BEX100 – Basic Electricity
Semiconductors
Transistors & SCR’s
Lesson Objectives
• To understand the basic construction elements and
•
•
•
•
•
schematic symbols of a transistor
Be able to identify the two main types of
transistors, (PNP, NPN types) and explain how
they operate
Understand what is meant by “Amplifier Current
Gain” of a transistor
Interpret wiring schematics containing transistors
Identify the various application uses for transistors
Understand the basic construction of an SCR and
how it functions
Introduction to Transistors
• A transistor is an electrical device that is used to
•
•
•
•
control current flow
It is made of three layers of semiconductor material
The construction is similar to the diode
Transistors often take the place of mechanical
switches and relays
A transistor can be thought of as two diodes that
share a common center layer
Introduction to Transistors
The terms used for the 3 terminals of a transistor are:
“the emitter”, “the collector”, and “the base”
Introduction to Transistors
•
•
•
•
The common schematic symbols used for transistors
The “emitter” always has a line with the arrow
The “base” is the heavy line at the bottom of the symbol
The “collector” is the line without the arrow
Transistor Construction
• Adding a second layer of “P-type” material to the basic
diode construction creates a “PNP” transistor
• Adding a second layer of “N-type” material to the basic
diode construction creates a “NPN” transistor
Basic Transistor Operation
Supplying a positive/negative voltage to the base
supplies the electrons needed for current flow
Transistor Operation
“Amplification” aspects of a transistor
Amplifier Current Gain
Transistors are small electrical controls that can perform
large electrical tasks. “Saturation” level occurs when the
maximum current flow of the circuit is reached.
The “NPN” Transistor
“NPN” Transistor Operation
• Supplying a small amount of
current to the base creates
current flow through the
transistor (switched on)
• Varying the amount of current
supplied to the base varies its
resistance and can vary the
current flow through the
emitter-collector
• No current to base means no
current will flow (switched off)
A Typical “NPN” Circuit
The “PNP” Transistor
“PNP” Transistor Operation
• Operates the same as the NPN
•
•
•
•
type of transistor
One difference being the bias
of the transistor in the circuit
Another being the polarity of
the voltage in the circuit
Has the same amplification
ability as the NPN
Operates as a electrical switch
in the circuit
A Typical “PNP” Circuit
Transistor Applications
The Darlington Pair
• Used when the current level supplied at the base is too
small to allow sufficient current flow through the circuit
• The first transistor is used to supply the necessary current
to the base of the second transistor which controls the
circuit
Solid State Relays
• Used in situations where a “relay-type” circuit is needed
• A “NPN” type transistor controls a “PNP” type transistor
• Benefits of solid state relays include faster operation, and
there are no moving parts to wear out
HEI Transistorized Ignition System
EST Electronic Spark Timing
The Hall Effect Switch
The Hall Effect Switch
•
•
•
•
With vanes “open” current flows to transistor
HEI reference voltage is less than 1 volt
With vanes “closed” no current flows to transistor
HEI reference voltage is 12 volts
SCR Silicon Controlled Rectifiers
SCR Silicon Controlled Rectifiers
“SCR” Silicon Controlled Rectifiers are a type of
transistor that by nature do not allow current to
flow in either direction until triggered
Unlike the transistors that act as switches, the SCR
will continue to conduct current even after the
trigger voltage has been removed
The main source of voltage must be removed from
the circuit to stop current flow through the SCR
The Silicon Controlled Rectifier
• Schematic symbol is
•
•
•
•
similar to diode
“The Anode” is positive
“The Cathode” is negative
“The Gate” is the trigger
A momentary voltage is all
that is required to initiate
and maintain current flow
SCR Operation
SCR Construction & Operation
• Constructed with alternating layers of semiconductor
material
• Similar to combining a PNP and a NPN type transistor
• Functions by applying voltage to the “gate” terminal
LET’S REVIEW
• WHAT ARE THE THREE LAYERS OF A
TRANSISTOR?
• WHAT IS THE SCHEMATIC SYMBOL
FOR A TRANSISTOR?
• WHAT ARE TWO FUNCTIONS OF
TRANSISTORS?
• WHAT ARE TWO THINGS THAT WILL
DAMAGE A TRANSISTOR?
Elizabethtown Technical College
BEX100 – Basic Electricity
Transistors
Spring Semester 2001