BEX100 Basic Electricity

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BEX100 Basic Electricity

• Semiconductors
• Transistors SCRs

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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

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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

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Introduction to Transistors

• The terms used for the 3 terminals of a
  transistor are the emitter, the collector,
  and the base

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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

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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

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Basic Transistor Operation

• Supplying a positive/negative voltage to the base
  supplies the electrons needed for current flow

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Transistor Operation

• Amplification aspects of a transistor

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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.

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The NPN Transistor
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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)

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A Typical NPN Circuit
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The PNP Transistor
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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

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A Typical PNP Circuit
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Transistor Applications
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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

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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

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HEI Transistorized Ignition System
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EST Electronic Spark Timing
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The Hall Effect Switch
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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

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SCR Silicon Controlled Rectifiers
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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

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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

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SCR Operation
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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

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LETS 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?

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Elizabethtown Technical CollegeBEX100 Basic
ElectricityTransistorsSpring Semester 2001