Tab Electronics Guide to Understanding Electricity ... - Sciences Club

Basic Electrical Concepts
29
types of commonly manufactured resistors are carbon film, metal film,
molded composition, thick film, and vitreous enamel. These different types
possess various advantages, or disadvantages, relating to such parameters
as temperature stability, tolerance, power dissipation, noise characteristics,
and cost.
The two most critical resistor parameters are value (measured in ohms)
and power (measured in watts). Resistors that are larger in physical size
can dissipate (handle) more power than can smaller resistors. If a resistor
becomes too hot, it can change value or burn up. Consequently, it is very
important to use a resistor with adequate power-handling capability, as
determined by the circuit in which it is placed. Unfortunately, there is
not a good, standard way of looking at most resistors and determining
their power rating by some standardized mark or code. Comparative
size can be deceiving depending on the resistor construction. For example,
a 10-watt wire-wound resistor might be about the same size as a
2-watt carbon composition resistor. Common power ratings of the vast
majority of resistors are 1 8
, 1 4
, 1 2
, 1, and 2 watts. Don’t worry about being
able to determine resistor power ratings at this point. It is largely an
experience-oriented talent that you will acquire in time.
The value of most resistors is identified by a series of colored bands
that encircle the body of the resistor. Each color represents a number, a
multiplier, or a tolerance value. The first digit will always be the colored
band closest to one end.
There are two color-coded systems in common use today: the fourband
system and the five-band system. In the four-band system, the
first band represents the first digit of the resistance value, the second
band is the second digit, the third band is the multiplier, and the
fourth band is the tolerance (resistors with a tolerance of 20% will not
have a fourth band). The five-band system is the same as the four-band one,
except for the addition of a third band representing a third significant
digit. The five-band system is often used for precision resistors requiring
the third digit for a higher level of accuracy. There are also a few additional
colors used for tolerance identification in the five-band system.
You will need to memorize the following color-code table:
Resistor Color Codes
Black 0 Green 5
Brown 1 Blue 6
Red 2 Violet 7