Canon SPEEDLITE 540EZ Service Manual page 41

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Part 2: Technical Information
When the shutter is released, the STSP signal is input to pin No.8 of IC7 from the
camera.
When the shutter is fully open, the X signal is input to pin No.10 of IC7 from the
camera.
A High signal (initial flash trigger) is output from pin No.35 of IC7 by executing an
AND operation on these STSP and X signals, and is input to pin No.88 of IC8 via
RQ18. A High signal (main flash trigger) is also output slightly delayed from pin
No.36 of IC7, and is input to pin No.85 of IC8 via RQ19.
A High signal is output from pin No.20 of IC8 by input to pin No.88 of IC8. Turning
ON of Q12 turns Q8 ON, and IT1 turns ON.
When IT1 goes ON, C22 discharges. The trigger is generated by T3, and XE2
flashes initially.
Next, a High signal is output from pin No.18 of IC8 by input to pin No.85 of IC8.
Turning ON of RQ12 turns Q19 ON, turning BCR1 ON.
With BCR1 ON, the charge stored to C49 is discharged in the loop C49(+) — R24 >
L2 — XE2 > IT1 > C49(-).
(At this time, XE2 is at a resistance of several Ohms due to initial flashing.)
By this current, High is input to the gate of SCR4, and SCR4 becomes ON.
When SCR4 goes ON, the change of main capacitor (C15) flows to XE2 via L2, and
main flash fires.
* The reason for initial flash being carried out before the main flash is in order to
smooth flash startup and to improve flash control precision.
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