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X-Ray Protection Circuit And Safety Protection Circuit; X-Ray Protection Circuit; Beam Current Protection Circuit; Ic701 Overcurrent Protection Circuit - NEC DPro2070SB Service Manual

Crt display nec-mitsubishi electric visual systems corporation september 2002
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1.7 X-ray protection circuit and safety protection circuit

1.7.1 X-ray protection circuit

This circuit prevents X-ray radiation from exceeding the dangerous level due to the abnormal rise of high
voltage.
Do not modify the high voltage circuit and the safety protection circuit.
The upper limit of the high voltage value and the beam current value are determined by the X-ray radiation
upper limit curve of CRT.
In the X-ray protection circuit, the X-ray protector activation voltage depends on the beam current. The
X-ray protector, however, is normally activated at approximately 30kV (when the beam current is
approximately 1mA). D709 and C704 rectify the increase in the pulse voltage output from pin 6 of T701.
Pin 24 of IC102 detects this rectified voltage. If the detected voltage exceeds the specified value, the
SUSPEND signal output from pin 5 of IC102 will be set to 'Low', and the P-OFF signal output from pin 42
will be also set to 'Low' (power-off mode). In addition, operation of IC701 will be stopped. This condition of
the protection circuit will be retained until the power switch is turned OFF.

1.7.2 Beam current protection circuit

When the current supplied to the high voltage generating winding of FBT exceeds approx. 1.5mA, the
protection circuit functions. The detection of the beam current is executed by the voltage fall of R722
connected between T901 pin 9 and the 12V.
Resistors R723 and R724 divide the potential of this voltage. The divided voltage is then input to pin 27 of
IC102 via IC703. If the input voltage exceeds the specified value, the SUSPEND signal output from pin 5
of IC102 will be set to 'Low', and the P-OFF signal output from pin 42 will be also set to 'Low' (power-off
mode). In addition, operation of IC701 will be stopped. This condition of the protection circuit will be
retained until the power switch is turned OFF.

1.7.3 IC701 overcurrent protection circuit

The peak value of the drain current of Q701 and the both-end voltages of source resistors R708 and R709
are detected by pin 2 of IC701. If the voltage of this pin exceeds 1.2V (typ), pin 9 of IC701 will stop
outputting the drive waveform. If the voltage of IC701 pin 2 drops below 1.2V (typ), pin 9 of IC701 will
output the drive waveform again.

1.7.4 IC701 overload protection circuit

If overload occurs consecutively and the overcurrent protection circuit is activated consecutively, this
overload protection circuit will enter the latch mode to stop operation. If the voltage of IC701 pin 2 exceeds
1.0V (typ), C709 will be charged. If the voltage of IC701 pin 13 exceeds 2.5V (typ), IC701 will enter the
latch mode to stop the control operation. This condition of the protection circuit will be retained until the
power switch is turned OFF.

1.7.5 IC902 overcurrent protection circuit

IC902 is equipped with an overcurrent protection circuit. R928 detects the drain current of the incorporated
FET. If the voltage of IC902 pin 1 exceeds approximately 0.7V, this overcurrent protection circuit will be
activated.

1.7.6 Short-circuit protection circuit on secondary power side

The output line of each secondary power (+215V, +80V, +15V, +8V, -15V) is equipped with a short-circuit
detection circuit. If a secondary line is overloaded and the output voltage drops by 30 to 40% of the normal
voltage, this short-circuit protection circuit will be activated.

1.7.7 Overvoltage protection circuit

The harmonic suppression circuit (active filter circuit) and the main power circuit are respectively equipped
with an overvoltage protection circuit. If the voltage between both ends of C911 rises by 10% of the normal
voltage, or if the voltage of the main power secondary output line rises by 30 to 40% of the normal voltage,
operations of IC901 and IC902 will be stopped.
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