Over Current Protection Circuit - Toshiba CN27E90 Technical Training Manual

first.
7. OVER CURRENT PROTECTION
CIRCUIT
1. Outline
If main power (125V) current increases abnormally due to
components failure, there is possible danger of the secondary
damage like failure getting involved in other part failure, and
abnormal heating. To prevent this, over current protection
circuit is equipped, which detects current of main B line to
turn off power relay in abnormal situation.
2. Operation
Fig. 11-19 shows over current protection circuit. When the
current of main B line increases abnormally due to the
shortage in load of main B line, voltage drop arises across
R470. By this voltage drop, when base-emitter voltage of Tr
8 in protector module (Z801) becomes appprox. 0.7V or
more, Tr 8 turns on, and the voltage by divided ratio of R15
and R16 is applied to cathode of ZD4. When this voltage
becomes higher than zener voltage of ZD4, ZD4 turns on to
supply base current to base of Tr 6 via R14. This causes Tr
5 ON and voltage at pin 16 of Z801 becomes Low. Therefore,
QB30 and Q843 turns off to set SR81 OFF. Tr 6 and Tr 7 in
Z801 are in thyristor- connection, and power 5V-1 supplied
at pin 15 keeps protection operation for standby power until
main power switch is turned off. During circuit operation,
power LED near main power switch blinks in red. Caution :
5V
MICON
QA01#7
RELAY
SR81
Q843
R830
R10
16
Tr6
Q830
R12
Tr5
R11
17
Fig. 11-19 Over current protection circuit
MAIN B
15
R9
ZD4
R16
Tr7
R15
R14
C1
Z801
PROTECTION MODULE
83
F470
R470
R472
R479
C472
2 1
Tr8
Z801
PROTECTOR MODULE
To T461