DEC VT220 - Page 191

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Q501

and

0502

are

coupled

to

the

pulse

width

modulator

in

reverse

polarity

via

T504.

Current

is

induced

in

T504

by

the

pulse

width

modulator,

with

the

effect

on

the

switching

transistors

determined

by

the

direction

of

the

current

flow.

When

current

in

T504

is

induced

from

pin

6

to

pin

5

and

pin

7

to

pin

8,

back

biasing

of

0502

occurs,

turning

this

transistor

off,

which

results

in

0501

being

turned

on

(with

charge

of

C505

speeding

up

the

transition).

When

current

is

the

reverse,

0501

is

back

biased

and

turned

off,

with

0502

turned

on

(with

charge

of

C506

speeding

up

the

transition).

The

result

is

an

ac

signal

to

the

T50l

primary

controlled

by

the

width

of

pulses

output

from

E505,

the

pulse

width

modulator.

9.2.2.3

DC

Output

Circuits

--

The

dc

output

circuits

(Figure

9-7)

consists

of

the

following

components/circuits.

•

-12

V

output

circuit

containing

a

rectifier

(0507

and

0508),

filter

networks

(L50l

and

C508,

and

C509,

R525,

and

R526),

and

a

voltage

regulator

(E504).

•

+5

V

filter

C513} •

output

circuit

containing

a

rectifier

(0509),

and

networks

{T503

and

C5l0,

and

L502,

C523,

and

T50,

D507

I

RG1J

L501

-1;2V

I

3

I

2

1.C508

0509

R525

R526

,

II

~

,

D508 I

T

330

T47

1K

1K

50V

RG1J

I.

I

14

i

r-j

I

1

0509

9

FE16D

I

8

I

I

I

I

+5V

L_J

1

I

0523

,-..,

470

16V

I

+12V

12

R5D8

I

68

10510

11

1/2W

0511

0510

IFE160

1000

1000

0512

0518

0513

C520 I

25V

10V

47

1

47

I

10

WOOP I I

l00V

IR507

I

L_J05

25W

~

TO

OVERCURRENT

PROTECT

...

ot.·06'Y,·

...

Figure

9-7

DC

Output

Circuits

Diagram

9-8

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