Over Current Protection Shutdown Circuits - Panasonic TC-26LX20 Technical Manual

Over Current Protection Shutdown Circuits

There are several protection (shutdown) circuits that monitor the voltage sources for
over voltage and over current conditions. They prevent the occurrence of catastrophic
failures by shutting down the unit. The circuit below is used to prevent catastrophic
failures if the Sub_5V, Sub_9V, Main_9V, or DR_12V becomes shorted. These are
voltage loss detection circuits that have the same method of operation. Let us analyze
the 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 Q832
high. However, if there is a short or excessive load on the Main 9V line that is being
monitored, the diode conducts, creating a current path for the base bias of Q832. The
transistor turns on and allows 3.3V to output at the collector. This voltage is then
provided to the base of transistor Q829 via the diode D874, forcing its collector to go
low. 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 to
the SUB POWER CIRCUIT of the P board to shut off the SUB Power (14V) and the
MAIN Power (15V) voltage sources.
SOS Shutdown
Figure 15
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