# led's

Discussion in 'Electronic Basics' started by [email protected], Jul 27, 2013.

1. ### George HeroldGuest

Yeah, all good questions. I'll have to send an email to Cliff. I'm guessing 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 reasonable. But I guess a bit slower would work too ~50 MHz. (Sometimes I make something, 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 and 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 list a maximum reverse bias of 30V, I've had 'em up to 60V and no problem. (Iran out of voltage.) I was wondering if I could make garden variety PD's avalanche.

George H.

2. ### Daniel PittsGuest

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
3.2v will cause 20ma. Diodes have an exponential curve relating voltage
to current, a slight change in voltage can have a significant change in
current. Which is why you want to have some other device (eg, a
resistor) to help set the current.
Ergo, Nope.

3. ### Mr. Man-wai ChangGuest

Is this 1mA LED military grade stuff?

--
@[email protected] Remain silent. Nothing from soldiers and magicians is real!
/ v \ Simplicity is Beauty!
/( _ )\ May the Force and farces be with you!
^ ^ (x86_64 Ubuntu 9.10) Linux 2.6.39.3
ä¸å€Ÿè²¸! ä¸è©é¨™! ä¸æ´äº¤! ä¸æ‰“äº¤! ä¸æ‰“åŠ«! ä¸è‡ªæ®º! è«‹è€ƒæ…®ç¶œæ´ (CSSA):

4. ### George HeroldGuest

Hey! That's interesting. Could I resonate the PD capacitance with some inductor? 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 past about using a T-coil* as part of a PD front end... only to find that Phil H. had already done it.)

George H.

*this was soon after reading about T-coils in one of the Jim Williams' books.

5. ### George HeroldGuest

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

So last week I was running this workshop on noise. I knew I'd have some spare 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 it. (The measruements were a bit of a pain, I had to abandon the inductor Iwas 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 tookthat 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 seemore 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.

6. ### George HeroldGuest

Yup, that's what I tried (inductor from voltage source) (Driving a light bulb with a current source is asking for trouble. IMHO)
Problem was I didn't have a big enough toroid and the coil inductor picked up magnetic noise big time. (I already knew this, but sometimes I need my face rubbed into something a few times till I remember.)

So I got some data with a resistor as bias, but at the highest temperaturesthe bias resistance was much smaller than the DC bulb resistance and it shunted most of the noise to ground.
Hmm.. OK I'll have to think some more... why dE/dI?

RE the thermal mass: The time constant for a bulb is something like a second maybe 10mS at the fastest. So only at low frequency is that going to be an issue.
Well I had the thing perched on bubble wrap to try and keep the building shake out of it. Better might be to suspend it. I would sometimes see a bunch of low frequency 'crude'. Which I assumed was adjacent filament loops bumping into each other. But there were long periods of relative quite. (Ialso could filter out the LF stuff.)

One issue is that the filament loops were also an inductor and they would pickup local magnetic field interference. (Mostly from the room lights, which I couldn't turn off because those in the workshop would have objected.) The pickup would increase at higher bulb currents.. which still has me a bit confused. So before I try this again I need some big torodial inductors and some mu metal to shield the bulb.

But this resistance question is great! What does resistance really mean? (Should I be thinking in terms of damping or energy?)

George H.

7. ### Jasen BettsGuest

Flashlight bulbs used to be "filled" with a vacuum, I think they're
using krypton in some now.

probably magnetic fields were moving the filament, changing is emission
pattern and therefore measured output.

8. ### Phil HobbsGuest

There was that few hundred pieces of the 1.3 um APDs surplused from
Terabeam that went for about 1 cent on the dollar three or four years
ago, but nothing interesting since.

We should still figure out something useful to do with those.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

Briarcliff Manor NY 10510 USA
+1 845 480 2058

hobbs at electrooptical dot net
http://electrooptical.net

9. ### George HeroldGuest

So there's some energy dissipated in the bulb given by the DC V*I.
But that doesn't come into the Johnson noise.
Then I was thinking of the derivation of Johnson noise that starts with a parallel RC and the equipartition theorem. (the capacitor holds 1/2 kT of energy = 1/2 CV^2) and then you integrate over the RC bandwidth to get the Johnson noise formula, So 'R' in this case is, as you say, the dynamic R.. And that's pretty cool!

So the data for my first crack at this was pretty crappy. (not much signal, since it was all lost in the small bias resistor.. big error bars.) But I got a number for the temperature of about 1500K for a bulb that was a little past orange and getting to yellow. And that number seemed small. ButI was using a DC resistance value. The dynamical resistance looks to be ~50% bigger..

Ah OK the drift velocity looks pretty small compared to all the other corrections.

George H.

10. ### George HeroldGuest

Grin, there's not that much gas inside the bulb. It really wasn't too bad..

George H.

11. ### George HeroldGuest

Yeah the workshop was at a university.. not my comfy lab.
Why what? Why try again? why a big toroid?, why shield the bulb?
Well there are other 'kinds' of electrical resistance.. but I agree electron scattering is the most common.

But if we stick to light bulbs, then there is a question of the DC resistance vs the dynamical resistance.

For instance if I was to use a bulb as part of small signal RC low pass filter. (The bulb is the R) Then I think it will be the dynamical resistance (dV/dI)

Well sure, even without the turns it has inductance and capacitance.

George H.

12. ### George HeroldGuest

You can still buy light bulbs from Newark.

George H.

13. ### George HeroldGuest

Newark lists 957 incandescent bulbs, isn't element 14 in europe?

http://www.newark.com/jsp/search/br...tt=incandescent+bulb&Ntx=mode+matchallpartial

George H.

Element 14 was called farnell. They are at least in the UK, Germany,
Austria and the Netherlands. I do not know if they have connections with
Newark.

Draw back is that as private person you can not buy there, atleast not
in Austria and Germany.
It took me a long time to get a Raspberry pi.

15. ### George HeroldGuest

Wow, that stinks. So where do you get electronics?

George H.

16. ### Tom BiasiGuest

As an NRA certified instructor I must advise against putting any
obstruction in front of the muzzle. As far as accuracy, just a small
nick in the crown at those distances will lead your bullet wide astray.
Just sayin"
Tom

17. ### Phil HobbsGuest

I recommend appropriate technology: a screened window with a box fan
sitting in it.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net

18. ### Fred AbseGuest

Still are. Premier Farnell, PLC.

They are at least in the UK, Germany,
Farnell have owned Newark for more than ten years.

Their instruments division (now gone) used to own Wayne Kerr.

Alan Farnell started the company selling components to UK radio repair
shops, out of a suitcase.

19. ### Phil HobbsGuest

Those are actually even cooler than you'd think--they have to use a
bunch of near-field acoustics tricks to reduce the size of the
microphone array and keep the angular resolution. BBN was working on
that sort of stuff five years or so when I visited them in Cambridge MA.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics