8. OVER CURRENT PROTECTION CIRCUIT
8-1. Outline
If main power (125V) current increases abnormally due to components failure, there is possible danger of the second- ary damage like failure getting involved in other part fail- ure, and abnormal heating. To prevent this, over current pro- tection circuit is equipped, which detects current of main B line to turn off power relay in abnormal situation.
8-2. Operation
Fig. 9-24 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 Tr8 in protector module (Z801) becomes approx. 0.7V or more, Tr8 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 Tr6 via R14. This causes Tr5 ON and voltage at pin 16 of Z801 becomes low.
Therefore, QB30 and Q843 turns off to set SR81 OFF. Tr6 and Tr7 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:
• To restart TV set, repair failure first.
5V
MICON
QA01#7
RELAY
SR80
Q845
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| F470 | R470 |
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| To T461 |
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| MAIN B | |
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| R471 | R479 | |
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| C472 |
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| 15 |
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| 2 | 1 |
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| R9 |
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| ZD4 | R16 |
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| R10 |
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| Tr7 |
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| R14 | R15 | D1 | Tr8 |
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16 | Tr6 |
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Q846 |
| R12 | C1 |
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| Tr5 |
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| R11 |
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| Z801 |
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| PROTECTOR MODULE | ||
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Fig. 9-24 Over current protection circuit
80
