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The basic principle of operation of all
chokes and transformers relies upon the fact that electricity
and magnetism are closely linked to each other. A wire carrying
an electric current will produce a magnetic field around the
wire. Conversely, a wire placed into a magnetic field or moving
through a magnetic field will have an electric current induced
in it.
A choke is simply a coil of insulated wire
of one or more turns. The magnetic properties of the choke or
its inductance is dependent upon the number of turns of wire,
the cross sectional area of the coil, its length and the magnetic
properties of the core. The unit of inductance is the Henry which
is actually quite a large unit, so it is more common to see component
values in one millionth of a henry or micro-henries, mH.
The core material of the choke can have
a large effect on the inductance. The simplest form of inductor
has an air core which, as its name suggests, doesn't actually
have any type of core. The inductance can be significantly increased
by winding the coil of wire on a magnetic material such as soft
iron or ferrite material.
Chokes are typically used in power supply
filter circuits or as part of a tuned or resonant circuit. In
a filter circuit, the inductance of the choke helps to smooth
out any sudden changes of current flow in the circuit. In simple
terms, the choke tries to oppose any sudden changes of current
flow in the circuit and tries to keep everything at a constant
level. The choke can also be used to filter out high frequencies.
The impedance of the coil is quite low at low frequencies but
increases as the frequency increases. This property can be harnessed
and used in a tuned resonant circuit by connecting a choke in
parallel with a capacitor. Since the impedance of the capacitor
behaves in an opposite way to the impedance of the choke, there
will be a particular frequency at which the impedance is at its
lowest, this is known as the resonant frequency. The frequency
response of a typical resonant circuit is shown opposite. This
type of circuit can be used as a filter for a specific frequency
band or as a tuning circuit in the front end of a radio receiver. |
