DEC VT220 Technical Manual page 195

9.2.2.5 Pulse Width Modulator Circuit The pulse width
modulator monitors the +12 V output potential, along with inputs
from the +5
V
output, overcurrent protect, and horizontal
processor in the monitor circuits, to control the width and
frequency of pulses passed through the primary of T501 by biasing
of the switching transistors in the input rectification circuit
(via T504).
The pUlse width modulator (Figure 9-9) consists of the following
components/circuits.
• Pulse width modulator device (E505)
• Power input components (0511 and C521)
• Summing network (R517-R520)
• Pulse control output network (R510, 0512, and 0513)
• Oscillator synchronization input network (R54, C54, and
054 )
•
Oscillator frequency control components
C514-C516 )
(RSll, R527,
• Biasing components (R515 and R5l6)
• Start-up delay capacitor (C517)
519
491(
%
054
1N4004
R520
5.11
I(
FROM
%
HORIZ
--PROCESSOR
054
IN
BBOP MONITOR
CIRCUITS
FROM +12V
OUTPUT
FROM
+SV
DC
OUTPUT
CIRCUIT
TO
OVERCURRENT
PROTECT
r
r
FROM +12V
l
OUTPUT
OS11
1N4004
RS17
R
rfFROM+12V
I
S 11
I(
6
STARTUP
1% 1
F
CS21
.47
9'
I
10
1
R518
511
I(
15
,.!l!
1%
,,1
r-;-
R510
I
13
~
3
68
1/2
,
11 4
1
E50S
~
14
SG3525A
1-
.0513
6 L
.4~ ~
0664
~
7
R51S R515
.4~
r-
33K 4701(
r--
R527
J
RS4
~'7
5008
lL.
~
22K
;;:
C515*
;F'
::: -
R511
C517
C514 C516
4.751<
47
1000P
.0047
01
~'7
1%
100V
"
FROM
DS12
0664
PART 0
TO T50\
INPUT
RECTIFICATION {
II
OVERCURRENT
PROTECT
Figure 9-9 Pulse Width Modulator Circuit Diagram
9-12