Panasonic TC-26LX20 Technical Manual

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Panasonic TC-26LX20 Technical Manual Manual pdf 22 page image

Over Current Protection Shutdown CircuitsThere are several protection (shutdown) circuits that monitor the voltage sources forover voltage and over current conditions. They prevent the occurrence of catastrophicfailures by shutting down the unit. The circuit below is used to prevent catastrophicfailures if the Sub_5V, Sub_9V, Main_9V, or DR_12V becomes shorted. These arevoltage loss detection circuits that have the same method of operation. Let us analyzethe main 9V source as an example.The circuit consists of a diode whose cathode is connected to a positive B+ source.Under normal conditions, the diode is reverse biased, which keeps the base of Q832high. However, if there is a short or excessive load on the Main 9V line that is beingmonitored, the diode conducts, creating a current path for the base bias of Q832. Thetransistor turns on and allows 3.3V to output at the collector. This voltage is thenprovided to the base of transistor Q829 via the diode D874, forcing its collector to golow. This low enters the SOS input of the MPU via pin 42 of the connector PAP2/DG2.The MPU reacts by having AC ON (H) go Low. This, as you may recall, is supplied tothe SUB POWER CIRCUIT of the P board to shut off the SUB Power (14V) and theMAIN Power (15V) voltage sources.SOS ShutdownFigure 1517