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From: firstname.lastname@example.org (Gregory L. Hansen)
Subject: Re: Camera Flashes
Date: Sun, 1 Dec 2002 00:20:40 +0000 (UTC)
Organization: Indiana University, Bloomington
References: <email@example.com> <firstname.lastname@example.org>
NNTP-Posting-Date: Sun, 1 Dec 2002 00:20:40 +0000 (UTC)
X-Newsreader: trn 4.0-test62 (21 February 1998)
In article <email@example.com>,
John Muchow wrote:
>>>>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.
Ah. Well, with a manual camera and flash units that will likely take
longer than half a second to recharge, I don't see a problem.
>I'm not at all sure, but do you need any series gate resistance for
>each FET to prevent possible oscillations?
I guess I'm not sure. I hadn't even thought of that. When I've selected
FETs to use I'll look up the data sheet for recommendations.
>>>>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. :-)
I didn't want to merely ensure I could fire the FETs, but fire them
quickly enough. But you don't think my design is wonderfully elegant? As
the battery wears down that's like increasing the resistance (internal
resistance of the battery) of the upper half of the voltage divider. With
a proper selection of resistors, it simply won't generate the 1.2 volts or
so needed to turn on the LED. It would still operate the FETs at that
point, but I'd know I'm pushing my luck if I don't replace it. For two
resistors and a diode that doubles as light and limit.
"A nice adaptation of conditions will make almost any hypothesis agree
with the phenomena. This will please the imagination but does not advance
our knowledge." -- J. Black, 1803.
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