RESISTORS
FIGURE 2. This is a model of
how a resistor actually behaves
in a circuit. The type of resistor
determines the importance of
each component.
used in a high voltage circuit.
Resistor Types
For a piece of conducting material to be made into a practical resistor, a pair of electrodes and leads are
attached so that current can flow. The
resistor is coated with an insulating
material to protect the conducting
material from the surrounding environment and vice versa. There are
several different resistor construction
methods and body styles or packages
that are designed for a certain range
of applied voltage, power dissipation, or other considerations.
Carbon Composition
Composition means that the
resistive material is a mix of carbon
and stabilizing compounds. The
amount of carbon in the mix determines the resistance of the material.
A small cylinder, like a pencil lead, is
held between the two electrodes and
coated with resin or phenolic, making a non-inductive resistor with low
LS that is often used in RF circuits.
Carbon comp resistors are
available with power ratings of 1/4 to 2
watts. They can also handle temporary
overloads much better than film resistors because the heat is distributed
evenly throughout the cylinder of resistive material. That makes them a good
choice for circuits that protect against
and absorb pulses and transients, for
example. Unfortunately, these resistors
are also strongly influenced by temperature and humidity and so are not
good for circuits that depend on
precise, stable resistance values.
it. These resistors are available with
very accurate and stable values.
A drawback of film resistors is
that they are unable to handle large
amounts of power because the film is
so thin. Overloads can also damage
the film by creating “hot spots” inside
the resistor, changing its value permanently. The value of film resistors
is sometimes adjusted before sealing
by cutting away some of the film with
a laser, a process called trimming.
If the film is deposited on the
inside of a tube, the trimming
process creates a coil-like current
path that raises the LS of the resistor.
If your circuit operates at high
frequencies, be sure the resistors you
select have a low value of LS.
Surface-mount resistors are
almost always film resistors. These
resistors have no leads at all, so LS is
very low. The film is deposited on a
ceramic sheet. Because of their
extremely small size, surface-mount
resistors have very low power ratings
— from 1/10 to 1/4 watt.
Wirewound
Common in power supplies and
other equipment where lots of power
is dissipated, a wirewound resistor is
made just as you might expect. A high-resistance wire is wound on an insulating form — usually a ceramic tube —
and attached to electrodes at each
Film Resistors
In a film resistor, the resistive
material is a very thin coating of
carbon or metal on an insulating
substrate, such as ceramic or glass.
The value of the resistance is determined by the thickness of the film
and the amount of carbon or metal in
CHOOSING RESISTORS
Here are some special applications that require special types of resistors.
These aren’t hard and fast rules, but can guide your initial selection. For most
circuits, plain old carbon film or carbon comp resistors will work just fine.
■ ESD and Transient Protection — Carbon composition, metal oxide (
withstand short pulse overloads and low inductance)
■ Audio and Instrumentation circuits — metal film (low noise)
■ High Voltage — wirewound and metal oxide in high-voltage body styles
■ RF — carbon composition and metal oxide (low inductance)
■ Precision circuits — carbon or metal film (fixed value) and cermet (trimmers
or controls)
Remember to consider what is important for your circuit — value, power or
voltage, stability, cost — then look for the resistor type that meets those
requirements.
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