Final Control

1
Final Control

• Introduction
• Thyristors
• SCR
• TRIAC
• DIAC
• Stepping Motors
• Summary

2
Thyristor SCR-Silicon Controlled
Rectifier Triac Diac etc

• A thyristor is a four-layer semiconductor device,
  consisting of alternating P type and N type
  materials (PNPN). A thyristor usually has three
  electrodes an anode, a cathode, and a gate
  (control electrode). The most common type of
  thyristor is the silicon-controlled rectifier
  (SCR).
• Thyristors are used in motor speed controls,
  light dimmers, pressure-control systems, and
  liquid-level regulators.

SCR Symbol
Hockey Puck SCR http//www.larkinpower.com/Thyrist
ors.htm
3
SCR Origins

• A commercially practical solid state rectifier
  has been operated by engineers at General
  Electric's Clyde, New York Rectifier Engineering
  Laboratory. This silicon-controlled rectifier
  operates in the same manner as a thyratron, and
  is capable of switching 1000 watts. A power
  rating of this magnitude is sufficient for most
  military and commercial applications. The device
  is expected to be first used in missiles.
• The size of the silicon controlled rectifier is
  approximately twice that of a signal type
  transistor and 1/100 the size of a thyratron. The
  predicted life of the device is over 300,000
  hours as compared to the 1000 hour life of a
  thyratron.
• (Electronic Design, Jan. 8, 1958, p. 7)
• The first SCRs were compared to thyratrons
  because that's what vacuum-tube era engineers
  understood. For today's engineer, we'd probably
  have to reverse the analogy--a thyratron, a
  gas-filled grid-controlled rectifier, was like an
  SCR.--Steve Scrupski

Technical Papers
Thyratron Power Supply at Fermilab
4
Operation of an SCR
http//www.tpub.com/neets/book7/26c.htm
5
SCR Characteristics
http//www.nteinc.com/Web_pgs/SCR.html
6
SCR V-I curve
VAK 1-1.5 volts
I
Very High
V
Very High Reverse voltage to break it down
http//www.tpub.com/neets/book7/26d.htm
7
SCR Circuit
Characteristics of 2N1595
Peak Reverse Blocking Voltage 50 volts RMS
Forward Current (max) 1.6 amps Typical Gate
Trigger Current 2 mA Typical Gate Trigger
Voltage .7 volts Turn-on time .8 usec Turn-off
time10 usec
8
SCR Output
9
SCR for Overvoltage Protection
Computer
Vin
10
Phase Control with an SCR
Characteristics of 2N1595
http//www.fairchildsemi.com/pf/1N/1N4148.html
11
SCR Phase Control
12
Diacs andTriacs
Diac
Triac

• The Diac and Triac are bilateral thyristors,
  meaning that they conduct current in both
  directions
• The Diac is designed to conduct when breakdown
  occurs in both directions The triac is like two
  parallel SCRs, with one in each direction.
• Triacs have less current carrying ability than
  SCRs

13
A Triac Full-Wave Circuit
?au RC .12 msec
14
A Triac Full-Wave Circuit
?au RC 3.8 msec
15
Diac-Triac Phase Control
Power Control with Thyristors and Triacs
16
Diac-Triac Light Dimmer
http//www.geocities.com/tjacodesign/dimmer/dimmer
.html
17
Stepper Motors

• A stepper motor system is an electro-mechanical
  rotary actuator that converts electrical pulses
  into unique shaft rotations. This rotation is
  directly related to the number of pulses. The
  speed is synchronous to the rate of pulsing.
• Stepper motors feature bi-directional control,
  built-in braking, variable torque, power control,
  precision accuracy, high resolution, open-loop
  control, and direct interface to digital systems.
  Stepper
  Motors - General Description
• A step motor converts electrical energy into
  discrete motions or steps.
• The motor consists of multiple electrical
  windings wrapped in pairs (phases) around the
  outer stationary portion of the motor (stator).
• The inner portion (rotor) consists of iron or
  magnetic disks mounted on a shaft and suspended
  on bearings.
• The rotor has projecting teeth which align with
  the magnetic fields of the windings. When the
  coils are energized in sequence by direct
  current, the teeth follow the sequence and rotate
  a discrete distance necessary to re-align with
  the magnetic field.
• The number of coil combinations (phases) and the
  number of teeth determine the number of steps
  (resolution) of the motor. For example, a 200
  step per rev (spr) motor has 50 rotor teeth times
  4 coil combinations to equal 200 spr.
• There are no brushes between the rotor and stator
  assembly a stepper motor is a multipole
  (polyphase) brushless DC motor.
• These multiple coil pairs can be connected either
  positive or negative resulting in four unique
  full steps. When the coils are sequenced
  correctly, the motor rotates for- ward. When the
  sequence is reversed, the motor rotates in
  reverse.

http//www.cs.uiowa.edu/jones/step/introduction
http//www.doc.ic.ac.uk/ih/doc/stepper/
18
A Stepper Motor
In the KP4M4-001 stepper motor, the permanent
magnet lies North - South along the shaft. It is
encased in two "stacks" each with 25 teeth round
the rim. The teeth on the South stack are out of
phase with the teeth on the North stack by half
the gap between teeth as can be seen in the photo
of the shaft shown above.
http//www.doc.ic.ac.uk/ih/doc/stepper/kp4m4/
19
SimpleStepper Operation
Permanent Magnet or can also be switched
http//www.st.com/stonline/books/ascii/docs/1679.h
tm
20
Summary

• Introduction
• Thyristors
• SCR
• TRIAC
• DIAC
• Stepping Motors
• Summary