Specific Information - Hitachi 51SWX20B Training

DP-2X LOW VOLTAGE POWER SHUT DOWN EXPLANATION
VOLTAGE TOO HIGH DETECTION
(See Figure 2)
Another circuit used is the Voltage Too High Detec-
tion circuit. In the example shown in Figure 2, the
zener diode D1 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.
EXCESSIVE CURRENT DETECTION
(See Figure 3)
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 particular power supply, (labeled B+ in the
drawing). The value of this resistor is determined by
the maximum current allowable within a particular
power supply. In the case of Figure 1, the value is
shown as a 0.47 ohm, however it could be any low
ohm value. 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 the transistor, the transistor will conduct, producing
a Shutdown signal that is directed to the appropriate
circuit.
NEGATIVE VOLTAGE LOSS DETECTION
(See Figure 4)
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 5, there are two resistors of equal value. One to the
positive voltage, (shown here as +12V) and one to the negative
voltage, (shown here as -12V). At their tie point, (neutral point),
the voltage is effectually zero (0) volts. If however, the negative
voltage is lost due to an excessive load or defective negative
voltage regulator, the neutral point will go positive. This in turn
will cause the zener diode to fire, creating a Shutdown Signal.

SPECIFIC INFORMATION:

In addition, there are 7 Hot Ground side Shutdown inputs that are specifically detected by the main power driver
IC I901. These sensors circuits protect I901 from excessive current, temperature or over voltage.
HOT GROUND SIDE SHUT DOWN SENSING CIRCUITS. (Specific to I901).
LATCHED SHUT DOWN MONITORS: (AC must be removed to recover).
1. (OVP) Pin 3 is monitored for Over Voltage Protection at pin 3 of I901.
2. (TSD) I901 itself is monitored for Excessive Heat. This block is labeled TSD. (Thermal Sensing Device).
3. (OLP) Over Load Protection monitors the difference between the Hot Ground and Floating Ground.
RECOVERING SHUT DOWN INPUT: (Driver IC will recover on it's own when trouble is removed.)
4. (OCP) Pin 5 monitors the low ohm resistors, R908, R909, and R910. If these resistors have an excessive
Voltage Too High
Detector
Shut-Down Signal
R1
0.47
B+
Current Sensor
Shut-Down Signal
Figure 3
Voltage
Detector
Figure 4
Any Positive
B+ Supply
Figure 2
Base
Bias
Shut-Down Signal
Loss
+12V -12V
(Continued on page 11)
PAGE 01-10