Electronics Principles & Applications Sixth Edition Chapter 14 Electronic Control Devices and Circuits (student version) ©2003 Glencoe/McGraw-Hill Charles.

ElectronicsElectronics

Principles & ApplicationsPrinciples & ApplicationsSixth EditionSixth Edition

Chapter 14Electronic Control

Devices and Circuits(student version)

©2003 Glencoe/McGraw-Hill

Charles A. Schuler

• The Silicon Controlled Rectifier

• Full-Wave Devices

• Feedback in Control Circuitry

• Troubleshooting

REVIEW

Dear Student:

This presentation is arranged in segments. Each segment is preceded by a Concept Preview slide and is followed by a Concept Review slide. When you reach a Concept Review slide, you can return to the beginning of that segment by clicking on the Repeat Segment button. This will allow youto view that segment again, if you want to.

Concept Preview• An NPN-PNP latch can be either on or off.

• Once gated on, the latch cannot be gated off.

• Silicon controlled rectifiers (SCRs) are latches.

• An SCR is turned on by applying a pulse to its gate terminal.

• With a dc source, the SCR stays on after it is gated.

• With an ac source, the SCR turns off at the zero crossing.

• Load power is controlled by gating the SCR earlier or later during the ac cycle.

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.

Gate

Anode

Cathode

Loa

dcu

rren

t

Time

Load…. or, notat all.

No gate pulses: ILoad = 0

0

Concept Review• An NPN-PNP latch can be either on or off.• Once gated on, the latch cannot be gated off.• Silicon controlled rectifiers (SCRs) are latches.• An SCR is turned on by applying a pulse to its

gate terminal.• With a dc source, the SCR stays on after it is

gated.• With an ac source, the SCR turns off at the

zero crossing.• Load power is controlled by gating the SCR

earlier or later during the ac cycle.

Repeat Segment

Concept Preview• An SCR, in conjunction with a bridge rectifier,

can provide full-wave control.

• Two SCRs can provide full-wave control without a bridge rectifier.

• A triac is equivalent to two SCRs.

• Solid state relays combine triacs and optoisolators in a single package.

• SCRs and triacs are both members of the thyristor family.

• A diac breaks down in both directions and is ideal for gating triacs.

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

Solid state relays provide optical isolation.

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.

Illustration courtesy Powerex, Inc.

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 gating the 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

Concept Review• An SCR, in conjunction with a bridge rectifier,

can provide full-wave control.• Two SCRs can provide full-wave control

without a bridge rectifier.• A triac is equivalent to two SCRs.• Solid state relays combine triacs and

optoisolators in a single package.• SCRs and triacs are both members of the

thyristor family.• A diac breaks down in both directions and is

ideal for gating triacs.

Repeat Segment

Concept Preview• Servos use negative feedback to provide

accurate velocity or positioning.

• A velocity servo compares tachometer voltage with a set voltage.

• A position serve compares potentiometer voltage with a set voltage.

• Servo response can be critically damped (most desired), overdamped, or underdamped.

• Servos are tuned for the best response by adjusting gain and phase.

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 control position or speed.

feedback

When servo response is sluggish, the response is __________. overdamped

When servo response is oscillatory, the response is __________. underdamped

The overdamped response can be caused by __________ gain. insufficient

The underdamped response can be caused by __________ gain. excessive

Concept Review• Servos use negative feedback to provide

accurate velocity or positioning.

• A velocity servo compares tachometer voltage with a set voltage.

• A position serve compares potentiometer voltage with a set voltage.

• Servo response can be critically damped (most desired), overdamped, or underdamped.

• Servos are tuned for the best response by adjusting gain and phase.

Repeat Segment

REVIEW

• The Silicon Controlled Rectifier

• Full-Wave Devices

• Feedback in Control Circuitry

• Troubleshooting