When U4 is activated the voltage across the U4 transistor output  
should drop from 65V to a least below 60V.
Gus


On Jul 16, 2009, at 8:49 PM, Jinx wrote:

Picked up my H11D1 optos for Figure 3 at

http://www.rory.co.nz/projects/lighting/beat_strobe.html

and been trying, unsuccessfully so far, to get a flash out of it

The problem appears to be that the SCR gate rises to only 200mV
when the opto is activated. Looking on the scope there don't seem
to be any gate pulses higher than 200mV and no activity at all on the
cathode. Voltage on the cold side of the 2k2 (opto collector) also
drops to near-zero

With the transformer out however, the gate sees 65V (V+ = 310V).
Using a 470R resistor as a light load instead of the transformer the
gate voltage again drops to 200mV

Transformer datasheet

http://www.jaycar.co.nz/products_uploaded/MM2520.pdf

It measures OK. Something under 1R on the primary and 17R on
the secondary. I've tried a couple of BT151 800R SCRs, rated at
100A peak. It seems a bit unlikely that a-k is broken, given the
energy available from C15

Any ideas ? The author's pdfs show no extra, omitted or changed
components so of course I wonder why his works and mine doesn't.
My only thought would be a separate 65V supply for the opto/gate
that won't drain C15 (if that's what's happening now)

TIA


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