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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