ElectronicsElectronics

Principles & ApplicationsPrinciples & ApplicationsFifth EditionFifth Edition

Chapter 14Electronic Control

Devices and Circuits

©1999 Glencoe/McGraw-Hill

Charles A. Schuler

• The Silicon-Controlled Rectifier• Full-Wave Devices• Feedback in Control Circuitry• Troubleshooting

INTRODUCTION

This two-transistor circuit isstable in either of two states.

On state Off state

Q1

Q2

Q1

Q2

p

p

n

n

A four-layer structure replacesthe two-transistor circuit.

Q1}{Q2

The silicon controlled rectifier (SCR)

Q1

Q2

Anode

Gate

Cathode

p

p

n

n

Anode

Gate

Cathode

Anode

Gate

Cathode

Gate

Anode

CathodeL

oad

curr

ent

TimeGate pulseoccurs here

LoadWith a dc source, the SCR stays

on after it is gated.

The SCR can be turned on at its gate terminal.

Gate

Anode

Cathode

Loa

dcu

rren

t

TimeGate pulseoccurs here

LoadWith an ac source, the SCR turnsoff at the

zero-crossing.

Turns off here

on

off

Gate

Anode

Cathode

Loa

dcu

rren

t

Time

LoadThe gate canbe pulsed foreach positivealternation.

Gate

Anode

Cathode

Loa

dcu

rren

t

Time

LoadThe averageload current

can bedecreasedby gating

the SCR later.

Gate

Anode

Cathode

Loa

dcu

rren

t

Time

Load…. and later.

Load

A full-wave rectifier allows use of both alternations.

Bridge rectifier

Load current

Gate

Load

Two SCRs can providefull-wave control.

Gated early for full power

Gated later for low power

Load

Main terminal 1

Main terminal 2

Gate

The TRIAC provides control of both ac alternations.

Gated early

Gated late

DIAC volt-ampere characteristic curves

+I

-I

+V-V

VP+

VP-

Load

A popular diac-triac control circuit

Decreasing R will gate the TRIAC

earlier.

Thyristor quiz

The three terminals of an SCR are anode,cathode and _________. gate

SCR turnoff in ac circuits occurs at the__________ crossing. zero

In ac control, load power is increased by gatingthe SCR ______ in the cycle. earlier

The device equivalent to two SCRs for full-wave control is the __________. triac

The diode often used to trigger triacs is the_________. diac

Servomechanisms use negative feedback to control velocity and/or position.

Motor

Tachometer

ErrorAmplifier

Negative feedback

Velocityset

VREF

If the mechanical load changes, causing the velocityto change, the error amplifier will respond by

adjusting the motor drive to reduce the change.

Motor

ErrorAmplifier

Positionset

VREF

VREFGearbox

In this servo,gears drive a

variable resistor to provide

position feedback.

Feedback

Pos

itio

n

Time

t1

Suppose, at time t1, a servo is commanded to a new position.

This is the ideal response

Pos

itio

n

Time

t1

This is the critically damped response

Pos

itio

n

Time

t1

This is the overdamped response

Pos

itio

n

Time

t1

This is the underdamped response

Pos

itio

n

Time

Tuning a servomechanism involves adjusting the loop gain and the phase

to achieve the best response.

Which response is the best?

Troubleshooting

• SCRs can fail by opening or shorting

• A shorted SCR means full load power

• An open SCR means no load power

• A defective gate circuit can produce either full power or no power

• TRIAC troubleshooting is much the same

Servo Troubleshooting• Mechanical problems can be confused with

electrical faults

• Slippage and excessive mechanical play can cause various symptoms and should be eliminated before tuning or troubleshooting

• The underdamped response is often caused by excessive gain

• The overdamped response is often caused by insufficient gain.

Servo quiz

Servos use negative _________ to controlposition or speed. feedback

When servo response is sluggish, theresponse is ___________. overdamped

When servo response is oscillatory, theresponse is ___________. underdamped

The overdamped response can be causedby _________ gain. insufficient

The underdamped response can be causedby _________ gain. excessive

REVIEW

• The Silicon-Controlled Rectifier• Full-Wave Devices• Feedback in Control Circuitry• Troubleshooting