Adjustable Voltage Power Supply Using LM317

In this tutorial, I will show how to make an adjustable voltage power supply with LM317 regulator ic which inputs AC supplies (220V-230V AC) and outputs DC voltage. An adjustable voltage power supply is commonly used for voltage regulation in today’s circuit industry. This ic has the ability to control over-voltage protection, internal current controlling, overload protection, and many more. Another most important thing is this adjustable voltage power supply can be made by using only a few components. So, the LM317 regulator ic is very efficient and cheap to use and assemble in a circuit. This ic has an adjustable output voltage of 1.28 V to 12 V and has a maximum of 1.5 A current. The step by step process of the circuit diagrams is given.

Circuit Diagram

adjustable voltage power supply

Components Required

  • 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

In this adjustable voltage power supply, a step-down transformer of the 12-0-12 AC transformer is taken. Anyone end of the secondary coil and center tape of the transformer is connected with a full-bridge rectifier. The full-bridge rectifier is made by connecting four diodes(1N4007) to each other designated as D1, D2, D3, and D4 in the circuit schematics. The cathode of D1 and the anode of D2 is connected to anyone end terminal and cathode of D4 and anode of D3 is connected to the center tap of the secondary coil. The cathodes of diodes (D2 and D3) are connected from which one terminal is taken out for output of rectifier and anodes of diodes(D1 and D4) are connected from which another terminal is taken out for output from the full-wave rectifier.

A 0.1 uF ceramic capacitor is connected between the output terminals of the full-wave rectifier. For voltage regulation, an LM317 regulator is connected in parallel to the capacitor. A 10 KΩ variable resistor is connected in a voltage divider with the regulator IC for voltage adjustment and a capacitor of 1 uF/50V is connected in parallel to the output for compensating transient currents. There is a protection diode connected between the input voltage and output voltage terminals of the voltage regulator IC for short circuit protection.

Working of Adjustable Voltage Power Supply

In this adjustable voltage power supply, the voltage of the main power supply is 220-230 V AC which further needs to be stepped down to 12 V. To reduce the 220 V AC to 12 V AC, a step-down transformer is used. The use transformer allows us to generate both positive and negative voltage at the input.

The stepped-down AC voltage needs to be converted to DC voltage for use. The rectification is the process of converting AC to DC voltage. In this adjustable voltage power supply circuit, a full-wave bridge rectifier is used for converting the 12 V AC to 12 V DC. In a full-wave bridge rectifier, four diodes (1N4007) are connected in such a way that current flows through them in only one direction. During full-wave rectification, the two diodes become forward biased and another two diodes get reverse biased.

During the positive half cycle, diodes D2 and D4 are in forward bias while diodes D1 and D3 are reverse biased and the current flow respectively through the output terminal D2, and D4. During the negative half-cycle, diodes D1 and D3 are in forward bias, and diodes D4 and D2 are in reverse biased and the current flows through D1 and D3. The direction of both current ways is the same.

Filtering is the process of the DC signal done by using a capacitor. The output from the full-wave rectifier is not a pure DC voltage. This will be mixed with some noises called ripples. Therefore, it needs to be filtered by connecting a capacitor in parallel to the output of the full-wave rectifier. The capacitor charges and discharges during a cycle giving a pure and rectified DC voltage as output. But we can’t remove all the ripples from the rectifier. So, a capacitor of 100 uF/25V is connected to the output of the rectifier circuit. This capacitor acts as a filtering capacitor which diverts all the AC through it to the ground. At the output, the mean DC voltage left is filtered and ripples free. A capacitor of 0.1 uF is used for filtering the AC signal.

LM317 is a positive voltage regulator IC. The IC has three pins: (1. Input pin which can be supplied about 40 V DC, 2. Output pin which drives output voltage in the range of 1.25 V to 37 V DC, and 3. Adjust pin which is used to vary the output voltage corresponding to the applied input voltage). For input up to 40 V, the output can vary from 1.25 V to 37 V DC. 

At this adjustable voltage power supply, for setting the desired voltage at the output of LM317, a resistive voltage divider(Variable Resistor) is used between the output pin and ground. The value of the resistive voltage divider is 100 KΩ that it can provide the required voltage range at the output. By adjusting the potentiometer, this adjustable voltage power supply circuit can obtain the desired voltage.

The LM317 provides a stable reference voltage of 1.25 V across the adjustment pin. The LM317 provides a minimum load current of 10 mA.

This adjustable voltage power supply circuit can be used in various types of voltage source applications.

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