DEC VT220 - Page 190

lN4oo4
C502
10
25V
TO
h--t~t--r--PUlSE WIDTH
MODULATOR
TO
OVERCURRENT
PROTECT
RETU RN , T502 4
{
-----.IIIC---..,
TO/FROM 230VAC 2 :
J50' I 'J-r-"'"'1
'15VAC 3 I, -.;..51---'
Figure 9-5 +12 V Start Up Circuit Diagram
T5D,
I
I
I
I
I
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I
I
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5
R5D6
56
2W
0502
2SC
2335
R503
lOOK C503
lW 220
250V
R501
R500
4
230VAC
PART OF
T504
~~~~E {"
WIDTH
MODULATOR 6 C507
1-2.:' 1~5::':VA:'::C'-~---'~-1~----~~::;C;52;:2~==+-.J I~~~P
R502 405J ~~~6
, OOK C504 PART OF
1W --r~5S T504 \
I ' 0505
I :~!~~ { 1171N~~
MODULATOR .8
TO/FROM RETURN 2
J50'
Figure 9-6 Input Rectification Circuit Diagram
9.2.2.2 Input Rectification Circuit -- The input rectification
circuit rectifies and filters the ac input to produce positive,
negative, and neutral "dc type" signals. These signals are then
used as a power supply for two switching transistors. The
switching transistors produce an ac signal applied across the
primary of T501, with the width of the ac signal controlled by
input to the switching transistors from the pulse width modulator.
The input rectification circuit
following components/circuits.
(Figure 9-6)
consists of the
• Rectifier (E503)
• Thermistors (R500 and R501)
• Filter networks (R503 and C503, R502 and C504, and R506,
C507, and C530)
• Switching transistor networks (Q50l and Q502, and related
components)
9-7