--- Chuck wrote about his problems with rf slave triggers :
> I ultimately determined that the sync problem is clearly
> due to a short delay imposed by the slaves
I am guessing but these slave modules likely use a low cost RF chip-set from a
company like Micrel
or Maxim (RF div) or even a module from Lynx (based on maxim chip) which can
introduce a delay of
something like 100-200us. At 100us I would not have expected the sync to be
affected for 1/200
shutter (5ms), since 100us is only 2% of 5mS and the delay of the normal flash
itself to peak
output is likely ~1mS or so. Maybe they rolled their own discrete
transmitter/reciever and it has
very long delays?
> and much to my surprise, I discovered that the slaves (all 5 of them of
> 2 different makes) almost don't work at all with the 540EZ.
......
> Is this somehow related to the trigger voltage of the flash unit? My
> T-32's and Sunpak 422D's are all in the 10-12 volt range.
The T32's are nominally ~11.4V as you mention (zenner regulated), but sometimes
seem more like
~7v(?) if I remember correctly, probably due to zenner leakage as current is
very low. They can
appear higher if used with another brand flash (on a connector cable say) as
they use an isolating
diode which allows sync pin to rise to the voltage of highest device .
The t20's are protected to 12v by a zenner but the resistor divider is set to
run it at ~7V and
there is no isolating diode. (This does mean the t20 should actually protect
the camera from a
higher than 12V on the sync pin if used with cable or sync connector.)
> the exact trigger voltage of the Alien Bees other than the spec says
> "less than 6 volts". The Canyon 540EZ, on the other hand, is about 3.4
> volts with alkalines and about 3.7 volts on the high voltage battery pack.
......
> I am intensely curious as to why this flash won't trigger with a slave that
> fires
> everything else.
>
It is likely your slaves just don't pull the pin low enough near ground, to
trigger the newer low
voltage canyon flashes OR they draw too much current at idle from the pin OR
maybe the scr in the
slave does not trigger properly at such a low terminal voltage.
Most slaves (and many cameras) use a low power scr to pull the sync pin low.
The scr typically
pulls the pin to ~1V but if a series diode is added to cause better scr switch
off (see my
description /circuit diagram on Mark Dapoz's hardware website : high voltage
trigger for Q310)
this becomes more like 1.6V. The change between trigger voltage and idle
voltage, is then only
1.8V. You could actually test the voltage required very crudely by hooking up
some small diodes
in series with a piece of wire and shorting the sync pin to ground via the
diodes. Add more diodes
to the string until it stops triggering. The voltage it stops triggering at is
then ~ the number
of diodes times about 0.5V (within 0.5V!). You would need a scope and a
variable supply to
figure if it was the scr not firing at low volts.
You could probably modify the slave trigger with an add-on to trigger the canon
flash more
reliably, by using a few extra parts.
Tim Hughes
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