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Confidential: FUJIFILM Service Center Use Only
FinePix S5800/S800 Service Manual
3-4.
Description of ST1 flash
circuit operation
3-4-1.
Charging circuit
The UNREG power is supplied to the charging circuit, and
when the CHG signal from the microprocessor is Hi (3.3 V),
the charging circuit starts operating and charges the main
electrolytic capacitor with a direct current high voltage supply.
When the CHG signal is Lo (0 V), the charging circuit does
not operate.
Power switch
When the CHG signal is Hi, Q5407 switches on and charging
operation begins.
Power supply filter
C5401 acts as a power supply filter to smooth out any current
ripples caused by oscillating transformer switching.
Oscillator circuit
This is a circuit that generates an AC voltage (pulses) that is
used to step up the low-voltage DC UNREG power supply.
This circuit generates a drive pulse with a frequency between
50 and 100 KHz. Because the oscillation is self-excited, the
oscillation frequency varies according to the drive conditions.
Oscillating transformer
The oscillating transformer converts the low-voltage AC
power supply generated by the oscillation control circuit to a
high-voltage AC supply.
Rectifier circuit
This circuit rectifies the high-voltage AC power supply
generated by the second stage of T5401 so that it becomes
a high-voltage DC supply and then stores it as charge in the
main electrolytic capacitor (C5412).
Voltage monitor circuit
This circuit is used to maintain the voltage stored in C5412
at a constant level. The charging voltage is divided by R5405
and R5406 to reduce it to a low voltage and is then output as
the monitor voltage (VMONIT). When the VMONIT voltage
reaches the stipulated value, the CHG signal is set to Lo and
charging stops.
3. Schematics
3-4-2.
Flash Circuit
When the FLCLT signal is input from the ASIC extended port,
the flash fires.
Flash control circuit
When the FLCLT signal input to the flash control circuit is Hi,
Q5409 is set to ON and flash firing commences. When the
FLCLT signal is Lo, flash firing stops.
Trigger circuit
At the same time that Q5409 is set to ON by the FLCLT
signal and preparations for firing the flash are carried out, a
high-voltage pulse of several kilovolts is generated by the
trigger coil and applied to the flash unit.
Flash element
When a high-voltage pulse is applied to the flash unit by the
trigger circuit, current flows to the flash element and the flash
fires.
* Take care to avoid electric shock.
3-5
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