Hitachi 51SWX20B Training page 24

DP-2X ADDITIONAL SHUTDOWN CIRCUITS EXPLANATION
Additional Power Supply Shut Down Circuit Diagram explanation:
(See DP-27 Additional Power Supply Shut Down Diagram for details)
Use this explanation and Diagram in conjunction with the following diagrams.
DP-2X Signal Power Supply (Low Voltage) Shut Down Circuit (Continuation A)
The following circuits are routed to the Lo Voltage Shut Down Circuit through connection point (A) de-
picted on the Circuit drawing:
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 in series
with the SW +115V. The value of this resistor 0.47
ohm. When the current demand increases, the voltage
drop across the resistor increases. If the voltage drop
is sufficient to reduce the voltage on the base of
Q905, the transistor will conduct, producing a Shut-
down signal that is directed to the appropriate circuit
indicated on the drawing 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 D937 to fire, creating a Shutdown Signal through D936
and on to the appropriate circuit indicated on the drawing as
point (A).
Note: The LED D940 used for visual trouble shooting is illuminated 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 D927 is connected to a voltage divider. 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 D926 and on to the appropriate circuit indi-
cated on the drawing as point (A).
R941
SW +115V
Current Sensor
Q905
Figure 1
Figure 2
Voltage
Detector
SW +28V
D927
D926
Figure 3
0.47
Base
Bias
Shut-Down Signal
Shut-Down Signal
Loss
D936
D937
D940
SW -28V
SW +115V
Voltage Too
High Detector
Shut-Down
Signal
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