| A transformer consists of two coils
(often called 'windings') linked by an iron core, as shown in figure
1. There is no electrical connection between the coils, instead they
are linked by a magnetic field created in the core.
Transformers are used to convert electricity from one voltage to
another with minimal loss of power. They only work with AC
(alternating current) because they require a changing magnetic field
to be created in their core. Transformers can increase voltage
(step-up) as well as reduce voltage (step-down).
Alternating current flowing in the primary (input) coil creates a
continually changing magnetic field in the iron core. This field
also passes through the secondary (output) coil and the changing
strength of the magnetic field induces an alternating voltage in the
secondary coil. If the secondary coil is connected to a load the
induced voltage will make an induced current flow. The correct term
for the induced voltage is 'induced electromotive force' which is
usually abbreviated to induced e.m.f.
The iron core is laminated to prevent 'eddy currents' flowing in
the core. These are currents produced by the alternating magnetic
field inducing a small voltage in the core, just like that induced
in the secondary coil. Eddy currents waste power by needlessly
heating up the core but they are reduced to a negligible amount by
laminating the iron because this increases the electrical resistance
of the core without affecting its magnetic properties.
Transformers have two great advantages over other methods of
changing voltage:
- They provide total electrical isolation between the input and
output, so they can be safely used to reduce the high voltage of
the mains supply.
- Almost no power is wasted in a transformer. They have a high
efficiency (power out / power in) of 95% or more.
Types of Transformer 
Mains Transformers
Mains transformers are the most common type. They are designed
to reduce the AC mains supply voltage (230-240V in the UK or
115-120V in some countries) to a safer low voltage. The standard
mains supply voltages are officially 115V and 230V, but 120V and
240V are the values usually quoted and the difference is of no
significance in most cases.
To allow for the two supply voltages mains transformers usually
have two separate primary coils (windings) labelled 0-120V and
0-120V. The two coils are connected in series for 240V (figure 2a)
and in parallel for 120V (figure 2b). They must be wired the correct
way round as shown in the diagrams because the coils must be
connected in the correct sense (direction)
:
Most mains transformers have two separate secondary coils (e.g.
labelled 0-9V, 0-9V) which may be used separately to give two
independent supplies, or connected in series to create a
centre-tapped coil (see below) or one coil with double the voltage.
Some mains transformers have a centre-tap
halfway through the secondary coil and they are labelled 9-0-9V for
example. They can be used to produce full-wave rectified DC with
just two diodes, unlike a standard secondary coil which requires
four diodes to produce full-wave rectified DC.
A mains transformer is specified by:
- Its secondary (output) voltages Vs.
- Its maximum power, Pmax, which the transformer can
pass, quoted in VA (volt-amp). This determines the maximum output
(secondary) current, Imax...
- ...where Vs is the secondary voltage. If there are
two secondary coils the maximum power should be halved to give the
maximum for each coil. Its construction - it may be PCB-mounting,
chassis mounting (with solder tag connections) or toroidal (a high
quality design).
Turns Ratio and Voltage
The ratio of the number of turns on the primary and secondary
coils determines the ratio of the voltages...
where Vp is the primary (input) voltage, Vs
is the secondary (output) voltage, Np is the number of
turns on the primary coil, and Ns is the number of turns
on the secondary coil.
Power and Current
The very small power loss in a transformer means that we can
assume that power in = power out. Power = voltage x current, so we
can use this to show that the current ratio is the inverse of the
voltage ratio. For equal power the current increases as the voltage
decreases
so...
where Ip is the primary (input) current, and Is
is the secondary (output) current.
The current in the primary coil Ip is determined
almost entirely by the current Is drawn by the load
connected to the secondary coil. With no load connected, Is
= 0 and Ip is very small indeed because the alternating
magnetic field created by the primary current induces a voltage in
the primary coil which almost exactly matches the supply voltage. If
a load is connected current will flow in the secondary coil,
creating a magnetic field which opposes and partly cancels out the
field created by the primary coil. The resulting weakened field
induces a smaller voltage in the primary coil to oppose the supply
voltage and this means that a larger primary current flows.
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