Centralion 400VA, Service Manual, Page: 12 / 33

E. Line And Zero-Crossing Detection
Please refer Figure S-4
1) Line Detection
The input voltage is full-rectified The signal passes through a voltage divider at R45
and R40//r47,feed into pin 8 of CPU. By monitoring this rectified sinusoidal voltage, the
CPU can find the unnaturalness of commercial line. The CPU has two ways to judge line
voltage loss
1.
Waveform detection:
If there is a rapid voltage drop occurs, the CPU is able to immediately detect it
and transfer to battery mode. The waveform detection has a short response time.
2.
RMS value detection:
CPU calculates input RMS value every cycles. If RMS value is not in range
of UPS specification for 3 cycles ,UPS will transfer to battery mode. The RMS
value detection has a more accurate voltage detection but it can not immediately
sense a rapid voltage transition.
2) Zero Crossing Detection
Zero Crossing Detection is used to minimize the phase difference between the
Inverter voltage and the input line voltage while UPS is switching from battery mode
to line mode. If the phase difference is too large, it can create a large amount of energy
which may damage the internal passive components such as relays.
The Zero Crossing signal is get by the following ways:
1.
Use the voltage reduced from input line voltage(by resistor R49) to drive
Q14(2222ASM).
2.
The Zero Crossing signal comes from the collector of Q14,goes to the CPU pin 1.
Refer to Figure W-2, Waveform measured from Q14 collector and line input.
When the line signal (reduced from input line) is applied to the base of Q14,the first
half cycle(positive half-wave) makes V
BE
<0 Volts , that turns off Q14. The second half
cycle (negative half-wave) causes V
BE
>0.5Volts and turns on Q14.
When Q14 is off, the +5 Volts is charging C23. Hence the V
CE
of Q14 changes from
0 to +5 Volts. As Q14 turns on, C23 discharges immediately. Thus, the line frequency
can be detected by measuring the time interval between the first falling and second falling
edge of V
CE
of Q14. Also It is used by the CPU to determine when to switch from battery
mode to line mode.
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