Epson L90P - TM Two-color Thermal Line Printer Manual de usuario descargar pdf (Pagina 54)

ANODE

GATE

ANODE

OA

CATHODE CATHODE

HOW

IT WORKS

Operation of

the touch control circuit

depends

on D5,

a silicon

unilateral

switch

(SUS).

This

semiconductor

is essentially a miniature

SCR

with

an anode gate

(instead

of

the usual cathode

gate) and a built

-in low

-voltage

avalanche diode

between

the

gate

and the cathode.

The SUS

switches

on when

its

gate

is

raised

to a

voltage

level in excess

of

that

required to cause the ava-

lanche

diode

to saturate.

When the avalanche

diode

is forced out of

conduction, the SUS cuts

off.

When

power is applied,

transistor

QI,

across

the gate and cathode

of

D5,

automatically

brings

D5

into

conduction.

This

applies a

negative

volt

-

age to the

gates

of

the SCR's cutting them off

and

removing power from the

load.

When contact

is

made to the

"turn

on" terminal,

Q2

conducts

to turn

D5

off.

This automatically

allows

both

SCR's

to turn on, on

the

next positive -going a.c.

alternation, thus providing

power

to the

load.

Contacting

the

"turn

off"

terminal causes

the

circuit

to revert to its original condition, thus

removing

power from the load

the

next

time

that the a.c.

line

alternation

goes

to

zero. The

gating voltage

for

both

transistors

comes

from

the

a.c.

field present in the human body when

the

person

is

in

the

presence of commercial a.c.

power

lines.

The

gates

of the SCR's

are

connected to the

power supply

(at

the junction of D3

and

D4)

through D5.

Since

the SUS can only turn off at

the

zero

point

of the a.c. waveform,

the

SCR's

are

turned on

only at that point.

This

character-

istic provides

minimum

clistortMn to the line cur-

rent (such as

that caused

by

the

opening

and

closing

of mechanical contacts) and therefore

pre-

vents

electrical

interference.

Resistors R7 and

123

prevent shock when

either of

the

touch

terminals is contacted.

will operate the

circuits. One way of

do-

ing

this

is to

make two

isolated metal

contact areas

for the on

-off operation,

with a

narrow

metal

strip

for the

resis-

tor contact

between

them. In this way,

contact

can be made to

turn the light

either

on or off.

Remember at all times that many por-

tions of the

circuit board are

"hot"

to

ground and avoid

getting

a

shock.

Besides a

lamp,

the touch control

can

be used to turn

on

any

117

-volt a.c.

re-

sistive or

inductive

load

whose power

requirements are

less than 200

watts.

Once

the

board

has been completed and

tested, it should

be

encapsulated

with

an

insulating

material

to

prevent

possibility of

electrical

shock. Only the two screws

at

left are

exposed

for

external

contacts.

58

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