October 1960 Popular Electronics
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
If you are looking for a very thorough treatise on transformers,
from AC line frequency up through audio and RF frequencies, but
without a lot of formulas to distract you, then this article by
Ken Gilmore is it. He begins with the fundamentals of a current
setting up a magnetic field, talks about mutual coupling and induction,
step-up- and step-down transformers, autotransformers, iron and
laminated core transformers, multi-tap voltage and impedance matching
transformers, and even touches on the mage-size transformers in
electrical distribution system substations. Transformers fundamentally
work the same way today as they did when first invented in the 1830s.
Equations for simulations have been refined to the nth degree, but
for the vast majority of mankind this article will suffice to provide
a better-then-layman's knowledge of the principles.
See all articles from
By Ken Gilmore
A fundamental coupling device, the transformer is one
of electronics' most capable magicians - here's what's behind
its electrical sleight-of-hand and how it performs its multitude
of valuable tricks.
What the Transformer Does
The electrical power that makes your light bulbs glow, runs your
refrigerator, and operates your hi-fi set comes into your home at
a potential of about 115 volts. Yet if you were to climb the utility
pole outside and measure the voltage there, it could turn out to
be as high as 6000 volts. If you kept on climbing poles at other
places around town, you might find voltages as high as 120,000 volts!
Even in your home, some appliances - air conditioners, clothes
dryers, electric ranges, and other heavy-duty equipment - may operate
at 230 volts instead of the usual 115 volts. And if you probe into
your television set, you'll find an even wider voltage range. For
although your TV draws its power from the wall plug - and power
there is at 115 volts - your set has the ability to change this
voltage into a number of different values, so that each tube and
circuit can operate under the exact conditions it likes best. Consequently,
in some places, you will find values as low as one or two volts;
in others, values as high as 15 or 20 thousand volts.
Electric power, one of our most useful servants, becomes tremendously
more useful when we can change it at will to dozens, or even hundreds,
of different voltages. Fortunately, we can make these changes easily
and economically with a device known as the transformer.
Transformers are all around us. One - the gadget about the size
of a large garbage can hanging near the top of utility poles - changes
the 6000-volt transmission-line power into the 115 and 230 volts
you need. Another - this one about as big as a flashlight - takes
6 or 12 volts from your car's battery, and changes it into the 10,000
or more volts needed to fire your spark plugs. Still another - a
square can a little bigger than your fist - channels high-fidelity
electrical signals into the speakers of your hi-fi set.
We'll talk more about these special applications - and others
like them - a little later. Right now, let's get down to the business
of seeing just how a transformer goes about performing this valuable
electrical sleight-of-hand-changing one voltage into another.
How the Transformer Works
an electric current flows in a wire, a weak magnetic field is set
up around it. If we twist the wire into a coil, the weak field around
each turn of the wire is reinforced by the fields around the other
turns; the result is a much stronger field.
If an a.c, current flows in the coil, the magnetic field builds
as the current flows in one direction; dies down, or decays, as
the current returns to zero; then builds in the opposite polarity
as the current flows in the other direction. You can think of the
building and decaying magnetic field as a pulsing, invisible force,
expanding and contracting as the current reverses its direction
of flow. As the field builds and decays, the magnetic flux lines
(the circular lines in the diagram) cut back and forth through the
suppose we put another coil of wire next to and in line with the
first, although not actually touching it. As the magnetic field
expands and contracts, the flux lines will cut back and forth through
the second coil as well as through the first one, and a voltage
will be induced in the second coil. This is called "mutual induction,"
and is the basis of all transformer action. Because of this property,
a simple transformer can be made - and many are - simply by placing
two coils of wire close together and applying an alternating current
to one of them.
The main value of a transformer lies in the fact that the ratio
of the voltages in the two coils can be controlled by the number
of turns of wire in each. To put it another way, if the secondary
(the coil into which voltage is induced) has ten times as many turns
of wire as the primary (the coil across which the original voltage
is applied), then the secondary voltage will be ten times the primary
voltage. In such a case we have a step-up transformer.
On the other hand, if the secondary has only one-tenth as many
turns as the primary, the secondary voltage will be one-tenth the
primary voltage, and we have a step-down transformer.
the above calculations, we have assumed that all magnetic lines
of flux, as they expand and contract, cut all turns of the transformer.
The magnetic coupling in such a case would be 100%. Of course, in
practical transformers a few lines of force manage to stray outside
the useful area. But by careful design, engineers are able to produce
transformers with efficiencies of 80%, 90%, and even more. In fact,
for the purposes of most calculations, transformer efficiency can
be considered to be virtually 100%.
Voltage vs. Current
Even though we can get a higher voltage from a transformer than
we put into it, the transformer is not capable of creating power.
What we gain in voltage, we lose in current. On the other hand,
if we step down the voltage, we get more current.
If the current flowing in the primary of the step-up transformer
in the diagram above is 5 amperes and the voltage 110 volts, the
power consumed in the primary is 550 watts. Since the output voltage
is 1100 volts, or ten times as much, we would have available only
one-tenth the current, or 0.5 ampere. Thus, even though we can juggle
voltages and currents at will, the output power is 550 watts - the
same as the primary input. (Actually, the output power would be
slightly less than 550 watts, due to the small losses in efficiency
far, we have described a transformer as two coils of wire, placed
close together along a common axis. Although some transformers are
actually built this way, most use other types of construction. Instead
of being placed side by side, the two coils are usually arranged
with one coil inside the other; this gives much closer and more
efficient magnetic coupling.
For use at low frequencies, designers wind the two coils around
a common iron core. Since iron is a much more efficient conductor
than air, the magnetic field built up is much stronger. That is,
almost all the magnetic lines of force developed by the primary
winding are gathered up by the
core and shaped so that almost all cut through the secondary winding.
Therefore the efficiency of the transformer is greatly increased.
The diagrams at left show the three principal types of iron-core
transformers. First is the open-core transformer which, while possible,
is never used because of its relative inefficiency - a large part
of the magnetic field would still have to be in air, rather than
in iron. The closed-core transformer is considerably more efficient;
and the shell core transformer is most efficient of all. The shell-core
type has another advantage: since the flux path is almost entirely
contained in the iron core, it is less subject to disturbances by
external magnetic fields than other types, and it doesn't disturb
other nearby circuits as much.
first transformer ever made was simply an iron ring with two 2-layer
coils of wire wrapped around it. Its inventor was Michael Faraday,
the great English electrical pioneer. He discovered electromagnetic
or mutual induction - the principle upon which the transformer works
- in 1831. When he connected his primitive iron-ring transformer
as shown, the galvanometer needle jumped as the switch was closed.
Faraday's device was a true transformer, its losses were high. Today's
modern, refined transformers have assumed a wide variety of sizes,
shapes, and characteristics as engineers have attempted to minimize
the losses that are a part of every transformer's operation.
losses come from many different sources. First, not every magnetic
flux line cuts the secondary - some simply travel out into space,
consuming energy from the primary, but doing no useful work. This
loss is called flux leakage. Designers minimize it by careful physical
arrangement of the coils and core. Sometimes the primary is wound
on the core first, then the secondary applied on top. At other times
the secondary is split into two layers with the primary in between.
The so-called copper losses are caused by the electrical resistance
of the transformer windings. Although copper is a good conductor,
it has a measurable resistance, as does any conductor. When current
flows through this resistance, heating takes place and power is
wasted. As a result, almost any transformer will feel warm to the
touch when operating normally, and some are actually hot.
the iron core itself, as well as the coils, is cut by the expanding
and contracting magnetic field, a current is induced here, too.
As this eddy current flows in the core, it steals energy from the
primary circuit and dissipates it as useless heat. The eddy current
flows at right angles to the magnetic flux. It can be reduced by
substituting several thin layers of iron for the solid core. These
thin layers - laminations - are separated by layers of glue which
electrically insulate the laminations from each other. In practice,
a small eddy current is set up separately in each lamination, but
the total loss is much less than for a solid-core transformer.
Still another core loss is caused by the alternating current
itself. Since this current reverses its direction 120 times a second,
the iron core - in effect, an electromagnet - must continually reverse
its polarity. And since the minute magnetic elements in the core
tend to resist this change, power must be expended to realign them.
This is called hysteresis loss. Engineers reduce it by building
transformer cores of steels which change magnetic polarity with
comparative ease, so that less power is consumed in making the switch.
the turns of wire in a transformer are close together, there is
some distributed capacitance between the turns, between different
layers of windings and between separate windings. This capacitance,
though small, is cumulative. Like a small capacitor connected across
the transformer, it shorts out some of the voltage developed across
the windings. At low frequencies (the usual 60 cps of house current,
for example) this loss is unimportant, but at higher frequencies
engineers must go to great lengths to minimize it.
