Diodes, Triodes, Thermistors, Opto-isolators, & Phototransistors

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Diodes, Triodes, Thermistors, Opto-isolators,
Phototransistors

• ME 6405 Spring 2005
• Danny Nguyen
• Wei Tan
• Qiulin Xie

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Presentation Outline

• Diodes Danny
• Triacs Thermistors Qiulin
• Opto-isolators Phototransistors Wei

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Diodes Overview

• Meet the Diode
• Junction Diodes
• Analysis and Applications
• Zener Diodes and Applications

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What is a Diode?

• Simplest semiconductor device
• Allows current to flow in one direction but not
  the other
• Symbols

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Junction Diodes

• Start out with Silicon or Germanium (Group IV
  elements)
• P-type - doping with Group III elements
• Boron, Aluminum, Gallium
• Adds positive holes to the region
• N-type - Group V doping
• Phosphorous, Arsenic
• Add electrons to the region

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Junction Diodes

• Due to thermal energy, some electrons diffuse
  into the p-type region, creating a depletion
  region
• No current flows through the diode at this point

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Junction Diodes

• Forward Bias
• Depletion region decreases
• Current flow when voltage is high enough (0.6-0.7
  Volts)
• Current sustained by majority carriers

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Junction Diodes

• Reverse Bias
• Depletion region increases
• Small leakage current by minority carriers
• Reverse saturation current (I0)
• On the order of 10-9 to 10-15 A

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Analysis of Diodes

• Mathematical Model
• Ideal Model
• On Off
• Constant Voltage Drop Model
• On
• Off

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Analysis and Applications

• Half-wave rectifier
• CVD Analysis
• On Replace diode with Von voltage source
• Off Replace diode with open circuit

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Analysis and Applications

• Half-wave rectifier
• CVD Analysis
• On
• Off

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Analysis and Applications

• Half-wave rectifier
• CVD Analysis
• On
• Off

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Analysis and Applications

• Full-wave bridge rectifier
• Peak Detector

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Zener Diodes

• Operated by reverse bias instead of forward bias
• All diodes have a breakdown region point where
  the diode can not handle anymore negative voltage
• Voltage remains nearly constant in the breakdown
  region (Vz Zener Voltage) under widely varying
  current for Zeners

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Zener Diodes I-V Graph
Reverse Breakdown Model
Schematic
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Zener Diodes Applications

• Ability to maintain a constant voltage allows it
  to act as a voltage regulator

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Zener Diodes Specifications

• VZ (Zener Voltage) Common range is between 3.3V
  and 75V
• Tolerance Commonly 5 to 10
• Power Handling ¼, ½, 1, 5, 10, 50 W

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Contents

• Shockley Diode
• Silicon-Controlled Rectifier (SCR)
• Triac
• Thermistor

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Shockley Diode

• Shockley diode after its inventor, William
  Shockley
• four-layer diode, also known as a PNPN
• on if applying sufficient voltage between anode
  and cathode
• Off if reducing to a much lower voltage

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Silicon-Controlled Rectifier (SCR)

• Shockley diode becomes SCR if gate addition to
  PNPN
• it behaves exactly as a Shockley diode If an
  SCR's gate is left disconnected.
• gate terminal may be used as an alternative
  means to latch the SCR
• SCRs are unidirectional (one-way) current
  devices, making them useful for controlling DC
  only

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Triode AC Switch (Triac)

• A triac can be regarded as a "bidirectional (AC)
  SCR because it conducts in both directions.
• 5 layer device
• Region between MT1 and MT2 are parallel switches
  (PNPN and NPNP)
• Allows for positive or negative gate triggering

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Triggering Quadrant
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Triac Characteristic Curve
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Triac Characteristic Curve

• VDRM refers to the maximum peak forward voltage
  which may be continuously applied to the main
  terminals and the highest voltage that can be
  blocked
• IDRM is the leakage current of the Triac when
  VDRM is applied to MT1 and MT2 , which is several
  orders of magnitude smaller than the on rating
• VRRM Peak Repetitive Reverse Voltage
• Maximum peak reverse voltage that may be
  continuously applied to the main terminals
• IGT Gate trigger current
• VGT Gate trigger voltage
• Latching Current the value of on-state current
  required to maintain conduction at the instant
  when the gate current is removed
• Holding current Value of on-state current
  required to maintain conduction once the device
  has fully turned on and the gate current has been
  removed. The on-state current is equal to or
  lower in value than the latching current

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Triac Advantages and Applications

• Advantages
• Controllable trigger
• Four quadrant device
• Triacs provide the lowest cost and simplest route
  to reliable, interference-free switching and
  power control.
• Application
• Light dimmer control
• Motor speed control (a phase-control circuit is
  used to vary the power to brush motors.)
• Reason
• Trigger pulse can control any percentage of half
  cycle

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Thermistor

• Thermistor - Temperature sensitive resistor
• Their change in electrical resistance is very
  large and precise when subjected to a change in
  temperature.
• Thermistors exhibit larger parameter change with
  temperature than thermocouples and Resistance
  Temperature Detectors (RTDs).
• Thermistor - sensitive
• Thermocouple - versatile
• RTD stable
• Generally composed of semiconductor materials.
• Very fragile and are susceptible to permanent
  decalibration.

