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[OM] Re: Indian flash slaves - where's Tim?

Subject: [OM] Re: Indian flash slaves - where's Tim?
From: ScottGee1 <scottgee1@xxxxxxxxx>
Date: Wed, 21 Sep 2005 09:12:35 -0400
Yep.  Where would we be without engineers?

ScottGee1


On 9/21/05, Chuck Norcutt <chucknorcutt@xxxxxxxxxxx> wrote:
> Thanks.  I knew you'd eventually explain it.
>
> Chuck Norcutt
>
> Tim Hughes wrote:
>
> > Chuck wrote:
> >
> > Tim never did respond to tell us what was going on with those diodes.
> >  <<
> >
> > Ok , I only just now caught my name being used in vain!
> >
> > The reference mentions soldering wires to short out the diodes
> > inside. Since that is all that was said, and I don't have a
> > schematic, I can only make an educated guess here:
> >
> > Ideally the slave needs to trigger either a positive or negative
> > polarity strobe flash unit, so as to operate with different camera
> > vendor's devices. (The OM flashes are all positive polarity)
> > Conventional transistors (or scr's) only conduct well in one
> > direction or have other limitations when operated inverted. It is
> > likely, that the slave unit has a diode bridge or other series diode
> > arrangements to allow it to trigger both positive or negative
> > devices.  The diodes add at least 0.6V and maybe 1.2V drop (4 diode
> > bridge). Quite often a small low power SCR is used as output switch
> > in the slave, to pull the flash trigger pin low. These drop about 1V
> > when "on". So potentially the output only gets pulled down to 2.2V
> > above ground, instead of ~0V. This may be above the threshold at
> > which the strobe triggers correctly.  This is particularly true if
> > the slave-trigger steals it's power from the flash trigger pin. It
> > then starts off lower than normal or even below the trigger level.
> > Many newer flashes have extremely small currents required to trigger
> > them, and stealing current from the trigger pins may stop them
> > working altogether.  Remember also, some new flash units have
> > unloaded voltages as low as 4V.
> >
> > Regards, Tim Hughes
>
>
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