Another small loss is caused by the imperfection of transformer
insulation. A small leakage current will flow through almost any
insulator, and thus absorb some of the transformer's power. This
is known as dielectric loss.
Then, too, particularly at high frequencies, a transformer can
begin to act as a small but efficient radio transmitter, and actually
radiate power like a broadcast antenna. This is called transmission
Most of these losses, under normal conditions, are minor, but
at times they become serious. For example, eddy current losses are
small at power-line frequencies, but at the high end of the audio
spectrum - say around 20,000 cps - they become significant. This
means that a poorly designed transformer in the output stage of
a hi-fi amplifier will operate much less efficiently at 20,000 cps
than at 1000 cps; the result is poor frequency response.
To minimize eddy currents designers specify thinner laminations.
Where laminations 20 to 25 thousandths of an inch thick are used
in power transformers designed to work at 60 cps, audio transformers
rarely have laminations thicker than 10 or 15 thousandths of an
inch. For really good hi-fi reproduction, lamination thicknesses
may range from ten thousandths of an inch all the way down to only
one thousandth of an inch.
Higher and Higher Frequencies
frequencies go still higher, even one thousandth of an inch is too
much, and eddy current losses become excessive. Consequently, r.f.
transformers frequently have cores made of minute grains of iron
suspended in an insulating material and compressed under high pressure
into a solid mass. Since the grains are insulated from each other,
they break up the eddy current path and help reduce eddy current
As might be expected, the size of the iron granules becomes important
as the frequency increases, since at high frequencies eddy currents
are even set up within the individual granules. Granules several
thousandths of an inch thick are satisfactory below 100,000 cps,
but as the frequency goes higher the particles cannot be larger
than several millionths of an inch thick.
A new type of magnetic core made of iron ferrite has recently
allowed designers to build iron-core transformers to operate at
frequencies higher than ever before. These ferrites - varieties
of iron oxide, or rust - are valuable because they have magnetic
properties, and yet are insulators and do not conduct current. Because
of the unusual construction of these transformers, no eddy currents
If you have bought an ultra-portable radio recently, you are
benefiting from ferrite-improved transformers. Miniature radios
of even a few years ago had loop antennas at least 8 to 10 inches
long and almost as high to collect enough signal to operate. Now
ferrite-core antennas, far more efficient because of their magnetic
core but not susceptible to eddy current ills, can be built as small
as a short pencil. As a result, portable radios can now be produced
smaller than they have ever been produced before.
many applications, particularly for very high frequencies, air-core
transformers are used. The coils are wound on a non-magnetic form
such as Bakelite or polystyrene. The coils may be concentric, or
end to end. Frequently one is movable, so that the degree of coupling
between them is adjustable.
One of the biggest problems in high-frequency transformer design,
particularly where multiple layers of winding are involved, is stray
capacitance. If a regular winding were used, with adjacent layers
lying parallel to each other, this capacitance could become intolerable.
Consequently, layers are frequently spiraled back and forth as in
the transformer shown in the drawing at right. This makes adjacent
layers cross each other almost at right angles instead of being
parallel, and stray capacitance is materially lowered as a result.
How the Transformer is Used
The transformer invented in the 1830's wasn't put to work outside
the laboratory until 1885 when William Stanley, an engineer who
worked for George Westinghouse, designed and tested a transformer
power-distribution system. He used a 500-volt generator and fed
the power directly into a 4000-foot transmission line. A transformer
to step down the voltage to 100 volts was used at the other end
of the line.
Westinghouse wasted no time in putting Stanley's superior transmission
system into operation. That same year he built the first plant especially
designed for transformer power distribution in Buffalo, N. Y. It
went into use on November 30, 1886. His generator produced a 1000-volt,
133-cps output which was fed directly into the transmission line,
and stepped down at the customer's home.
In spite of its obvious superiority, however, high-voltage transmission
with transformers did not gain immediate acceptance. Thomas Edison,
for one, was violently opposed to a.c. power, and he used his tremendous
prestige to gain support for his own d.c. system. Consequently,
it was not until many years later - after the turn of the century
- that high-voltage a.c. power distribution became common. Even
today there are a few places - some areas of New York City, for
example - still receiving Edison's legacy of d.c. power.
