Electronics > Beginners
Photodiode circuit design
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jlmoon:
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Zero999:

--- Quote from: stcoso on March 31, 2019, 12:23:00 am ---
--- Quote from: StillTrying on March 30, 2019, 07:40:00 pm ---I've found the light shape from reds to be slower and curved, and super bright green and blue the fastest and squarest.
I won't be trying to get the fastest speeds out of the green and blues because I've got a 100W white COB waiting for >>100W testing!

--- End quote ---

I've tried also with some green led and found them slower. I used an IR led and it was the fastest.
I'm trying to build this little probe because i need it to measure a UV source (20W 365nm) that i've already built and rise/fall time are really important.

I suppose :D white LED are really bad (microseconds?) at turning on/off because of phosphorescence (turn off especially).

--- End quote ---
I've tested a variety of LEDs and had mixed results.

White is interesting. The initial turn off is very fast. There is some afterglow, but it's very weak, several orders of magnitude dimmer than when driven at the rated current, so it's not a problem, Even the warm white Cree CMA3090 I tested was fast enough for use in a 1µs strobe application.



Some phosphor converted LEDs are slow. The amber one I tested was extremely slow. I can't remember the exact part number. It was a Lumiled/LUXEON brand.
https://www.lumileds.com/uploads/161/DS58-pdf
https://www.luxeonstar.com/assets/downloads/ds62.pdf

How bright is the source shining on the photodiode? If it's bright, you need a large revere bias voltage to prevent saturation, as well as speed it up. At very high intensities, no amplification is required. As you can see from the above schematic, I connected the photodiode directly to the 50Ohm input of an oscilloscope and applied a bias voltage to it.
stcoso:
My source is 6/7W of radiated power over a 25cm^2 area but it's 365nm. BPV10 sensitivity at that wavelength is about one tenth of the sensitivity at 900nm. So I decided to dick around with op-amps  :D



Zero999:

--- Quote from: stcoso on March 31, 2019, 09:28:07 am ---My source is 6/7W of radiated power over a 25cm^2 area but it's 365nm. BPV10 sensitivity at that wavelength is about one tenth of the sensitivity at 900nm. So I decided to dick around with op-amps  :D

--- End quote ---
That's quite intense, don't look at the beam and consider UV googles.

Yes the sensitivity of photodiodes declines fairly linearly past the their peak response. One reason is because past the a peak sensitivity the current is proportional to the number of photons per second and UV photons have much more energy, than IR photons, so the current vs radiant power is much less. Another is not all photons hitting the photodiode result in current flow, some ware absorbed and reflected,  which also gets worst, with increasing energy.

At high intensities photodiodes start to saturate. An increase in intensity no longer causes the current to increase and there's a storage time i.e. a delay in it turning off, after the light source is removed.
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