Variable Voltage Power Supply Using LM317

A widely used and necessary electronic device in the fields of electrical and electronic engineering is the Variable Voltage Power Supply with LM317. The need for dependable and adaptable power sources is become more and more important as technology develops. A key element in accomplishing this objective is the integrated circuit voltage regulator LM317, which offers an ideal basis for building adjustable voltage power supply.

In order to develop a variable voltage power supply, this project will explore the possibilities of the LM317. One important aspect that makes this power supply design so useful in a variety of electronic circuits, from basic hobby projects to complex electronic systems, is its ability to control and alter the output voltage within a set range. The basis for achieving a reliable and adaptable power supply is the LM317, which is known for its accuracy, stability, and user-friendliness.

In this study, we explore the operational properties of the LM317 and the theoretical basis of voltage regulation. It is essential to understand the LM317 datasheet since it offers information about the pin layout, voltage regulation system, and current limitations of the device. Electronics hobbyists and engineers may build power supplies that are customised to meet their individual requirements by fully utilising the LM317 by understanding these qualities.


img 20231220 0029012550234515326277769

Circuit Schematic



  • LM317 Voltage Regulator
  • 12-0-12 Step Down Transformer
  • 1N4007 Diode
  • 220 Ω Resistor
  • 10 KΩ Variable Resistor
  • 0.1uF Ceramic Capacitor
  • 1uF/50V Electrolytic Capacitor

Circuit Connection

The 12-0-12 AC transformer is used as a step-down transformer in this variable voltage power supply. A full-bridge rectifier is linked to the centre tape of the transformer and either end of the secondary coil. Connecting four diodes (1N4007) to one another—designated as D1, D2, D3, and D4 in the circuit schematics—makes a full-bridge rectifier. The cathode and anode of D1 and D2 are linked to any end terminal, while the cathode and anode of D4 and D3 are connected to the secondary coil’s centre tap. The anodes of diodes (D1 and D4) are connected from which another terminal is removed for output from the full-wave rectifier, and the cathodes of diodes (D2 and D3) are connected from which one terminal is removed for rectifier output.

Between the full-wave rectifier’s output terminals is a 0.1 uF ceramic capacitor. An LM317 regulator is linked in parallel to the capacitor to control voltage. To modify the voltage, a voltage divider with a 10 KΩ variable resistor and a 1 uF/50V capacitor connected in parallel to the output would compensate for transient currents. For short circuit protection, a protection diode is attached between the voltage regulator IC’s input and output voltage terminals.

About LM317 Regulator IC

An IC for positive voltage regulation is called LM317. The integrated circuit consists of three pins: 1. the input pin, which can supply about 40V DC; 2. the output pin, which drives the output voltage between 1.25V and 37V DC; and 3. the adjust pin, which modifies the output voltage in accordance with the applied input voltage. The output can range from 1.25 V to 37 V DC for input up to 40 V.

What is Filtering?

The procedure of filtering a DC signal involves employing a capacitor. A pure DC voltage is not what the full-wave rectifier produces as its output. Ripples are the noises that will be combined with this. Consequently, a capacitor must be connected in parallel to the full-wave rectifier’s output in order to filter it. During a cycle, the capacitor charges and discharges, producing an output of pure, rectified DC voltage. However, we are unable to eliminate every ripple from the rectifier. Therefore, the rectifier circuit’s output is connected to a 100 uF/25V capacitor. All of the AC that passes through this capacitor is directed towards the ground in its filtering capacity. The remaining mean DC voltage is filtered and free of ripples at the output. The AC signal is filtered by a 0.1 uF capacitor.

Working Principle of Adjustable Voltage Power Supply

The main power source in this adjustable voltage power supply has an AC voltage of 220V–230V, which must be further reduced to 12V. An apparatus called a step-down transformer is used to convert 220V AC to 12V AC. We can produce both positive and negative voltage at the input by using the transformer.

To be used, the stepped-down AC voltage must be changed to DC voltage. The process of changing AC voltage to DC voltage is called rectifying. A full-wave bridge rectifier is utilised in this adjustable voltage power supply circuit to convert 12V AC to 12V DC. Four diodes (1N4007) are coupled in a full-wave bridge rectifier such that current passes through them only in one direction. Two diodes become forward biassed and two more diodes become reverse biassed during full-wave rectification.

While diodes D1 and D3 are reverse biassed and allow current to flow through the output terminals of D2 and D4, respectively, diodes D2 and D4 are under forward bias during the positive half cycle. The current passes through diodes D1 and D3 during the negative half-cycle while they are in the forward bias and diodes D4 and D2 are in the reverse bias. Both present routes are going in the same direction.

A resistive voltage divider, also known as a variable resistor, is utilised between the output pin and ground of this adjustable voltage power supply to establish the required voltage at the LM317’s output. The resistive voltage divider has a value of 100 KΩ, which enables it to give the output with the necessary voltage range. This adjustable voltage power supply circuit may achieve the required voltage by varying the potentiometer.

A steady reference voltage of 1.25V is supplied across the adjustment pin by the LM317. The minimal load current of 10 mA is offered by the LM317.

Why Need to Use Zener Diode in Power Supply?

A common component for driving the voltage across a tiny circuit and serving as a voltage reference is the zener diode. It can be applied to different load current circumstances to generate a minimally rippled, stabilised voltage output. when the diode’s breakdown voltage (VZ) is reached by the load voltage. When the diode is conducting, the series resistor reduces the surplus voltage and restricts the amount of current that flows through it. so that the circuit as a whole can stay stable.

Applications of Adjustable Voltage Power Supply

A variable voltage power supply can be used in a wide range of electrical and electronic applications. Typical applications include the following:

  • Electronics Prototyping: Excellent for electrical circuit testing and prototyping, since various components could need different voltage levels.
  • Laboratory Experiments: Adjustable power supply are essential for powering and testing electrical circuits, components, and gadgets in educational and research contexts.
  • Battery Charging: It is helpful for supplying varying battery-powered devices with flexibility by meeting the voltage requirements of the batteries.
  • Testing and Calibration: When exact voltage levels are required, testing and calibration of electronic equipment are important.
  • Custom Power Requirements: Any application, including specific research projects or specialised electrical installations, that requires a flexible power supply.

Leave a Comment