But giant power transformers and their complex distribution stations
now dot the landscape all over the country. The one shown on the
next page, one of the largest ever built, can handle enough electric
power for a city of 500,000 inhabitants.
use transformers for power distribution? The efficiency of transmission
is tremendously increased by stepping up the voltage to as much
as several hundred thousand volts. Also, a given size of wire can
carry far more power at high voltage than low, saving money in transmission
costs. Let's see why.
As an example, let's take a transmission line of No.1 wire 10
miles long - that's a conductor about the size of your little finger.
The resistance of one such wire 10 miles long is about 7 ohms. (Actually,
the resistance of each wire in the transmission pair is 7 ohms but
for the sake of illustration let's consider just one.) Now let's
say that we transmit a current of 120 amperes at 120,000 volts (a
common transmission-line voltage) over the 10 miles. The total power
fed into the line at the generating station is 14,400,000 volt-amperes.
With 120 amperes flowing in the 7-ohm line, the voltage drop
over the ten miles will be 840 volts. Thus, the output voltage will
be 119,160 volts; 120 amperes at 119,160 volts gives a 14,299,200
volt-ampere output. Along the line we have lost 100,800 volt-amperes,
dissipated by the resistance of the transmission line. This seems
like a lot of power, but if we figure it in terms of percentage,
the loss amounts to a negligible 0.7% of the total fed into the
Now let's see what happens if the supply voltage is reduced to
only 12,000 volts. The power input is now 1,440,000 volt-amperes.
We will assume that the transmission line is still carrying 120
amps - its maximum load under any conditions. Since the current
and resistance are the same, the voltage drop over the 10 miles
will also be the same - 840 volts. The loss in the transmission
line will still be 100,800 volt-amperes, but now this represents
a whopping 7% of the total fed into the transmission line.
Obviously, the high-voltage transmission is far more efficient.
As also demonstrated in this example, the transmission line can
carry far more power under high-voltage conditions. For these reasons,
all transmission lines operate at higher voltages than those delivered
to your electric meter by the power companies.
At Niagara Falls, N. Y., for example, hydroelectric generators
produce power at 6000 volts. It is immediately stepped up by transformers
to 120,000 volts and fed to long-distance transmission lines. At
various points it is stepped back down to 6000 volts for distribution
over local areas, then stepped down once again to 230 and 115 volts
for home use.
a power-distribution transformer is more spectacular, you're much
more likely to be familiar with the ordinary power transformer used
in radios, amplifiers, and TV sets. Such devices have a primary
winding and usually several secondary windings to meet the various
voltage and current requirements of a receiver or amplifier; a drawing
of a typical power transformer is shown at right, above. The primary
is usually designed for 115 volts; the high-voltage secondary may
produce anywhere from 250 to as high as 600 or 700 volts (higher
for some purposes). The other secondaries, usually rated at 5.0
and 6.3 volts, are for tube filaments.
Power transformers are available with a wide variety of windings
and current capabilities. They may have four, five, six, or even
more windings, each rated at a different voltage for some specific
purpose. The high-voltage winding of a light-duty power transformer
may be capable of producing perhaps only 30 or 40 ma., while a heavy-duty
unit may turn out 300, 400, or even 500 ma. Transformers for high-power
transmitters produce voltages and currents far in excess of these
values, but for such applications separate transformers are generally
used for high-voltage and filament supplies.
far, all the transformers we have talked about in detail are designed
for use in power circuits which operate at 60 cps. But transformers
can operate on a wide variety of frequencies - every audio amplifier
uses at least one transformer of this sort, and many include several
Although the same basic principles of step-up and step-down are
used in audio transformers, this is usually of secondary importance
to the transformer's ability to serve as an impedance-matching device.
Take, for example, an input transformer. Here it may be necessary
to match a phonograph pickup, a microphone, or other input source
of as little as 200 or 300 ohms (even less, in some cases) to a
grid circuit of as much as several hundred thousand ohms. If the
pickup or microphone were connected directly to the grid, a serious
mismatch would occur, which would not only reduce the efficiency
of the circuit but upset frequency response as well. The input transformer
matches the components so that each operates properly.
interstage transformer is another variety of the audio transformer
and performs much the same kind of job: matching the output tube-several
thousand ohms-to a grid circuit of a much higher impedance.
