Page 120 - Electromechanical Devices and Components Illustrated Sourcebook

82 Electromechanical Devices & Components Illustrated Sourcebook

power supply and motor controller designers. Once the SCR in AC motor controllers, such as ceiling fan speed controls.
is turned on it will remain conductive until the voltage across Figure 4-142 shows a typical triac package.
the anode and cathode reaches zero, even after the gate volt-
age has been removed. This attribute makes these devices Transistors
ideal for switching AC because the voltage in an AC signal
drops to zero 120 times per second.
A transistor is essentially a semiconducting diode that has the
ability to control the current flow throughput. These devices
have a base, emitter, and collector. The emitter and collector
Triacs make up the primary terminals and the base acts as a variable
trigger. The amount of current that flows through the device is
A triac is simply a pair of SCRs that are placed back-to-back proportionally dependent on a lesser current applied to the
to form a solid state AC switch. Triacs are commonly found base. Figure 4-143 shows a typical commercial transistor that
may be found in ordinary electronic assemblies.

Base

Emmiter Collector
Schematic Symbol
Schematic Symbol

Package
Package

Lead Wires
Primary Terminals
Base
Figure 4-142 Triac Figure 4-143 Transistor