Hi, folks:-
I am looking at using an array of BPW34S photodiodes to detect a visible red laser.
Reverse biased by about 0.5V.
The sensitive area is 3mm x 3mm.
What happens to the photocurrent if only part of the PD sensitive area is illuminated?
--Spehro Pefhany
red laser.
illuminated?
What ought to happen is that illuminated bits will photoconduct and the unl it areas won't.
If you illuminate a reverse-biased diode, individual photons generate indiv idual charge-carrier pairs in the reverse biased layer, and the pairs separ ate and the two elementary charges involved head off to their preferred ele ctrode and constitute a charge transfer between the two electrodes, also kn own as a photocurrent. It looks exactly the same as dark current, but - if you've got enough incident light - there will be more of it.
I suppose if you had very intense and very localised photo-illumination you might have a problem with local space charge in the reverse-biassed layer, and some of the charge carriers might take a bit longer to get to the elec trodes, but only Phil Hobbs would know about that.
-- Bill Sloman, Sydney
You're more vulnerable to nonlinearity that way, but otherwise nothing. Oh, and you should use the Osram ones rather than Vishay--their series resistance is much lower.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
I've never looked in detail, but for a big PD as I move an edge across it, I just see the expected photo current. (that's DC) I think the unlighted area is just some capacitance that has to be dragged along.
George H.
On a sunny day (Tue, 20 Nov 2018 16:48:51 -0800 (PST)) it happened speff wrote in :
It is probably not what you want, but for a 3D array I'd use a cheapo analog NTSC CMOS camera and a PIC to give precise x, y position at 30 fps. I have some PIC 18F14K22 asm and ciruit if you want.
What is the application ?
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Detecting a 5mm shadow at 500 m/s
~us. I hope it's not on the end of a cable. (without preamp) If it is slow, more reverse bias could help. (though I figure you know that.)
George H.
Instead of voltage mode feeding a virtual open, how about trying current mode, feeding a virtual short? That way, (theoretically) only the internal "capacitance" of the diode affects the speed; not cable capacitance, op-amp capacitance, or other bushwah. PD biasing can be almost anything you want.
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