LM317 Voltage Regulator with Pass Transistor – 4.3A Build

Description

The LM317T output current can be augmented through the incorporation of an auxiliary power transistor to manage a portion of the total current flow. The degree of current division is determined by a resistor placed in series with the LM317's input terminal and a resistor positioned in series with the transistor’s emitter. As illustrated below, the pass transistor commences conduction when the LM317’s current reaches approximately 1 amp, due to the voltage difference across the 0.7 ohm resistor. Current limiting occurs at roughly 2 amps for the LM317, resulting in a voltage drop of around 1.4 volts across the 0.7 ohm resistor and a 700 millivolt decrease across the 0.3 ohm emitter resistor. Consequently, the total current is restricted to around 2+ (.7/.3) = 4.3 amps. The input voltage must be roughly 5.5 volts above the output at maximum load, and heat dissipation at full load would be approximately 23 watts, necessitating a substantial heat sink for both the regulator and the pass transistor. The filter capacitor's size can be calculated using the formula C=IT/E, where I represents the current, T signifies the half cycle time (8.33 mS at 60 Hertz), and E is the voltage decline during one half cycle. To maintain ripple voltage below 1 volt at 4.3 amps, a capacitor with a capacitance of 36,000 uF or greater is required.

More about components

The LM317T is a three-terminal adjustable post-regulation shunt regulator IC. It is a versatile device used in a variety of power supply applications. This IC allows for the adjustment of the output voltage within a specified range, typically from 1.2V to 33V, depending on the external resistors connected to the trim and adjust pins. It consists of a Zener diode, a PNP transistor, and a current-limiting resistor, all integrated into a single package. The LM317 is commonly employed in applications such as voltage regulators, power supply stabilization, and current limiting circuits. It is a popular choice due to its small size, low cost, and ease of use. Its adjustable output makes it suitable for diverse power supply designs where a precise output voltage is needed. The internal Zener diode provides the desired output voltage, while the PNP transistor regulates the current flow, ensuring stable operation. The IC's robust design and wide operating voltage range contribute to its reliability and versatility.

Circuit diagram