How Transformers Work
are many sizes, shapes and configurations of transformers from tiny to
gigantic like those used in power transmission. Some come with
stubbed out wires, others with screw or spade terminals, some made for
mounting in PC boards, others for being screwed or bolted down.
Transformers are composed of a
laminated iron core with one or more windings of wire. They are
called transformers because they transform voltage and current from one
level to another. An alternating current flowing through one coil
of wire, the primary, induces a voltage in one or more other coils of
wire, the secondary coils. It is the changing voltage of AC
current that induces voltage in the other coils through the changing
magnetic field. DC voltage such as from a battery or DC power supply
will not work in a transformer. Only AC makes a transformer work.
The magnetic field flows through the iron core. The faster the
voltage changes, the higher the frequency.
The lower the frequency, the more
iron is required in the core for the efficient transfer of power.
In the USA, the line frequency is 60 Hertz with a nominal voltage of 110
volts. Other countries use 50 Hertz, 220 volts. Transformers
made for 50 Hertz must be a little heavier than ones made for 60 Hertz
because they must have more iron in the core. Line voltage can
vary a little and usually runs between 110 volts and 120 volts or
between 220 and 240 volts depending on country or power connections.
A house in the USA has 220 volts coming in but is split to two legs of
110V by grounding the center tap (see configuration section below)
The ratio of input voltage to
output voltage is equal to the ratio of turns of wire around the core on
the input side to the output side. A coil of wire on the input
side is called the primary and on the output side is called the
secondary. There can be multiple primary and secondary coils.
The current ratio is opposite the voltage ratio. When the output
voltage is lower than the input voltage, the output current will be
higher than the input current. If there are 10 times the number of
turns of wire on the primary than the secondary and you put 120 volts on
the primary, you will get 12 volts out on the secondary. If you
pull 2 amps out from the secondary, you will only be using 0.2 amps or
200 milliamps going into the primary.
Transformers can be built so they
have the same number of windings on primary and secondary or different
numbers of windings on each. If they are the same, the input and
output voltage are the same and the transformer is just used for
isolation so there is no direct electrical connection (they are only
linked through the common magnetic field). If there are more
windings on the primary side than the secondary side, then it is a step
down transformer. If there are more windings on the primary side,
then it is a step up transformer.
A transformer can actually be used
in reverse and will work fine. For example, if you have a step up
transformer built for transforming 120 volts to 240 volts, you can also
use it for a step down transformer by putting 240 volts into the
secondary side and you will get 120 volts on the primary side.
Effectively, the secondary becomes the primary and vice versa.
Transformer Power Ratings
Voltage is measured in volts,
current is measured in amps, and the unit of measure for power is watts.
Watts is equal to the volts times the amps. There is a little loss
of power in a transformer due to the combination of resistance and
reactance. Reactance is similar to resistance except it is the
resistance to an AC current or more technically, the resistance to
change in a change in current due to the change in the field created.
This heat is what limits the amount of current or power a transformer
can handle. The higher the current, the more heat is produced.
When the wires get too hot, the insulation breaks down and shorts with
adjacent wires which causes more heat which eventually melts wires and
ruins the transformer.
A basic transformer has no
additional components and so nothing to protect it from overloading.
If you were to connect the two output wires directly together, that will
constitute a short circuit and cause far too much current to flow in
both the primary and secondary and you will burn out the transformer.
In the same way, if you use the transformer to power a hot wire foam
cutter and you are using a wire with too little resistance for your foam
cutter, you will burn out your transformer if you don't have it
protected by a proper value fuse or breaker. You have to make sure
that the wire resistance, in other words, the gage or diameter, and the
length is correct to limit the amount of current to under the rating of
The higher the current, the larger
the wires need to be that carry that current. When the wires are
larger, there is less resistance and so less heat. The power that
is changed to heat and lost can be calculated as P=I2R.
That means that if you double the current, the power lost to heat
increases by four times. If the transformer is a step down
transformer, then there will be more current on the output and so the
wire in the secondary windings will be heavier than the primary.
