DEC VT220 - 9.2.2 AC Conversion Circuits

9.2.2
AC Conversion Circuits
The ac conversion circuits convert ac input into three dc voltage
outputs: +5 V, +12 V, and -12 V. The ac conversion circuits,
(Figure 9-4) which are all located on the PS and monitor board,
are as follows.
• J501 is the connector for ac input from voltage
se1ectionswitch.
• +12 V start up -- provides operational voltage potential
for overcurrent protect circuit, and also provides
initial power for pulse width modulator during power up
sequence (until +12 V circuit is operational).
• Input rectification -- consists of components responsible
for rectification and filtering of ac input into the
pulse width modulator controlled switching transistor
circuit, which provides ac input to primary of T501.
J501
I
PART
OF
J2
TO
LOGIC
BOARD
C
K
/FROM
ONITOR
RCUITS
I--
El
-12V
~
...
i230VAC,
21
-12V
1
INPUT
OUTPUT
I
I--
115VAC
'--
RECT1FJCATJON
I
~
7
+SV
2
-
RETURN
~
~
!
I-
8
+SV
3
I--
-
GND
~
U U
+5\/
I-
18
+5V
4
...
OUTPUT
fJ-
-
I,'5VAC'
~
r-
19
+SV
5
TS04
I
I
-
(230VACi
~
I
I
20
+12V
I
I
i
-
I
• 9
GND
~
-
i
it
+12V
+
....
• 10
GND
I
OUTPUT
-
I
• 16
GND
I
---
I
• 17
GND
i.:~!_ .J
---
-'
I I
-
-
f-
~
+12V PULSE
OVERCURRENT
START
WIDTH
PROTECT
UP MODULATOR
+SV
}r
--
-12v
...
,
t
I
+12V
{SYNCI :1
+5V
TO
-12V >M
CI
+12V
FROM{
S2
TO
Fl
FROM CHASSIS
GND
FROM(
52 \...
Figure 9-4 AC Conversion Circuits Block Diagram
9-4