A resistor is a passive two-terminal electrical component that limits the
current flowing in electrical or electronic circuits. Its property to resist the
flow of current is called resistance, expressed in ohm (Ω), named after
German physicist Georg Simon Ohm.
Resistors are available in different sizes. Its size is directly proportional
to its power rating. The power rating is the maximum amount of power
that a resistor can dissipate without being damaged by excessive heat
build-up. The larger the surface area covered by a resistor, the more
power it can dissipate.
TYPES OF RESISTORS
There are actually two types of resistors: fixed and variable.
Fixed resistors are designed to set the right conditions in a circuit. Their
values should never be changed to adjust the circuit since those were
determined during the design phase. It can have a carbon composition
or chip-and-wire wound type. It can also be made with a mixture of
finely ground carbon or be very small in size and for high power rating.
Variable resistors have fixed resistor elements plus a slider. The slider
taps onto the main resistor element so there will be three connections;
two are connected to the third element and one to the slider. Examples
of this are potentiometers, rheostats, trimmers, and so on.
HOW DO RESISTORS WORK?
Wiring a resistor in a circuit will reduce the current by a precise amount.
If you look at resistors from the outside, they most likely look the same.
However, if you break it open, you’ll see an insulating ceramic rod
running through the middle with copper wire wrapped around the
outside. Resistance depends on those copper turns. The thinner the
copper, the higher the resistance since it’s harder for the electrons to
pass through it. As we’ve found out, it’s easier for the electrons to flow
in some conductor materials than insulators.
George Ohm studied the relationship between resistance and the size
of the material that was used to make the resistor. He proved that the
resistance (R) of a material increases as its length increases. This
means that the longer and thinner wires offer more resistance. On the
other hand, resistance decreases as the thickness of wires increases.
Having said that, Georg Ohm came up with an equation that explains
this relationship:
where ρ = resistivity (Ω-m)
Note: Conductors have much lower resistivity than insulators. At room
temperature, aluminum comes in at about 2.8 x 10-8 -Ωm, while copper
is significantly lower at 1.7 x 10-8 Ω-m. Silicon has a resistivity of about
1000 Ωm and glass measures about 1012 Ω-m. Resistivity varies for
different materials.
RESISTOR COLOR CODING
SMD RESISTORS
SMD means Surface Mounted Device. It is used to create Surface
Mount Technology. SMDs have small leads or pins that are soldered to
pads on the surface of the board, instead of wire leads that go through
the PCB. This eliminates the need for holes in the board and lets both
sides of the board be more fully used. Since SMDs are too small, there
is no room for traditional color band code to be printed on them. For this
reason, new SMD codes were developed.
EIA-96 SYSTEM
This system is based on the E96-series, thus aimed at resistors with 1%
tolerance. Values are denoted by two (2) numbers, to indicate the
resistor value and one (1) letter for the multiplier. The two numbers
represent a code that indicates a resistance value with three significant
digits. The tables below shows the value of each code. For example,
38C = 24300 Ω ±1%.
SMD Resistor Code
Table of Values for EIA-96 System
THREE- AND FOUR-DIGIT SYSTEM
In this system, the first two or three digits indicate the numerical
resistance value of the resistor, and the last digit gives a multiplier– the
power of ten by which to multiply the given resistor value. For example:
273 = 27 Ω x 103 or 27,000 Ω (27 kΩ)
7992 = 799 Ω x 102 or 79,900 Ω (79.9 kΩ)
Note: The letter “R” is used to indicate the position of a decimal point for
resistance values lower than 10 ohms. For example, 0R5 would be
0.5Ω, and 0R01 would be 0.01Ω.
RESISTOR POWER RATING
Everytime a current passes through a resistor due to the presence of a
voltage across, electrical energy is lost in the form of heat. The greater
the current flow, the hotter the resistor will be. A resistor can be
functional at any combination of voltage and current as long as it does
not exceed the power rating that a resistor can convert into heat or
absorb without any damage.
Resistor Power Rating is defined as the amount of heat a resistor can
handle without sacrificing its performance in no definite time. In Ohm’s
law, when a current flows through a resistance, a voltage is dropped
across it producing a product that relates to power. In other words, if
resistance is subjected to a voltage, or if it conducts a current, then it
will always consume electrical power. Given this, we can say that these
three quantities– power, voltage and current, are in a power triangle.
Resistor Power Triangle
Using the Resistor Power Triangle is the best way to calculate the
power dissipated in a resistor if we know the values of the voltage and
current across it. Additionally, Ohms law allows us to calculate the
power dissipation given the resistance value of the resistor. We can
obtain two alternative variations of the above expression for the resistor
power if we know the values of at least two among the three– voltage,
current, and resistance.
Based on the power triangle, the electrical power dissipation of any
resistor in a DC circuit can be calculated using one of the following
three standard formulas:
where V is the voltage across the resistor in Volts, I is current flowing
through the resistor in Amperes, and R is the resistance of the resistor
in Ohmss (Ω).
TYPES OF RESISTOR MATERIALS
Below are different types of resistor materials, their pros and cons, and
their uses:
Carbon film resistors are made up of a pure carbon film enclosed in an
insulating cylindrical core, cut in a spiral to increase the resistive path. It
is more accurate than carbon composite. However, in applications that
require high pulse stability, special carbon film resistors are used.
Metal film resistors are produced with tantalum nitride but more often,
they are made using Nichrome. A combination of ceramic and metal is
used as the resistive material. It has better stability, temperature
coefficient, and tolerance than carbon films. Typical tolerances are
between 0.5% and 2% with a temperature coefficient between 50 and
100 ppm/K. Stability is lower than wire-wound, but its high-frequency
properties are better.
Wire wound resistors are created using a winding resistance wire that
has a spiral non-conductive core. The resistance wire is made up of
nickel-chromium and the core is ceramic or fiberglass which has a
coating protected with vitreous enamel. It is not suitable for applications
higher than 50kHz since the spiral winding has capacitive and inductive
effects. It is best used for high precision or for high power applications.
Precision resistors have a thin bulk metal foil that is cemented on a
ceramic substrate. It is the most accurate and stable type and it features
a very low-temperature coefficient of resistance that is used for
applications with high precision requirements.
Metal oxide film resistors. The resistive material is usually a metal oxide
such as tin oxide. It is useful in applications requiring higher endurance
because it has a higher operating temperature that makes it more
reliable and stable.
Carbon composite resistors are made up of a mixture of fine carbon
particles and a non-conductive ceramic material pressed in a cylindrical
shape and baked. The resistance value depends on the dimensions of
the body and the ratio between carbon and ceramic material. The more
carbon you add, the lower the resistance. Carbon composition resistors
are remarkably reliable but have poor accuracy with a maximum
tolerance of around 5%.
Summary of Key Performance Indicators for each Resistor Material