64 Fundamentals of Electricity and Electronics
an actuator similar to a push button switch. These break-
ers are pushed in to reset after tripping. Circuit breakers
and resets require a waiting period to allow the internal
trip mechanism to cool down. Most homes today use cir-
cuit breakers as the safety device to prevent overloads.
Overloads could result in house fires.
Circuit breakers are produced with two different trip-
ping methods. One method uses bimetallic strips. A
bimetallic strip is a metal strip made of two different
types of metal. Different metals expand at different rates.
Heat generated from the overload condition causes the
bimetal trip to expand. The different metals expand at dif-
ferent rates. This causes the breaker’s trip mechanism to
bend and break contact. Some trip mechanisms are
adjustable to allow for a more precise trip current.
A second tripping mechanism uses magnetism to
operate. The circuit current runs through a coil. As the
current increases through the coil, the amount of magnet-
ism in the coil increases. When a predetermined point is
reached, the tripping mechanism operates and opens the
circuit. The magnetic circuit breaker is much faster and
more accurate than the bimetallic circuit breaker. The
magnetic circuit breaker, however, is more expensive than
the bimetallic type.
Review Questions for Section 3.3
1. What type of switch is best for controlling lighting
from two different locations?
2. What does the abbreviation DPDT mean?
3. What advantage does a circuit breaker have over a
fuse?
4. How are fuses sized?
3.4 LIGHTING
There are many different methods in which light is
produced. Light is generated when burning a variety of
gases (methane, propane), liquids (gasoline, fuel oil), and
solids (wood, paper). Certain chemicals will luminesce
when mixed together. Necklaces and bracelets filled with
these chemcials are often sold at nighttime celebrations.
Various minerals and glow-in-the-dark toys supply light
through phosphorescence. They absorb energy (from sun-
light or a lamp) and release this energy in the form of
light when the energy source is removed. However, for
most of our lighting systems, the energy source is elec-
tricity. There are two very different types of electric light-
ing systems. One is the incandescent lamp and the other
is the discharge lamp.
Incandescent Lamp Principles
In 1879, Thomas Edison developed the first success-
ful incandescent lamp. In an incandescent lamp, light
and heat are created from current flowing through a fila-
ment. The first lamp was of simple construction with a
carbon filament inside a glass envelope. The air was
removed from the glass envelope to prevent the carbon
from igniting. Soon afterward the tungsten filament
replaced the carbon filament. The tungsten filament
proved to be a much more efficient method of producing
light than the carbon filament. In Figure 3-20, the basic
construction of the incandescent lamp is illustrated.
Halogen lamp
In a halogen lamp, a tungsten filament is inserted
through a glass tube, and the tube is filled with halogen
gas. This type of lamp will economically produce a great
deal of light. It is commonly used in automobile head-
lights. The halogen gas makes the tungsten filament last
longer. The tungsten filament in most lamps evaporates
over a long period of time due to the tremendous heat
generated by tungsten filament lamps. The halogen
Figure 3-19. A typical circuit breaker.