The reverse is true for a step up transformer.
A transformer may be rated in Amps,
Volt-Amps (VA), or Watts (W). For small transformers, VA and Watts
are the same thing for all practical purposes. In large industrial
transformers, power factors get involved and the two can be different.
If the transformer is rated in amps, it usually says X amps at X volts
and is rated on the output or secondary side. A 120V transformer
with 24V out rated at 2 amps means that you can only safely pull 2 amps
from the secondary side. You can find the power rating of the
transformer by multiplying the rated amps times the output voltage so 2
X 24 = 48 watts.
If the transformer is rated in VA
or watts, you can calculate the maximum allowable output current by
dividing the VA or watts by the output voltage. So if the
transformer is rated at 48 VA with 24 volts output, the allowable output
current is 48 / 24 = 2 amps.
120 volt transformer with two wires in and two wires out is very simple.
You hook up the two wires on the primary side, the 120V side, to a wall
outlet and your output voltage is on the two wires coming from the
When a transformer is shown in an
electronic circuit, it is shown as a diagram like shown here. The
parallel lines represent the laminated iron core, the curved lines
represent the primary and secondary windings, the circles represent the
terminations whether terminals or short wires.
A common configuration is a center
tap or CT. The secondary side has three wires out. The
middle wire on the output side is attached to the secondary coil,
usually at the middle. If the winding ratio is 5 to 1, then with
120V input, you get 24 volts output on the two outside wires but if you
connect an outside wire and the center wire, you get 12 volts
because you are using only half the secondary winding making the
connection a 10 to 1 ratio. If the transformer is rated at 2 amps,
you still can only use 2 amps output whether you use 12 volts or 24
volts. Often the center tap is grounded so you then have two 12
volt sources that can be used to make + and - 12V DC after running
through a converter (rectifier and filter).
dual output configuration is similar to the center tap except that
instead of connecting a wire to the center of the coil, the coil is
separated into two separate coils with wires with terminals or wires
coming out from both ends of both coils so four wires come out of the
secondary side instead of three.
If the transformer is a 110V input
with two 12V outputs, you can connect the two secondary coils in series
to get 24 volts out, or you can connect them in parallel to get 12V out.
You have to be careful to connect the right ends of the two secondary
coils in both the series and in the parallel connections. If you
reverse the connections, you will get 0 volts out because the two
voltages will cancel each other out.
If the transformer is rated at
48VA, then you can use up to 2 amps out for the 24 volt connection which
is no different than the center tap or single 24V output configuration.
However, when connected in parallel, you get 12 volts out but double the
output current available so you can get 4 amps out. You get the
full 48VA output where with the center tap 12V output, you can only get
half the rated output or 24VA. This is an advantage in hot wire
foam cutters because you have a wider range of wire diameters and
lengths depending on whether you connect the outputs in parallel or
series. The series and parallel connections are shown below.
dual input transformer is often used to make the transformer able to be
used in both countries with 120V line voltage and 240V line voltage.
The primary is separated into two separate windings with terminals at
each end of both windings so there are four wires or terminals on the
To use it with 110 volts input, the
two primary windings are connected in parallel as in the left diagram
below. Care must be taken to connect the correct ends together.
If they are reversed, the fields cancel each other out because the
fields generated by each section of the primary are opposite.
Normally, terminals are labeled with numbers or letters and a diagram is
provided on the transformer or in an accompanying data sheet showing how
the connections must be made for 110V and 220V.
If the transformer is to be
connected to a 220V supply, then the two coils are connected in series
and again, care must be taken to connect the correct terminations
together. Parallel connections for 110V and series connections for
220V is shown below.
Dual Input and Output
And of course, you can have both a
dual input and a dual output so you have four wires in and four wires
out which gives even more flexibility to the use of the transformer.
Some specialized transformers may
have several secondary taps or several secondary windings to provide
different voltages and they need not be even numbers. A
transformer could have a 3V, 5V, 12V, and 24 volt output for example.
