Build Your 1K Adjustable Voltage Source Circuit

Description

An uncomplicated, albeit less effective, method for regulating a direct current voltage involves utilizing a potentiometer and a transistor emitter follower arrangement. The schematic below demonstrates employing a 1K potentiometer to determine the base voltage of a medium-sized NPN transistor. The collector of the NPN feeds into the base of a larger PNP power transistor, which delivers the majority of the current to the load. The output voltage will be approximately 0.7 volts beneath the voltage of the wiper of the 1K potentiometer, allowing for adjustment from 0 to the full supply voltage minus 0.7 volts. The utilization of two transistors achieves a current gain of approximately 1000 or more, necessitating only a few milliamps of current to be drawn from the voltage divider to supply several amps of current at the output. It is important to note that this circuit is less efficient than the 555 timer dimmer circuit employing a variable duty cycle switching approach. The figure below shows that a 25 watt / 12 volt lamp draws about 2 amps at 12 volts and 1 amp at 3 volts, resulting in energy loss when the lamp is dimmed as (12 - 3 volts * 1 amp) = 9 watts. A sizable heat sink is required to prevent the PNP power transistor from exceeding its operating temperature. The power consumed by the lamp is solely (3 volts * 1 amp) = 3 watts, yielding an efficiency factor of only 25% when the lamp is dimmed. The principal benefit of the circuit lies in its simplicity, and additionally, it doesn't generate any radio frequency interference as a switching regulator does. This circuit can be utilized as a voltage regulator if the input voltage remains constant, however, it won’t compensate for variations at the input as the LM317 does.

LM317

The LM317 is a three-terminal adjustable post-regulator integrated circuit. It’s widely used to generate a precise, adjustable output voltage from a variable supply. This IC is a versatile component for a broad range of analog circuits and is commonly found in power supply designs and voltage reference applications. The LM317 is a versatile and popular adjustable voltage regulator IC, known for its easy use and wide availability. Its adjustable output enables it to tailor the voltage to specific requirements in various electronic circuits, contributing to accurate and stable voltage delivery.

Circuit diagram