At 03:37 PM 25/11/2009, you wrote:
>The flash in my Canon Elph is probably a fairly typical example of a
>modern flash. It flashes twice in quick succession, the first time is
>for exposure and maybe focus. The second time it's for taking the
>picture.
>
>In old-fashioned flashes, the flash had an inverter charging a big
>capacitor that had a Xenon flash tube across it, and a little SCR that
>hit a second, smaller transformer to trigger the flash tube. The big
>capacitor is drained pretty heavily and takes time to recharge, it can't
>flash twice in a tenth of a second.
>
>Any idea how they get two quick flashes in my camera? I'd love to be
>able to use my cheap slave flash to augment the one in the camera, but I
>don't see how any of the circuitry can be reused. And I already tried
>putting a flip-flop in, so that it would only fire the second time. My
>test pictures were overexposed because the first flash(the one that sets
>the exposure) wasn't augmented by the slave flash.
>
>Thanks,
>
>Bob

I think they use IGBTs to control the flash current (interrupt it before the
photoflash cap is discharged too much). IGBTs are generally a more appropriate
choice than MOSFETs in this kind of application.

>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com



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