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From: jmuchow@SPAMMENOTcamlight.com (John Muchow)
Subject: Re: Camera Flashes
Date: Sat, 30 Nov 2002 21:33:30 GMT
Organization: MindSpring Enterprises
References: <firstname.lastname@example.org> <email@example.com>
X-Server-Date: 30 Nov 2002 21:35:22 GMT
X-Newsreader: Forte Free Agent 1.21/32.243
>>>Uh-oh. Are there some timing issues here? I think my circuit was going
>>>to be a lot simpler than yours; when the camera sync is on the FETs are
>>>on, when the camera sync goes off the FETs turn off because the battery
>>>would be disconnected and the gate charges drain through a resistor, I
>>>was thinking 10K. Battery in series with the camera's sync port, to a
>>>bunch of FET gates in parallel, then to ground through a resistor.
Sounds like that should work great ( as long as the FET voltage rating
is greater than the open circuit voltage of the flash sync input). We
needed to have our circuit "release" the sync input of the flash as
soon as possible because they don't start to recycle until that
happens. The flash units were being used by sport photographers who
were shooting 1-2 frames per second so any delay became a concern.
So, we just gave the triac a tiny pulse and let the flash sync input
drop as quickly as it could so the triac could turn off and the flash
unit could start to recycle ASAP. An extra 50-100mS either way made a
1/4-1/2 stop difference in the brightness of the light from the flash
units...a huge amount for these guys.
IMHO, you shouldn't have any timing problems.
I'm not at all sure, but do you need any series gate resistance for
each FET to prevent possible oscillations?
>>>Plus a "test" button, and a battery check button that would fail to light
>>>an LED under conditions I haven't really decided yet, but connected to a
>>>voltage divider that would fail to generate enough voltage to turn on the
>>>LED when the battery is weak enough.
Perhaps a LDO 5V regulator with a "battery low" output? At 5V, you'd
still be able to fire logic-level FETs and you could add (if needed) a
transistor to buffer the output of the regulator so you can have the
LED always light up bright (only when the test button is pressed
though or the LED would drain the battery to zero, with possible
leakage I believe, otherwise).
Or maybe dedicate another FET to light the LED? If you match the
current through the LED with the current through the sync citcuit when
it's fired, the LED won't light when the battery voltage is too low to
fire the FETs.
There's probably a wonderfully elegant way to do what you want that
involves a lot fewer components and cost than my method though. :-)
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