Lee H. asked why there is a recommended limit of 9 for the number of Tseries
flashes that can be
connected simultaneously?
Having modified a number of 3rd party flashes for use with Oly gear, I am
fairly familiar with the
T series ccts. The OM2N service manual has some of the best published data on
the interface. Here
is my best guess:
The short answer is the LED flash ready circuit probably constrains the number
of flashes, as it
uses different current levels for the "flash connected" but **not ready** vs
"flash ready" state.
When you attach many more than approx 9 flashes the flash ready current level
is reached from the
sum of the flash connected signals before the flashes are ready.
With more than 9 flashes the LED ready current total exceeds 20mA (2.3mA*9)
**when used on an OM2
or OM2N**. Often small LEDs like these are only rated to handle 20mA
continuously, so this might
shorten their life. In practise this is probably not a problem because the
relatively small total
number of flash ready usage hrs. On some versions of OM2S's and newer models
the LED current comes
from the camera battery so this last item is not an issue at all. (Advantage to
flash/battery
eater OM2S!)
Here is a more detailed discussion:
The T32 and T20 ccts are very similar, although not identical in the interface.
1) The main camera trigger cct has lots of margin to trigger multiple flashes,
as either a 400mA
scr (OM2S-OM4) drives this pin or a mechanical switch (OM2N and earlier). So
you can **trigger**
lots more than 9 flashes. With a TFlash connected the trigger switch must pass
current from a 33nF
trigger cap charged to between 7 and 12V, a tiny load for the scr or mech sw.
2) The quench trig circuit needs to sink an estimated 16-20uA (@ ~1.1V) per
Tflash, so a total of
less than 180uA for 9 flashes. The limiting factor here actually depends on a
10kOhm limit
resistor in the case of the original OM2 body (originally designed for Q310
flash). In the OM2N it
depends on a buffer ic (ic103) which likely can drive more current but we don't
know for sure. If
we use the OM2 as worst case (has lowest drive current), then it should be able
to drive more than
9 flashes since (3V-1.1V)/10k = 290uA > 180uA. So it should be able drive maybe
up to at least 5
flashes more ( (290-180)/20=5 ). This is a rough estimate.
3)The LED flash ready cct/pin: Uses a 5V source with a 120k resistor which
drives ~30uA to the
ready LED in the viewfinder. With 9 flashes connected this current totals
270uA. The spec for the
threshold of flash ready from the OM2N service manual is 350uA. It is probably
not very
accurately controlled but there needs to be some margin between these levels.
In theory if
everything is as designed, you could connect another (350-270)/20= 4 flashes
before you reached
the threshold. The TFlashes also need to short out the LED briefly during
flash exposure. They do
this with a transistor switch. It appears the drive current to the transistor
should allow it to
drive quite a bit more than 20mA, so this is probably not the limiting factor.
If you did not care about flash ready LED being meaningful, then the number of
connectable flashes
is limited by the drive current from the quench pin. You could do some
empirical testing to
determine the drive available by loading the pin with a resistor and checking
waveforms on a
scope. You could then make a rough estimate of the number of flashes like I did
above. This is
quite likely different for each model of camera.
Regards,
Tim Hughes
TimHughes@xxxxxxxx
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