Power protection circuit works: When the + B voltage is higher than 140V, Q834 trigger conduction, + B voltage sharp drop, switch power off mode, CPU power failure, Q814 deadline, Q810, Q811 have been turned on, the machine into the protected status, standby indicator lights. Q834 is not an ordinary thyristor (SCR), and between its internal control electrode and an anode and a Zener diode connected. When the + B voltage is higher than 140V, zener diode breakdown is triggered thyristor conduction. When the 12V power loss of pressure or load short, D851 loss of pressure or negative terminal voltage drops conduction, Q851 are biased conduction, Q814 deadline, Q810, Q811 have been conducting, switching power off, standby LED is lit, the machine enters protection status. From the wiring schematic 12V circuit protection point of view, when the C870 (470μF) decreased capacity, can also cause instant boot protection circuit, the output of the CPU due to 6 feet high while driving Q814 charge for C870, C870 use the voltage across not to delay the Q851 mutation conduction, 12V protection circuit to prevent malfunction. Standby power load current increases, the voltage drop across the resistor R815 when sampling ends increases, the voltage drop is greater than O.6V, Q812 is turned on, Q810 emitter current weakened, Q812 deadline, because of the over-current protection circuit can not self-locking, Q810 and conduction, and so on, the standby indicator is blinking light and shade. Such protection only works when the moment and standby power. According to the above off (to be) working process and analyze the protection circuit can exclude + B voltage and 12V power supply protection circuit may be due to the boot only 6 feet instantly detects 0.2V voltage of the CPU, there is no description of the switching power supply start, it does not exist + B voltage and 12V power protection possible. If the instantaneous power supply protection circuit malfunction 12V, Q851 to Q814 is turned on pulling the base potential to around 0.3V, CPU 6 feet should be kept high, so the possibility of malfunction of the protection circuit 12V is excluded.

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