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Low-Voltage Adjustable Regulator

Need to operate a CD, DVD or MP3 player from the cigarette lighter socket in your car? Or perhaps run a digital still or video camera or some powered speakers from the power supply inside your PC? This Low-Voltage Adjustable Regulator will step the voltage down to what's needed. It has jumper shunts to select one of six common output voltages (from 3-15V) and depending on the input voltage and the heatsink(s) you use, it can deliver an output current of just over 4A.


Considering the price of batteries and the ever-growing array of small items of electronic gear designed to run from low-voltage battery
power, it’s not surprising that one of the most common requests from SILICON CHIP readers is for an adaptor so this kind of equipment can be run from either the power supply inside a PC or a cigarette lighter socket in a motor vehicle.

Click for larger image
Fig.1: the circuit is based on an LM317 adustable regulator and a PNP Darlington transistor (Q1) to boost the output current capability. The output voltage is set by the resistive voltage divider string on the regulator's OUT and ADJ terminals and depends on the jumper shunt installed.

Most of the battery-operated equipment we’re talking about is designed to operate at 3V, 6V or 9V whereas the voltages available from vehicle batteries or PC power supplies are rather more restricted. For example, there’s usually only either 12V or 24V available from vehicle batteries, while most PC power supplies only have 5V and 12V supplies readily available.

In addition, the voltage available from a vehicle battery can vary over a fairly wide range depending on whether the engine is running, the battery is being charged and whether the lights and/or air conditioning are on. This sort of voltage variation can cause problems for electronic circuits, as these generally perform much better and more reliably when operated from a regulated power supply.

This low-voltage adaptor has been designed for use in virtually any of these common DC voltage step-down applications. It can be connected to any convenient source of input voltage up to about 28V and is "programmed" using a push-on jumper shunt to deliver one of six output voltages: 3V, 5V, 6V, 9V, 12V or 15V. In each case the output voltage is well regulated, remaining very close to the selected voltage despite broad changes in both input voltage and load current level.

Circuit description

The circuit is shown in Fig.1. The heart of the adaptor is an LM317T adjustable 3-terminal regulator which comes in a TO-220 package. The LM317 is designed to maintain the voltage between its output (OUT) and adjustment (ADJ) terminals at close to 1.25V. At the same time, the current level through its ADJ terminal is maintained at a very low level (typically 50mA) and varies by less than 5mA over the full rated load current range (10mA - 1.5A) and the input-output voltage range of 3-40V.

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