An autotransformer is often
referred to as a Variac which is actually one company's trade name for
their autotransformer. It has a continuous output voltage from
zero to a little over the input value. It works similar to a
potentiometer or rheostat except the change in the voltage is due to the
field change rather than resistance. Another difference is that a
potentiometer or rheostat is very inefficient because it converts the
current flowing through it to heat (Watts = Amps X Volts). As in
all transformers, the resistance is low so the amount of heat generated
is much less and so much more efficient at transforming voltage
An autotransformer has only one
winding which serves as both the primary and the secondary winding.
Because there is only one winding, there is no electrical isolation
between the input and the output but if isolation is not required, then
it provides an alternative to multiple winding transformers in some
This transformer has the input
wires connected to one end of the winding and the other a little ways
from the other end. The secondary is connected the same point as
the input side that is on the end. The other secondary connection
is to a wiper that rides on the top of the windings where the insulation
has been removed so the wiper can make contact with the windings at any
point on one surface. The wiper is connected to a knob on the top
of the autotransformer so a person can turn the knob to get the voltage
they want. Because one primary wire is connected a ways from the
end of the winding, the wiper can go past that point and so provide a
voltage higher than the input, typically a 110V output can go up to
around 130V on the secondary side.
Because the autotransformer has
only one winding, there is only one wire size and so the maximum input
current is also the maximum output current. If a 110 Volt
autotransformer is rated at 10 amps, then the maximum output current is
10 amps regardless of the voltage. If it is rated in Watts or VA,
then the amps is calculated by dividing the Watts or VA by the rated
The autotransformer is a good
alternative to a step down transformer when the range of desired
voltages is on the high end or the whole range of voltages is needed but
becomes more expensive if the range is on the low end because you have a
lot of unused windings. A step down transformer is more
For hot wire foam cutting, an
autotransformer is much more expensive than step down transformers in
most applications. If the voltage required is more than 24 volts,
then an autotransformer might be considered.
Phases and connecting multiple
For simplicity's sake, I have not
mentioned phase but when connecting two or more windings together, the
phase becomes very important. AC current is a sine wave and the
voltage changes from positive to negative and back in a sinusoidal
rhythm many times per second. How often the voltage changes is
called frequency and used to be called cycles per second but is now
called Hertz (abbreviated Hz). Household current in the USA and
some other countries is 60 Hz, in other countries is 50 Hz. When
talking about two wave forms such as you have in two windings, the
relationship between the two sine waves is the phase. If the sine
waves line up, they are in phase, if the positive peak of one wave lines
up with the negative peak of the other wave, the two waves are 180° out
of phase. The phase between one end of a coil and the other are
also 180° out of phase. When one end is at the positive peak, the
other end will be on the opposite peak. Since there must be a
difference in voltage between two points for current to flow, the two
ends of the winding must be opposite voltage at any point in time.
The phase difference between two
windings depends on the direction of windings and how they are connected
so in electrical schematics a dot at one end of the winding indicates
the beginning of that winding. For simplicity's sake, I have left
the dots off the schematics in this article. However, when
connecting two coils together, it is very important to connect them
For a series connection you must
connect the end of one winding to the start of the other winding
(windings for multiple coils are always wound in the same direction).
If you connect the start of one winding to the end of the other winding
in a series connection, the fields will cancel out and you will get zero
output. This will not hurt the transformer but you will get no
When connecting two windings in
parallel, you must connect the start of one winding to the start of the
other winding and the two ends of the windings together. In
a parallel connection, connecting the wires in reverse will burn up your
transformer if not properly protected (proper current rating) by
a fuse or circuit breaker. Be very careful when connecting
two coils together.
This has basically just been an
overview for a layperson. Although physically a transformer is a
fairly simple device with few parts, how it actually works is pretty
complicated. I recommend Rod Elliot's excellent articles if you
are interested in understanding them better:
Transformers - The Basics
Transformers - The Basics
He also has many other articles on
electronics including power supplies.