led's

zmann tail' that have enough extra thermal energy to make it into the deple tion region. (I'll have to try I-V curves for LED's at low temperatures so meday, dunking into LN2 should be pretty easy.)

energy.

EV than

ght out of an led than power in.. at some ridiculously low current.

re are a few with many kT's of kinetic energy.)

ngth and forward voltage drop for a bunch of different color leds and plot things up to get a measure of Planck's constant. (A bit of a bogus experim ent when you look at the details.)

ner cases,

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'character'.) Was visiting the other day. He does a bunch of educational outreach and uses a modulated diode laser, beam splitter, and cheap corner cube reflector to measure the speed of light. But what he needs is a cheap (fast) photodiode detector. I put him on to Phil's book... but I've been thinking it might be a nice project.

Yeah, all good questions. I'll have to send an email to Cliff. I'm guessi ng it'll be easier to modulate with a sine wave. So at 100MHz I'd get a ful l 2*pi phase shift with a path lenght of 3 meters. That seems reasonable. But I guess a bit slower would work too ~50 MHz. (Sometimes I make someth ing, measure it, and then define the spec.)

I'm not sure we can use ebay as a source, unless I can buy a few build it a nd then get several hundred more. And do I need APD's? I've been reverse biasing all sorts of diodes lately. The optoelectronics PD's I'm using lis t a maximum reverse bias of 30V, I've had 'em up to 60V and no problem. (I ran out of voltage.) I was wondering if I could make garden variety PD's avalanche.

George H.

You've got cause and effect backwards here. Forcing 20ma through it will cause ~3.2v drop across the LED. It is not necessarily the case that

Ergo, Nope.

Cool, Digikey can quit listing resistors by ohms and start listing them all by voltage. That will save a lot of people doing hard math.

It's meaningless and circular (and not predictive) to force "Ohm's Law" onto a nonlinear device. It is a good way to blow up LEDs.

Ohm's Law isn't even a law. It's a statement that some devices more or less sometimes have a linear relation between voltage and current.

And innocent LEDs.

A modulated laser, ballpark 1 milliwatt, will make gobs of signal into an ordinary photodiode, no need to avalanche. If you're using sine waves, and can use a tuned amp, even better.

TBF - I blew a few up back in the days when the Candela rating was a challenge to get some visible light out of the damn things.

Back then there were only red ones, and if you could get one to be visible in sunlight - it didn't do it for long!

What makes sense is to plot current vs voltage.

When you plotted resistance, was it E/I or dE/dI?

That's goofy. It's not a law, it's just two numbers that you enjoy dividing because you think it means something.

When we started using the original Cree SiC LEDs, we ran 50 mA through them. I used two 74F38 sections in parallel, and something like 39 ohms to +5. Nowadays, 1 mA is about all you want for a blue panel indicator.

Lots of people find blue, especially bright blue, to be annoying.

Trick: if all you have is 3.3 volt logic,

because when the logic is 3.3, the available LED voltage is only 1.7, and that won't turn on a blue LED.

The way you use it, it's not a law, it's a definition.

Is this 1mA LED military grade stuff?

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ssing it'll be easier to modulate with a sine wave. So at 100MHz I'd get a full 2*pi phase shift with a path lenght of 3 meters. That seems reasonabl e. But I guess a bit slower would work too ~50 MHz. (Sometimes I make som ething, measure it, and then define the spec.)

t and then get several hundred more. And do I need APD's? I've been rever se biasing all sorts of diodes lately. The optoelectronics PD's I'm using list a maximum reverse bias of 30V, I've had 'em up to 60V and no problem. (I ran out of voltage.) I was wondering if I could make garden variety PD 's avalanche.

Hey! That's interesting. Could I resonate the PD capacitance with some in ductor? I could even tune it a bit with the PD reverse bias. (Or were you thinking of a tuned stage after the PD?) (I had this 'crazy' idea in the p ast about using a T-coil* as part of a PD front end... only to find that Ph il H. had already done it.)

George H.

Hey this is kinda interesting. (But let's not have a big John vs John conf rontation.)

So last week I was running this workshop on noise. I knew I'd have some sp are time while the attendees were doing stuff. So I took along a setup to measure the Johnson noise of a light bulb with a DC current going through i t. (The measruements were a bit of a pain, I had to abandon the inductor I was using as a bias element and go with a simple resistor...anyway that's not important.) So at some voltage across the light bulb I measured the current. And I took that ratio to be the resistance of the bulb. And then I assumed that the bulb would be making Johnson noise given by v^2 = 4kTR*BW. Where I'd see more noise because of increased temperature of the bulb. (The idea was to try and measure the temperature.) Do you think there is something wrong with this 'theory'? Does the light bulb have resistance? Does it have Johnson noise? What's the 'correct' relation between them?

George H.

Yeah, photodiode front end design is often an exercise in creative desperation. Not a lot of stones remain unturned.

Cheers

Phil Hobbs