A third variety is the output transformer, whose main task is
again impedance-matching. The plate circuit of the output tubes
may have an impedance of many thousands of ohms, while most loudspeakers
are 4, 8, or 16 ohms. To accommodate various tube-speaker combinations,
most output transformers have a series of "taps" on the secondary
winding, and perhaps on the primary as well, so that windings of
the proper impedance can be selected. Only the part of the transformer
windings actually used (in 'the diagram, the portion between the
first and second terminals) affects the circuit's impedance values.
One form of output transformer - known as a "universal" type - is
so designed that it is capable of matching virtually any possible
tube and speaker combination.
Better and Better Design
progress has been made recently in audio-transformer design. Just
a few years ago it was difficult to get a transformer with any appreciable
output above, say, 10,000 to 15,000 cps. Today, transformers with
flat outputs up to 20,000 cps are common, while units flat to 50,000
or even 100,000 cps are available.
Tremendous problems had to be overcome to produce today's outstanding
transformers. In addition to the losses mentioned earlier, a transformer
has inductive reactance which varies according to frequency (remember
that a transformer is also a coil). At frequencies of 100 and 1000
cps, the inductive reactance of the primary will be 10 and 100 times,
respectively, its value at 10 cps. The inductive reactance appears
to the output tube's plate as a load resistance, and thus various
amounts of amplification take place at various frequencies. As a
result, the gain of the amplifier is about two-and-a-half times
higher at 200 cps than at 10 cps. At 3000 cps it would be three
times higher. At still higher frequencies, distributed capacitance
becomes an important factor, and gains fall off rapidly.
Engineers go to great lengths to compensate for these effects;
by means of special core materials, unique coil designs, special
wrapping patterns, interlaced layers, and other techniques, they
have produced a variety of audio transformers with unbelievably
even response over an extremely wide range of frequencies.
mentioned earlier, transformers are also widely used in r.f. circuits.
Even the simplest five-tube a.c.-d.c. radio will usually have as
many as four transformers, in addition to its audio output transformer.
A typical radio, for example, might have an antenna coil (actually
a small transformer which couples the antenna's output into the
grid of the first amplifying tube), an oscillator coil (a transformer
which supplies feedback for the oscillator), and two i.f. transformers
which couple the various stages.
These transformers are likely to be air, powdered-iron, or ferrite-core
transformers, since a regular iron core would cause intolerable
eddy-current losses. The windings will also probably be of a special
spiral design calculated to minimize capacitance effect,
Special Purpose Transformers
Although the transformers we have been discussing make up the
bulk of those used, there are many other types, all of which perform
their useful, specialized jobs.
autotransformer, for example, uses only one winding instead of two,
but accomplishes an effect similar to that of a regular transformer.
If the whole coil is used as the primary and only a portion as the
secondary, then it is a step-down unit. Hooked in reverse, it is
a step-up device. This transformer, of course, cannot be used in
circuits which must be electrically isolated from each other. But
it serves very well in your automobile where it draws current from
the 6- or 12-volt battery or generator and puts out the 10,000 or
more volts needed to fire your spark plugs.
we're on the subject of your automobile, let's take a look at the
car radio which uses another kind of specialized device, the vibrator
transformer. This device effectively "transforms" d.c. As the vibrator
element moves back and forth touching each contact in turn, current
flows through each half of the primary alternately, with each pulse
going in a different direction. With the proper turns ratio, the
output from the original 6- or 12- volt d.c. source can be as much
as several hundred volts a.c.
Photoflash transformers, used to operate photographer's electronic
flash or "strobe" units, are also vibrator-operated. They can take
the vibrator-interrupted output from a 1 1/2-volt battery and turn
it into several thousand volts a.c.
Pulse transformers are used primarily in radar. They range from
tiny units (several of which can fit in a thimble) that put out
a few millionths of a watt to huge, multi-ton giants that transmit
powerful million-watt pulses. These transformers are designed to
step up odd-shaped waveforms without changing the waveshape.
of the newest types - transistor transformers - are similar to those
used in regular r.f. and a.f. circuits except that their impedances
and voltage ratings are calculated to match the operating requirements
of transistors. Some of these units, by the way, can fit in a cube
three-eighths of an inch square, and they weigh only a fraction
of an ounce.
Thus, through the ingenuity of the design engineer, the transformer
- though always operating on the same simple principle discovered
by Faraday - can be adapted to perform hundreds of useful and important
Posted May 14, 2014