The resistor is a
component that offers
resistance to the flow of
electrons(
current),
this
resistance is
measured in
ohms. A resistor's
value is
indicated
by a
colour code or numbers.
Linear resistors:
A perfect
linear resistor has a
constant resistance independent
of
current,
voltage,
temperature and other
factors.
In
reality no resistor is
prefect and even two resistors with the
same marked value may have several
percent difference if
measured.
Therefore, in addition to their
value, resistors are marked
with a
indication of inaccuracy, called
tolerance, measured
in percent. The most used resistors have a tolerance of 5%,
but if needed resistors down to 1% are available.
The first resistors were made of a
solid rod or tube of
carbon,
with a
metal clamp on each end. This resistor has a low
inductance
and can withstand
overload for a
short time, but has a
high
capacitance(0.2pF-1 pF) and the resistance is higly dependent on
temperature and
voltage.
This resistor has been
replaced by other
types in most
applications,
but is sometimes used in
pulse circiuts due to the low
inductance.
The carbon
film resistor is made of a
ceramic core with
a thin
film of
carbon on the outside. To vary the resistance
of these resistors, the film is cut into a spiral by a
diamond edged blade,
or by a
LASER cutter. This resistor type have a high
capacitance(~0.2pF),
medium to
high inductance and low
temperature stability. But they
are
cheap to
manufacture.
The
metal film resistor is
similar to the
carbon film resistor,
but uses a more stable
metal film as the resistive
layer.
These resistors have low capacitance, high
voltage and
temperature stability.
But their
pulse resilience is low, and therefore metal film resistors
should not be used instead of
carbon resistors in
pulse circiuts.
The Thick film/cerment resistor, is made from a
mix of
metaloxides
and
ceramics that is
fastened on a
ceramic base. This
is the most
common type of
surface mounted resistors.
They have
medium capacitance, a good long time stability
and can '
survive' relatively high
temperatures.
The
wirewound resistor: This type of resistor
consists of a
metal wire
that is
wound around a
ceramic core. The wire is made from a highly
stable alloy. This resistor is usually used in high
effect applications, but
have a high
inductance.
Nonlinear resistors:
Some times the nonlinearety of a resistor is desirable.
This can be like the
thermistor and the
light dependent resistor(
LDR) that is used as sensors
Or as voltage or current dependent resistors(
VDR/
CDR)that is used to protect against high
voltages or
currents.