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Interesting find with optocoupler

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StillTrying:

--- Quote from: T3sl4co1l on November 27, 2019, 01:04:16 am ---Xenon photoflash is too slow, some microseconds, but air sparks can be nanoseconds.
--- End quote ---

I've looked at lots of xenon flashes over the years, and even the total light shape of multiple slave flashes. I don't think even air sparks can get close to the clean ~15ns rise of a small 5mm LED.

200mA through Super Bright green and blue LEDs were the fastest rise and fall times and lowest propagation times I got here. The light->current shape should still be quite linear at ~X9 the LED nominal current.
https://www.eevblog.com/forum/chat/20w-halogen-bulb-viewed-by-a-photodiode/msg2411274/#msg2411274

Despite all the theories of the light spreading into the wrong areas of the photo diode and causing the curves and long tails, I still think most of that is in the emitter, >:D because I could improve the curves and long tails by increasing the LED current, and increasing the LED->PD distance so that the light level at the PD was still the same.

T3sl4co1l:
Same thing affects PDs, apparently some have so much internal resistance that it trashes the bandwidth and noise floor, much as Rbb' in BJTs does the same.  I'm not entirely sure if that's a sheer bandwidth thing (if it can be overcome with current, as in your example), or if it's a noise optimization problem (which was where I heard this from, Phil Hobbs).

Tim

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