TRANSFORMERS
Cross-country power lines operate at very
high voltages, as much as 300,000volts in
some cases. Although this reduces the
amount of power lost to resistance as seen
2 2
2
by the power loss formula Ploss=P R /E , a
very high voltage makes it difficult to use
the power in a practical way without first
lowering it. Electricity can jump 1 inch
across dry air for every 1,000 volts of
pressure. So a long distance power line
could have an arc of as much as 300
inches, if it is able to find a convenient
grounding source nearby. That high
voltage would not work for a home or
office (or a theatre!) for obvious reasons.
Most high voltage lines are kept far off the
ground, and the conductors themselves are
not insulated because of the added weight,
and the fact that rubber insulation dries
and cracks in harsh sunlight.
Fortunately, the same process of induction
that is used to form alternating current in
a generator can also be used to reduce the
voltage pressure when the lines reach the
consumer. You will recall that a coil of
wire with a current running through it can
form a magnetic field, and that conversely
a magnetic field can be used to induce
current flow in a coil of wire. What will
happen when both of these processes are
used in tandem as in a transformer?
Transformers are aptly named, because
their function is to “transform” the
voltage in a circuit. The coil of wire in a
transformer that is in series with the
incoming voltage is known as the primary.
The coil of wire that is in series with the
outgoing, altered voltage is known as the
secondary. Both of these coils of wire are
wrapped around the same iron core. As
the alternating current gains voltage
strength in the first part of its cycle, a
building magnetic force is engaged. As
the alternating current cycle continues,
the magnetic field collapses to zero, and
then re-forms with its magnetic poles in
the opposite direction as the AC cycle
reverses its current flow. This creates a
magnetic field that is constantly moving,
but due to electronic forces, not physical
forces as are found in a generator. It is
important to keep the field moving in
order to induce a current in the secondary
A transformer that has the same number
of turns of wire in both the primary and
secondary is know as a 1:1 transformer,
because the ratio of the input to the
output is equal. Transformers of this type
are generally used to isolate equipment
from unwanted transient DC voltages.
They are often found in sound equipment
as a means of protecting PA speakers.
Audio signals are AC voltages that have a
variable period of cycle, usually between
30Hz and 30,000Hz. Since amplifiers use
DC to power most of their circuitry, 1 to
1 transformers are used to isolate speakers
from DC current that might accidentally
form in the output line to the speakers.
When a transformer is placed in the
circuit, DC current is stopped, while the
AC audio signal passes through.
coil. The polarity of the flow in the
secondary is exactly opposite to the
polarity in the primary. The movement of
the magnetic field in the primary induces
an opposite, but equal current flow in the
secondary if the coils of wire are equal in
all aspects; but most importantly the
number of turns around the core, and the
proximity of the coil of wire to the iron
core.
This principle is used in a transformer for
alternating current but will not work for a
steady direct current. Even though DC
develops a magnetic field in the coil of
wire forming the primary as its voltage
builds, once it has established the field it
will remain in a steady state. Since the
field is static and not moving, it will not
continue to induce a current in the
secondary coil. Transformers will pass an
AC current, but will stop a DC current.
Several other types of transformers are
used to alter current flow in different ways.
Probably the most common of these is the
step-down transformer.
TYPES OF T RANSFORMERS:
The schematic drawing of a transformer
gives us some clues to their construction.
Power transformers are wrapped around a
core made from sheets of laminated steel
that help to focus the magnetic energy.
The coils of wire at either side are shown
relative to their size in the transformer.
The primary is always shown to the left,
and the secondary to the right.
Step down transformers do the job of
reducing voltage using the same physics
principle of induction, first producing a
magnetic field from electricity, and then
electricity from a magnetic field. The
voltage coming in to the primary is higher
than the usable voltage at the secondary
terminals. The change in voltage pressure
results from a difference in the number of
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windings that are in each of the coils. In a
step-down transformer, there are more
turns in the primary than there are in the
secondary. Reducing the size of the
secondary coil means that the coil cannot
create as high a voltage. In a schematic
drawing a step-down transformer is shown
with fewer curls on the secondary side.
The smaller, 6v coil supplies the lighting
system, while the 24v output is used for
relays and solenoids.
Step-up transformers are just the opposite
of the step down. They are used when a
higher voltage is required for some specific
piece of equipment. Fluorescent lights are
a prime example of this, as are Neon lights.
Both of these fixtures require a high
voltage because they are types of arc lights,
and have no filament.
Instead the
electrons must “arc” or jump across inside
the lamp from one terminal to another.
You will recall that the higher the voltage
pressure, the more likely electrons are to
jump across an open gap.
TERMS USED IN THIS CHAPTER
Transformer
Primary
Secondary
Iron core
Polarity
Step-up transformers have more turns of
wire in the secondary than they do in the
primary.
1:1 Transformer
Step-up Transformer
Step-down Transformer
Some transformers have more than one
coil of wire in the secondary. Generally,
one of the coils is much longer than the
other. That allows one transformer to
supply more than one output voltage in
the secondary. This schematic drawing is
for a transformer used in a pinball
machine, which requires different voltages
for different devices inside the cabinet.
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