Canon 580EX - Speedlite II - Hot-shoe clip-on..., Service Manual, Page: 32 / 97

Technical Information
. Wireless Communication, FP, TTL Flash, and External flash metering
(1) For FP "Flat Firing" an inductance of about 60μH is necessary, so for flat firing IC107S
pin 118 goes low, turning off BRC1 and SCR2 and placing both L2 and L3, a total of
60μH, in series with the Xenon tube.
(2) For ordinary M flash, wireless communication flash pulses and external flash metering
firing, IC107S pin 118 goes high turning RQ30, and RQ33 on which turns BCR1 and
SCR2 on. At the same time RQ31 turns IT1's 30V on.
(3) When SCR2 is on the circuit is through L2 (30μH), and SCR2, bypassing L3 (30μH), so
the total inductance is only 30mH.
The reasons for this circuit are as follows:
• When using ordinary TTL, with an inductance of 60μH the extra energy in the coil
makes it difficult to cut off the Xenon tube quickly, resulting in a longer than necessary
flash and overexposure.
The Speedlite 380 and 220EX have a single 60μH coil. Precedence is given to
flat firing at the expense of TTL operation. (Since E-TTL is basically manual flash,
compensation is made within the flash.)
The Speedlite 580EX II will likely be used with the EOS-1N in TTL mode so
performance must be maintained at the same level as the Speedlite 540EZ which has a
30mH coil.
The dual 30mH coil design makes this possible.
• In E-TTL compensation could be made using 60μH, as in the Speedlite 380 and
220EX, at a small sacrifice in maximum power (Guide No.), but the dual 30μH coil
design makes this unnecessary.
Speedlite Coil Inductance
540EZ 30μH
380/220EX 60μH
550EX 30μH x2
580EX 30μH x2
580EX II 30μH x2
• When used in the wireless communication mode, only one coil is used to keep the time
constant as small as possible for short pulses.