by Mark Balch
Just For Starters
Basics For Beginners
Just For Starters
Easy Ways to Power Your Project
Figure 1. AC sine wave showing peak and RMS levels
Every electronics project
requires power to operate. Though power systems can get quite complex,
many projects are well served
with a few basic solutions.
Designing a power system is
easier when high voltages and
currents are not required.
Power systems are also simplified when
the circuits being driven are not operating at high speeds and when components are not very sensitive to electrical
noise. This article discusses power system design for a typical electronics project with low to moderate power needs.
The techniques presented here can be
applied in more demanding applications, though more analysis and care is
required. A basic power system can be
considered in three sections: raw power
source, power regulation, and power distribution. Safety concerns dominate all
three areas. This article stresses the use
of off-the-shelf building blocks that
reduce risk and design effort.
In the US, AC power is nominally
110-120 volts. (AC power is nominally
220-240 VAC in Europe.) When was the
last time you saw a logic chip run at 100
volts?! The step-down process generates
a lower voltage that is not directly usable
by ICs, but that is low enough for a voltage regulator to handle. Batteries offer
less complexity here as well, because
they are already low-voltage sources.
Typical batteries have cell voltages from
one to two volts, allowing you to combine as many as necessary to generate
higher voltages. Working with AC is a
safety headache. High voltages, step-down transformers, and rectifiers all
pose problems that are best avoided.
However, this doesn't mean that you
should be restricted to batteries. A famil-
iar "wall-wart" AC-to-DC power
module is a great way to use
AC power with all the high-voltage safety problems taken
care of for you.
Wall-warts are available in
varying output voltages and
currents. Always make sure
that the module is certified by
Underwriters' Laboratories (UL) for safety. Whether you use a wall-wart or batteries, the power design task is made easier by working with low-voltage DC. It is
easiest to select a wall-wart or battery
configuration that provides the lowest
practical voltage. If your voltage regulator requires 5. 5 V, it is better to use a 6 V
module rather than a 12 V module. We'll
talk about a regulator's voltage requirements later.
If you're using batteries, you have to
determine how many are required based
on the nominal cell voltage of each battery's chemistry. Alkaline batteries have
a 1.5 V nominal cell voltage.
The most common forms of raw
power available to most of us are the
AC wall outlet and batteries. Solar cells
run a close second for some and, for
the purposes of this discussion, may be
considered as batteries. AC-powered
devices must somehow convert AC to
DC because integrated circuits (ICs)
run on DC. (Refer to the sidebar, "AC
versus DC Power" for more information.) The conversion process is called
rectification. Batteries, on the other
hand, natively supply DC power.
AC-powered devices must also
step-down the high voltage AC to a
more manageable DC voltage.
AC Versus DC Power
Direct-current — or DC — power is delivered at a static voltage. A battery
provides DC power.Alternating-current — or AC — power alternates its voltage
level between positive and negative polarity on each half of the sine wave.
Figure 1 shows an AC power signal that might be measured from a standard
wall outlet. The 110-120 VAC nominal level is actually a mean value because the
voltage is constantly changing. The nominal level of an AC sine wave is its root-mean-square (RMS) value. The RMS value of an AC sine wave is equal to the peak
sine wave amplitude divided by one half the square root of two, or approximately
0.707 times the peak amplitude. A 120 VAC RMS level corresponds to a peak
voltage of nearly 170 V. An AC power signal is rectified using diodes, or rectifiers,
to achieve a constant polarity. Figure 2 shows a rectified version of the power
signal in Figure 1. This rectified, but non-static signal is then filtered and regulated
to provide a static DC power supply.