AEG Electrolux ERN 2922 User Manual download pdf (Page 19)

A couple of

degrees by

itself

would

be of

little consequence

if

it

were

not

for the fact that

objects,

including

ambient

air, do

not cool

or heat

in-

stantaneously.

(Each

type of

mater-

ial

has its own

thermal constant.)

Hence,

large temperature

excursions

above and

below

the setpoint

are a

common

phenomenon.

So the

first

use

of electronic

controllers

was

as

electronic

thermostats

or

on /off

controllers.

By replacing the

bulky power

relay

with

a thyristor

and

the thermostat

with

a solid

-state sensor,

hysteresis

can

be set to

a minimum,

while

at the

same

time eliminating

contact

arcing

and

its contribution

to system

fail-

ure. With

this type of

arrangement,

heating elements

can be switched

faster.

Shown

in Fig.

1 is a typical

elec-

tronic thermostat

circuit

built

around

a CA3059

zero -voltage

inte-

grated-

circuit

switch.

The CA3059

has an

internal rectifier

and

voltage

regulator that are

supplied

from the

ac power

line, a circuit

that detects

the

zero crossing of

the ac

waveform,

a current -limiting

triac

driving cir-

cuit, and

a protection

network that

monitors the

sensor for

an open or

shorted condition.

A basic controller

circuit

is

very

simple

in terms of

both component

PARTS LIST

Semiconductors

A I -CA3059

zero -voltage

switch

Q1- MAC2236

or similar

400

-volt,

20-

ampere

triac

specified

for

four -quad-

rant triggering

(see text)

Capacitors

C

1- 100 -µF,

10 -volt aluminum

electro-

lytic

C2-

0.05 -µF,

200 -volt

Mylar

Resistors

R1-See

text

R3 -1,800

ohms, '/z

watt, 5% toler-

ance

R2

-5,000 -ohm

NTC

thermistor

P1- 10,000

-ohm,

linear -taper

potenti-

ometer

TEMP

SET

POINT

TIME

Fig. 2. The electronic

on

/off controller

suffers from

large temperature

fluctua-

tions

above and

below

the setpoint.

count

and complexity.

Though

it

can

be built

with

only

a few discrete

com-

ponents and

a single

integrated

cir-

cuit,

this type

of circuit provides

a

high

-performance

on /off control-

ling device.

By firing the

triac inside

the

IC

at

the

zero crossing

point

of the

ac

waveform, noise pulses

produced

by

random turn

-on are avoided

and

triac

stresses that

become

more meaning-

ful

with

larger

loads are decreased.

Operation of

the

Fig. 1

circuit

is

as

follows. Ac line

voltage is dropped

by

R1,

which feeds the

IC's internal

limiting circuit.

By selecting

the pro-

per resistor

value,

any ac

supply

po-

tential

from 24 to

250

volts

can

be

used

to power the circuit.

This limit-

ing circuit

feeds

the internal

supply

and zero -crossing

networks.

A regulated

6.5

volts

is available

at

pin 2 of

the CA3059,

which is

filtered

by Cl.

An

internal comparator

is

driven

at its

noninverting

input by

the

voltage from

the divider

network

made up of

potentiometer

PI and

temperature-

sensing

thermistor

R2.

The pin

9 inverting

input

is fed by the

internal biasing

resistors

and

is avail-

able

at pins

10 and

11.

Because a

negative

temperature

coefficient

(NTC)

resistor

is used

in

the

Fig. 1 circuit,

a

rise in tempera-

ture

will

decrease

the

voltage

applied

to pin

13 of the

CA3059.

This turns

off the

internal comparator

that sup-

plies a

signal

to an

internal

AND

TEMP

T

IME

Fig.

3.

Tighter control

is achieved

in the proportional

band,

between

the 0 and

100-

percent

power

levels,

when power

is gradually

decreased.

Say You Saw

It In

Modern Electronics

February

1988 / MODERN

ELECTRONICS / 23

1 2 ... 14 15 16 17 18 19 20 21 22 23 24 ... 79 80

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