Panasonic KX-NS500AG, Service Manual

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KX-NS500AG
4.11. Protection Circuit
This section describes the Overvoltage Protection Circuit and Overheat Protection Circuit.
1) Abnormal Condition Detection Circuit
Abnormal condition detection circuit is a thyristor circuit which is comprised of Q306 and Q307. When power supply operation
becomes abnormal, a signal is sent to the abnormal detection circuit and Q307 is turned ON. A current flows through the diode
of PC50, which is sent to the control circuit of main converter. The operation of main converter is then stopped.
2) Overheat Detection Circuit
When internal temperature of power supply rises abnormally, resistance values of PTC thermistor PT301 increase, transistor
Q305 turns ON, and signal is sent to the abnormal detection circuit through D312.
3) Overvoltage Protection Circuit
When 15V or 41V output becomes abnormal, that voltage is sent to the abnormal detection circuit through zener diode D311
4) Reverse Connection Protection Circuit
When the battery is connected in reverse, + terminal of the battery becomes negative voltage, D167, D168, D169 start to con-
duct and transistor Q313 turns ON.
Dropper Circuit voltage (in section 5) is sent to the abnormal detection circuit through zener diode D319.