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Thermistor Probe

• One of many available probe assemblies

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Thermistor Characteristics

• Most thermistors have a negative temperature
  coefficient (NTC) that is, their resistance
  decreases with increasing temperature.
• Positive temperature coefficient (PTC)
  thermistors also exist with directly proportional
  R vs. T.
• Extremely non-linear devices (high sensitivity)
• Common temperature ranges are 100 F (-75 C)
  to 300 F (150 C)
• Some can reach up to 600 F

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Thermistor R-T Curve

• An individual thermistor curve can be very
  closely approximated by using the Steinhart-Hart
  equation

T Degrees Kelvin R Resistance of the
thermistor A,B,C Curve-fitting constants

• Typical Graph

Thermistor (sensible)
V or R
RTD (stable)
Thermocouple (versatile)
T
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Thermistor Applications

• Resistor is set to a desired temperature (bridge
  unbalance occurs)
• Unbalance is fed into an amplifier, which
  actuates a relay to provide a source of heat or
  cold.
• When the thermistor senses the desired
  temperature, the bridge is balanced, opening the
  relay and turning off the heat or cold.

Temperature Control
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Phototransistor

• Introduction
• Package and Scheme
• Operation
• Advantages
• Example and applications

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Phototransistor Introduction

• A transistor which is sensitive to the input
  light intensity
• Operation similar to traditional transistors
  Have collector, emitter, and base
• Phototransistor base is a light-sensitive
  collector-base junction
• Dark Current Small collector can emit leakage
  current when transistor is switched off.

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Phototransistor Packages
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Phototransistor Scheme

• Photocurrent The electrons are amplified by the
  transistor and appear as a current in the
  collector/emitter circuit.
• The base is internally left open and is at the
  focus of a plastic lens.

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Phototransistor Operation

• The phototransistor must be properly biased
• A light sensitive collector base p-n junction
  controls current flow between the emitter and
  collector
• As light intensity increases, resistance
  decreases, creating more emitter-base current
• The small base current controls the larger
  emitter-collector current
• Collector current depends on the light intensity
  and the DC current gain of the phototransistor

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Why Use Phototransistors?

• More sensitive than photodiodes of comparably
  sized area
• Available with gains form 100 to over 1500
• Moderately fast response times
• Available in a wide range of packages
• Usable with almost any visible or near infrared
  light source such as IREDs, lasers, sunlight, and
  etc
• Same general electrical characteristics as
  familiar signal transistors

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Application Example Avoiding Obstacles
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Phototransistor Applications

• Computer/Business Equipment
• Write protect control floppy driver
• Margin controls printers
• Industrial
• LED light source light pens
• Security systems
• Consumer
• Coin counters
• Lottery card readers

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Optoisolator

• Introduction
• Scheme and Package
• Optocoupler Interrupter Example
• Advantages and applications

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Optoisolator Introduction

• A device that uses a short optical transmission
  path to accomplish electrical isolation between
  elements of a circuit.

Note 1 The optical path may be air or a
dielectric waveguide Note 2 The transmitting
and receiving elements may be contained within a
single compact module.
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Optoisolator Scheme

• The light emitted form the LED is detected by a
  photodetector which sits across from the LED
  inside the chip, and output a current.
• Since the input signal is passed from the LED to
  the photodetector, and cannot be passed form the
  photodetector to the LED, the input device is
  optically isolated from the circuit connected to
  the output side.

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Optoisolator Package

• An IRED is typically a controllable light source
  and a phototransistor employs as the detector
  element.
• The input and output sides have separate grounds
• Optoisolators sensitive to input voltages.

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Optocoupler Interrupter Example

• Integrated emitter and detector pair
• Setup Similar to Lab L3
• Used to calculate speed or distance

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Optoisolator Advantages Applications

• Advantages
• Output signals have no effect on input
• High reliability and high efficiency
• Noise isolation
• Small size
• Applications
• Optical switch
• Signal transmission devices
• Used to control motors, solenoids, etc.

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• Questions?

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References

• Introduction to Mechatronics and Measurement
  Systems, 2nd Ed. by D.G. Alciatore and M.B.
  Histand
• http//www.semiconductors.philips.com
• http//www.omega.com
• Microelectronic Circuit Design, 1st Ed. by
  Richard C. Jaeger
• Fall 2000 Slides