Protection Circuit Section; Lamp Current Detection Protection Circuit; Overvoltage Protection Circuit - NEC LCD1550X Service Manual

1.8.4.3

Protection circuit section

The protection circuit is configured of a lamp current detection protection circuit (lamp current
limiter) that detects errors by detecting the current in the lamp return wire, and setting a
threshold for the upper and lower limits of that value. In addition, this circuit has an overvolt-
age protection circuit that detects the power voltage value, and a fuse. A timer circuit is pro-
vide before the protection latch circuit, and malfunctioning of the protection circuit by the
transient response and noise is prevented. By using individual return wires for each lamp,
errors can be detected in each lamp. This allows the ballast capacitor's short-circuits and the
open state for one lamp to be detected.
1.8.4.3.1 Lamp current detection protection circuit (lamp current limiter)
(1) Outline of operation
The current that flows to lamp 1's return wire is converted into voltage at R706, and is input
into IC701 as the DC voltage rectified by D708 and C714. IC701 compares this voltage with
the threshold (upper/lower limit range) set by the D706 reference voltage. If the lamp current
is an abnormal value and exceeds this threshold, the IC701 comparator output is short cir-
cuited (the IC701 output type is an open collector).
The IC701 comparator output remains open while the lamp current is normal, and the protec-
tion circuit does not function.

1.8.4.3.2 Overvoltage protection circuit

IC703
FET switch
An overvoltage in the power voltage is detected, and the timer circuit and latch protection
circuit are activated in the same manner as the lamp current detection protection circuit ex-
plained above.
12V
R713
C703
SL6.3kV 15p-J
D706
Lamp 1
R721
D708
Vr
R706
C716
D717
12V
D707
R1
R2
IC701
R712
comparator
12V
R753
F701
C707
F702
C708
Q707
Timer circuit
1 - 45
Timer circuit
Q717