 is another positive adjustable regulator that can output a regulated voltage of from 1 to 125 V, with a maximum output current of 700 mA.
The LM337T, unlike the previous two regulators, is an adjustable negative voltage regulator. It can output a regulated voltage of from −1.2 to −37 V, with a maximum output current of 1.5 A.

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