Adjustable Voltage Regulator Battery Charger Circuit using LM317

Introduction

Charging batteries efficiently and safely requires a regulated voltage supply. The LM317-based adjustable voltage regulator battery charger is a versatile and cost-effective solution for charging a variety of battery types, including lead-acid, Li-ion, and NiMH. This circuit is capable of adjusting the output voltage between 1.25V and 30V depending on the battery requirements. With a simple configuration, it offers protection and control, making it ideal for DIY electronics enthusiasts, students, and hobbyists.

This circuit utilizes the LM317 voltage regulator IC, which can supply up to 1.5A of current and has built-in protection features like thermal overload, current limiting, and safe operating area (SOA) protection. With the addition of capacitors for stability, resistors for voltage control, and a potentiometer for fine adjustment, this setup provides a reliable output suitable for charging.

The versatility of this design lies in its ability to adapt to different charging needs by merely adjusting the potentiometer. It not only ensures battery safety by preventing overcharging but also improves charging efficiency by delivering a stable voltage. Moreover, its compact design makes it suitable for portable and space-constrained applications. Whether you’re powering up small Li-ion cells or larger 12V batteries, this circuit provides an excellent platform for controlled and adjustable power delivery.

In this article, we will explore the schematic diagram, list of components, working principle, and application tips for building your own adjustable battery charger.

Components Required

Component	Value/Part Number	Description
Voltage Regulator	LM317 (TO-220)	Adjustable voltage regulator IC
Resistor R1	220Ω	Fixed resistor for voltage setting
Potentiometer RP1	10kΩ	Variable resistor for output voltage control
Capacitor C1	100µF	Input filter capacitor
Capacitor C2	0.1µF	Bypass capacitor for voltage stability
Capacitor C3	100µF	Output filter capacitor
Input Terminal	H1	DC input (3V to 30V)
Output Terminal	H2	Regulated DC output

LM317 Pinout

Circuit Diagram

Adjustable Voltage Regulator Battery Charger Circuit using LM317

Working Principle

The LM317 adjustable voltage regulator maintains a constant output voltage by comparing the voltage between its output and adjustment terminals. The output voltage is given by the formula:

Vout = 1.25V (1 + R1 / RP1) + Iadj × RP1

In most applications, the adjustment pin current (Iadj) is very small and can be neglected. So, the simplified formula becomes:

Vout ≈ 1.25V (1 + R1 / RP1)

In this circuit:

R1 (220Ω) sets the reference resistance.
RP1 (10kΩ Potentiometer) allows you to adjust the output voltage by varying the resistance.
Capacitors (C1, C2, C3) are used to stabilize the input and output voltages and prevent unwanted oscillations.

When a DC voltage between 3V and 30V is applied to the input (H1), the LM317 regulates and adjusts this voltage according to the settings of R1 and RP1. The regulated voltage is then available at the output terminal (H2).

Construction Steps

Gather all the components as listed in the table.
Place the LM317 on a breadboard or solder it to a PCB, ensuring pin orientation (IN – pin 3, OUT – pin 2, ADJ – pin 1).
Connect the input capacitor (C1 = 100µF) across the input pin and ground.
Add the bypass capacitor (C2 = 0.1µF) between the adjustment pin and ground.
Connect R1 between the output pin and the adjustment pin.
Connect the potentiometer (RP1) between the adjustment pin and ground.
Attach the output capacitor (C3 = 100µF) between the output pin and ground.
Add input and output connectors (H1 and H2) for easy power input and battery connection.
Optionally mount the LM317 on a heat sink if you are drawing high current to prevent thermal shutdown.
Double-check all connections before powering the circuit.