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

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 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