Canon SPEEDLITE 540EZ Service Manual page 37

Part 2: Technical Information
4.2 Charging
* The 540EZ uses two-stage charging. The main pre-trigger capacitors (C22, C16) and
the initial flash capacitor (C20) are charged first. When these reach 240V or more,
the main and pre-flash capacitors (C15, C19) are charged.
(1) When oscillation starts, C22, C16 and C20 (main pre-trigger and initial flash)
charge.
(2) At this time, the charge voltages to C22, C16 and C20 are divided by R17, R16,
R116 and VR1, and input to pin No.100 of IC8.
(3) When the input to pin No.100 of IC8 of 2.5V or less (charging voltage 240V or less
of C22, C16, and C20), pin No.14 of IC8 outputs High, and Q6 turns ON.
(4) In other words, the gate of SCR1 is grounded via D5 and Q6, and SCR1 is OFF, so
C15 and C19 do not charge.
(5) Charging of C22, C16 and C20 progresses. When each of their voltages reach 240V
or more, and the input to pin No.100 of IC8 is 2.5V or more, pin No.14 of IC8 goes
Low, turning Q6 OFF.
(6) Next, SCR1 turns ON, and C15 and C19 are charged.
(7) C22, C16, C20 self-discharge by R16, R17, R116 and VR1 unless the charging
voltage to C15 and C19 reach 240V or more.
(This is because the secondary voltage of Tl drops due to the charging to C15 and
C19, and charging to C22, C16 and C20 stops.)
(8) Self-discharging of C22, C16 and C20 progresses. When their voltage falls below
240V, and the input to pin No.100 of IC8 falls below 0.6V, pin No.14 of IC8 goes
High again.
(9) This stops charging of C15 and C19, and C22, C16 and C20 are charged again.
* By repeating steps (1) to (9), C15 and C19 are charged while the voltages of C22,
C16 and C20 are held at 240V.
(10) When both C15 and C19 have reached 240V or more, all capacitors are charged
simultaneously.
¢ Flow of Operations: ii (1) > (2) > (3) > (4) > (5) > (6) > (7) > (8) > (9) pp (10)
This cycle is repeated until both reach 240V or more.
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2-15