differential across the bulb’s filament, the fila-
ment has an engineered amount of resistance,
and the resistance generates enough heat to
make it glow brightly. The relationship between
the voltage delivered and the resistance of the
filament determines the current draw through
the circuit.
Nonlinear loads, which work on the princi-
ple of inductance, include devices such as
transformers, motors, solenoids, and relays.
These have a more complex relationship be-
tween volts and amps.
First, a magnetic field develops around any
wire or conductor through which current is flow-
ing. The strength of the magnetic field is propor-
tional to the amount of current (i.e., amperage).
Second, we also know that we can induce
electrical current flow through a conductor by
moving a magnet rapidly in close proximity to
the conductor. A transformer is simply a pair of
wire coils in close proximity to one another, with
an iron core in the middle to contain the mag-
netism. As we pass alternating current through
one of the transformer’s coils (the primary wind-
ing), an alternating magnetic field is created. The
field rapidly rotates around the secondary wind-
ing, and electrical current flow is induced (at 60
times per second, or 60 Hz, in North America,
and 50 Hz in most of the rest of the world).
Sounds perfect, right? Well, it isn’t quite.
In fact there is a loss through the trans-
former, resulting in some inefficiency. The
level of this inefficiency is a function of har-
monic voltage levels at a transformer’s primary
winding versus load-generated harmonic cur-
rent (nonlinear) levels at the output terminals
on the secondary side of the transformer, and
a phase relationship is established. To explain
more simply, remember that both voltage and
amperage in AC circuitry are not constant as
in DC circuitry. Alternating current (and volt-
age) is delivered as waves—there are peaks and
valleys. When we measure 120 VAC with a
multimeter, the actual peak voltage is usuallyanalyzing harmonic distortion 77and will smooth out the curve, or wave. It is
important for boatowners and technicians to
be aware of this because some equipment
requires true-sine-wave output to function prop-
erly, as discussed on page 75.Sine waveSquare waveModified sine wave120V170V150VPulse width varied to
maintain 120 volts RMSA comparison of inverter waveforms. (Reprinted
with permission from Boatowner’s Illustrated Elec-
trical Handbook,second edition, by Charlie
Wing)Load Types
AC loads can be divided into two categories:
linear and nonlinear. Linear loadsbasically
draw current in proportion to the voltage
delivered, much like a DC load where Ohm’s
law applies. That is:
amperagevolts ohms
A basic incandescent lightbulb is a good
example of a linear load. There is a voltage