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From: John Popelish
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X-Mailer: Mozilla 4.7 [en] (Win98; U)
Subject: Re: Photo Flash Trigger Circuit Help
Date: Thu, 14 Nov 2002 03:03:03 GMT
NNTP-Posting-Date: Wed, 13 Nov 2002 22:03:03 EST
> Hello everyone, newbie here.
> I am photographer with a keen interest in electronics and I have a design
> question I was hoping someone could help with.
> I recently purchased a new (to me) strobe system but was dismayed to find
> that the trigger voltage was 98 volts DC. In the interest of safety, and
> longevity of my equipment I devised a circuit that would trigger the strobes
> but only put 10 volts DC across my camera sync contacts. The circuit can be
> found here: http://home.cogeco.ca/~camera911/trigger/trigger.jpg The two
> black dots in the center of the graphic are the points that I connect to my
> camera. Also I was thinking putting a .1uf capacitor across the 98V to
> filter out any AC.
> I mocked up the circuit on a breadboard and it actually works most of the
> time but every once and a while it fails to trigger the strobes. Can anyone
> suggest what I might do to improve reliability?
> Thank you for your time.
> Please respond to camera911 at cogeco dot ca
This circuit has two weaknesses that I can see. The 5.6 k resistor
limits the gate current to about 17 ma peak and this may not be enough
of a thump to cleanly trigger the SCR. If you parallel the zener with
a microfarad in series with a 47 ohm resistor, the peak current will
near 200 ma. This will also allow you to raise the 5.6 k to an even
higher value to keep it from draining as much power from the 98 volt
source. There is also no need for a 5 watt zener. A 1/4 watt would
work as well, and give a better voltage regulation.
The other problem is that the gate is very sensitive to leakage
current and hum pickup without a bit of resistance from cathode ot
gate to drain this away. With the above capacitor thumper, you could
easily tolerate 100 ohms cathode to gate to make sure the SCR is
completely off till the gate pulse arrives without loosing much of the
gate drive pulse. This assumes that you are not using an SCR that
already includes such a shunt resistor, internally.
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