16-08-2012, 11:40 AM
Resistor Types
1Resistor.pdf (Size: 226.91 KB / Downloads: 121)
Resistors are the most common passive electronic component (one that does
not require power to operate). They are used to control voltages and currents.
While a resistor is a very basic component, there are many ways to manufacture
them. Each style has its own characteristics that make it desirable in certain
types of applications. Choosing the right type of resistor is important to making
high-performance or precision circuits work well. This bonus chapter covers the
resistor types and helps with picking the right one for your project.
Meet the Resistors
All resistors are basically just a piece of conducting material with a specific value
of resistance. For that piece of conducting material to be made into a practical resistor,
a pair of electrodes and leads are attached so current can flow. The resistor
is then coated with an insulating material to protect the conducting material from
the surrounding environment and vice versa. There are several different resistorconstruction
methods and body styles (or packages) that are designed for a certain
range of applied voltage, power dissipation, or other considerations. The construction
of the resistor can affect its performance at high frequencies where it may act
like a small inductor or capacitor has been added, called parasitic inductance or
capacitance.
Carbon-composition resistors
These are also known as carbon-comp resistors. “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 (one with very low parasitic inductance) that is often used
in RF circuits.
BC2 Circuitbuilding Do-It-Yourself For Dummies
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 (more about those
in a moment) because the heat is distributed evenly throughout the cylinder of resistive
material. That makes this type of resistor a good choice for circuits that protect
against and absorb pulses and transients (short bursts of excess voltage or current),
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.
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 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.
Surface-mount resistors are almost always film resistors; 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 resistors
Common in power supplies and other equipment that dissipates lots of power, wirewound
resistors are made just as you might expect: A high-resistance wire is wound
around an insulating form — usually a ceramic tube — and attached to electrodes at
each end. These are made to dissipate a lot of power in sizes from 1-watt to hundreds
of watts! Wirewound resistors are usually intended to be air cooled, but some
styles have a metal case that can be attached to a heat sink or metal chassis to get
rid of undesired heat.
Because the resistive material in these resistors is wound on a form, they also act
like small inductors. For this reason, wirewound resistors are not used in audio and
RF circuits. Be careful when using a resistor from your junk box or a grab bag in such
a circuit! Small wirewound resistors look an awful lot like film or carbon-comp resistors.
There is usually a wide color band on wirewound resistors, but not always. If
you’re in doubt, test the resistor at the frequencies you expect to encounter.
Ceramic and metal oxide
If you need a high-power non-inductive resistor, you can use cermet (ceramic-metal
mix) or metal oxide resistors. These are constructed much like carbon-comp resistors,
substituting the cermet or metal oxide for the carbon-composition material.