Hitachi 46W500 Training page 13

DP-3X POWER SUPPLY SHUT DOWN EXPLANATION
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).
SW+115V
Current Sensor
C949
Q905
Figure 1
Voltage Loss
Detector
R964
D954
15K
SW +28V
D931 R952
R953
D930
Figure 3
R944
0.39
R946
Base
Bias
R947
Shut-Down Signal
Shut-Down Signal
D953
D952
R965
15K
SW -28V
Figure 2
SW +115V
Voltage Too
High Detector
Shut-Down
Signal
(Continued on page 6)
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