Hitachi 46W500, Тренировка

Страница: 13 / 162

DP-3X POWER SUPPLY SHUT DOWN EXPLANATION
PAGE 01-05
GENERIC SHUT DOWN CIRCUITS EXPLAINED:
The following circuits are commonly used in Hitachi product and relate to the drawing on page 01-10:
SW +115V EXCESSIVE CURRENT DETECTION
(See Figure 1)
One very common circuit used in many Hitachi tele-
vision products is the B+
Excessive Current Sensing
circuit. In this circuit is a low ohm resistor
R944
in
series with the SW +115V. The value of this resistor
is
0.39 ohm . When the current demand increases, the
voltage drop across the resistor increases. If the volt-
age drop is sufficient to reduce the voltage on the
base of
Q905 , the transistor will conduct, producing a
Shutdown signal that is directed to the appropriate
circuit indicated on the drawing on page 01-10 as
point
(A) .
NEGATIVE VOLTAGE LOSS DETECTION
(See Figure 2)
The purpose of the Negative Voltage Loss detection circuit is to
compare the negative voltage with its’ counter part positive volt-
age. If at any time, the negative voltage drops or disappears, the
circuit will produce a Shutdown signal.
In Figure 2, there are two resistors of equal value, (15K). One to
the positive voltage SW +28V and one to the negative voltage
SW –28V. At their tie point,
(neutral point) , the voltage is effec-
tually zero (0) volts. If however, the negative voltage is lost, the
neutral point will go positive. This in turn will cause the zener
diode
D952 to fire, creating a Shutdown Signal through D953
and on to the appropriate circuit indicated on the drawing on
page 01-10 as point
(A)
.
Note: The LED D954 used for visual trouble shooting is illumi-
nated by the current draw from +28V to the –28V supply.
VOLTAGE TOO HIGH DETECTION
( See Figure 3)
Another circuit used is the Voltage Too High Detec-
tion circuit. In the example shown in Figure 3, the
zener diode D931 is connected to a voltage divider
R952 and R953 . If the voltage source rises too high,
the voltage at the divider center point will rise as well
and trigger or fire the zener diode which produces a
Shutdown signal through
D930 and on to the appropri-
ate circuit indicated on the drawing on page 01-10 as
point
(A) .
Figure 1
SW +28V
Shut-Down Signal
D954
D953
D952
SW -28V
R964
15K
R965
15K
Voltage Loss
Detector
Figure 2
SW +115V
Voltage Too
High Detector
Shut-Down
Signal
D931
D930
R952
R953
Figure 3
(Continued on page 6)
Current Sensor
Base
Bias
Shut-Down Signal
R944
0.39
Q905
SW+115V
C949
R947
R946