Safety Protection Circuit And X-Ray Protection Circuit; X-Ray Protection Circuit; Beam Current Protection Circuit; Primary Leakage Current Protection Circuit - NEC MultiSync FE001SB Service Manual

6. Safety Protection Circuit and X-ray Protection Circuit

This is a protection circuit intended to prevent the radiation of X-ray from exceeding a dangerous level,

possibly caused by an unusual high voltage rise.

Please do not modify either the high voltage circuit or the safety protection circuit.

The high voltage upper limit value and the beam current value are defined by the upper limit curves of X-ray

radiation in the CRT.

The X-ray protection circuit operates at 27.5kV. The detection and protection circuit comes in the two circuits.

In Circuit 1, the rise of the pulse voltage at T301 Pin 7 is controlled by comparing the reference voltage with

the voltage rectified at D308 and C312 in U302. In an error mode, a high level output from U302 is applied to

U701 Pin 3 (X/XRAY-IN). When an error is sensed, U701 suspends the high voltage deflection for protection

in the completely stopped mode.

In Circuit 2, the rise of the pulse voltage T301 Pin 7 is controlled by inputing the voltage which is rectified at

D308 and C312, at U501 Pin 2 (XRAY). When an error is sensed, U501 suspends the oscillation. At the

same time, the HUNLOCK is turned HIGH to send the information of abnormal oscillation to the

microcomputer. Q301 is simultaneously turned ON to stop the driving U301 for protection.

Operation of this circuit is maintained until the AC power supply is turned OFF, resetting this circuit.

The protection circuit operates when the current flowing in the high voltage generator winding of the FBT

exceeds the specified level. The detection of the beam current is conducted in terms of the voltage drop

measured by the circuit elements of VR302, R321, and R320, connected in between T301 Pin 8 and 12V.

When the inrush current flowing in Pin 8 exceeds the specified level, the voltage is lowered at T301 Pin 8.

This voltage input to U302 via R236 and is compared with the reference voltage. In any error mode, the

output is turned LOW and this output is input to U701 Pin 13 (MEAN-IN). At this stage, high voltage

deflection is suspended in the completely stopped mode.

The protection circuit operates when the current flowing into the primary winding of the FBT exceeds the

specified level. The detection of the primary leakage current is conducted in terms of the voltage drop

measured at R301 that is connected in between the 83V power supply and T301 Pin 2. When the inrush

current flowing into Pin 2 exceeds the specified level, the voltage of R301 is lowered at T301 Pin 2. Q309 is

then turned ON and the potential at U501 Pin 2 is turned HIGH via R318. When an error is sensed, U501

suspends oscillation. At the same time, the HUNLOCK is turned HIGH to send the information of abnormal

oscillation to the microcomputer. Q301 is simultaneously turned ON to stop driving U301 for protection.

7-15

Table of Contents