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From: email@example.com (Gregory L. Hansen)
Subject: Re: Camera Flashes
Date: Tue, 3 Dec 2002 21:13:19 +0000 (UTC)
Organization: Indiana University, Bloomington
References: <firstname.lastname@example.org> <email@example.com>
NNTP-Posting-Date: Tue, 3 Dec 2002 21:13:19 +0000 (UTC)
X-Newsreader: trn 4.0-test62 (21 February 1998)
In article <firstname.lastname@example.org>,
John Muchow wrote:
>>>>The flash really is triggered by shorting the PC cable, right? I can fire
>>>>it by shorting the connector, and it works fine when the PC cable is in my
>>>>camera, but if I manually short the end of the cable nothing happens. I
>>>>really don't know why.
>In my experience, yes. If the cable is functioning properly and the
>flash has recycled, I can't think of any reason why shorting out the
>cable wouldn't fire the flash. There must be something weird in the
>layout/arrangement of the parts of the cable's connector that makes it
>hard to reach them properly to short together.
I got together with an electronics tech and we took it apart for a look.
And it was as simple as it could possibly be. The red wire attached to
metal parts that included on part of the plug and one of the shoe
electrodes, the blue wire attached to metal parts that included the other
part of the plug and the other shoe electrode. Short them and it fires.
Which doesn't explain why it doesn't fire when I tried manually shorting
the shoe electrodes or the end of the cable. But it still works fine on
the camera. To a first approximation I'm thinking some tarnish might need
to be cleaned up, but I still don't know why it would work perfectly fine
on the camera but not for a manual short.
>>>>What kinds of voltages and current would be running down the cable for an
>>>>old flash like the Vivitar 2600 or 2800? The rep said something about 150
>>>>volts, but I find it hard to believe there's 150 volts across the signal
>>>>and ground of the cable and being fed into the camera.
>Very high voltage isn't uncommon for older flash units. This is the
>reason why I had to come up with my "buffer" circuit because the high
>sync voltage of some flash packs was frying the sync circuits in the
>more delicate electronic cameras.
>The current varies but in my experience it's usually been a
>discharging capacitor that supplies any current flowing through the
>camera's sync circuit.
Once we got the flash open we were able to find spots to get a voltage
reading from, and it was around 140V. I suppose that made more sense when
all the cameras had mechanical switches to close the circuit.
So what components could you recommend? The FETs I saw at Radio Shack top
out at 25V, which is clearly not sufficient.
>>>>How does the hot shoe work? I can slide the flash unit into a metal
>>>>bracket and it won't fire, so it seems the shoe mount isn't triggered by
>>>>shorting the connections. Is there some voltage applied across the
>>>>terminals? I didn't want to experiment too much for fear of putting too
>>>>much voltage the wrong way through it.
>The camera sync circuit is just paralleled with the hot-shoe contacts.
>There can be other contacts for additional auto-exposure features, but
>two of the contacts should short together when a picture is taken.
>AFAIK, no voltage is applied to the hot-shoe contacts by the camera.
>Your unit may not be compatible with the camera's flash shoe. Though,
>I thought that any flash unit would work in any shoe (but perhaps
>without certain features that would only work when a same-brand camera
>and flash were used together). There is probably a contact that is
>not touching another properly or a portion of the camera's sync
>circuit may be damaged.
>What is the open-circuit voltage across the flash unit's sync
Zero volts, until we took the flash apart and found some good surfaces.
But the flash still functions on a camera...
"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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