AEG Electrolux ERN 2922 User Manual download pdf (Page 18)

Project

Electronic

Heat

Control

How

electronics provides

more

efficient

utilization

of

available

energy resources

and

some

controller

circuit

examples

By

Fernando

Garcia Viesca

Electronic

"intelligence"

is

frequently

used

to manage

energy resources.

With elec-

tronic management,

control

of

all

types of heating,

particularly

elec-

trically generated,

can maximize

ef-

ficiency.

Electronic

energy manage-

ment

can be put

to effective

use in

business /commercial

and residential

heating,

maintaining

a

specific

water

temperature

range in

tropical

fish

aquariums,

even fractionating

crude

oil

into

gasoline

and

other petroleum

products.

Our discussion will

be

limited

to

electronic

management

of electrical

heating

systems in

which

tempera-

ture control

up to 150 °

C (302 ° F) is

required.

This range is

easily handled

with

low -cost

thermistors,

which

provide

a large

"gain"

without

the

need for

amplification.

Coupled

with

the thermistor's

small heating

constant,

an accurate

controlling

system

can be built

with

only simple

circuitry.

Accompanying

our discussion will

be schematic diagrams

for three

dif-

ferent

types

of electronic

controllers

you can build

and experiment with

or

use in

specific applications.

Before

we

begin, however,

a

word

of cau-

tion is

in order: When working with

these

circuits, exercise

extreme

care,

since you

will

be dealing with

poten-

tially lethal 117

or

220 volts

ac.

Basic

Controllers

In

a typical

electromechanical

ther-

mostat

arrangement,

the heating

load

is energized

through

a power

re-

lay

or "contactor."

The main

disad-

vantage

of this

setup is that

arcing

between

the

contacts of the relay

or

contactor

eventually

leads

to system

failure,

due

to erosion

of the metal

that makes

up the

contacts. To mini-

mize

on

/off

cycles, thermostats

have

built

-in hysteresis,

meaning

that the

temperature

must

drop two

degree:;

or so below

the setpoint

before

the

relay

energizes.

+..

P3

. -: 1 P'.7:

F'1. 51.,:.

PLTC

gl_1'- _'I.J 1 .::-:L.,::

1':1"..L::::

F'1'.»>

1

_'Ç.

RESISTOR

Rl

1

1 '

4 3

1

6

1

_ =1 '1'

1

4

F1 1

I NPLIT

VOLTS

-t

0--

LOA ll

Sht.tt:off

+

WfTT

1tAT

I NG

24

1_>

4

Q 1

t°1!_::._

_11Ñ

Fig.

1. This

simple

high - performance

on /off electronic controller

can be matched

to the ac input voltage

by

selecting

an appropriate

value

for

RI.

22

/

MODERN

ELECTRONICS / February

1988

Say You

Saw It In Modern

Electronics

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