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.
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.
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.