Low current LED 7-segment

Yup. That's what I'm trying to do, understand.

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Rick

That is "simple" physics. But there is no heat energy that is being collected into a photon or other form of low entropy.

Here's a thought experiment. The photon emitted by this LED has more energy than was delivered by the voltage of the circuit, right? Some of the energy was from heat of the LED. These photons can be collected by a solar cell. If we allow each of these two steps to be highly efficient, the energy collected by the solar cell can be greater than the energy required to drive the LED and we could close the loop and get a net gain of energy violating the 1st law.

I know the LED and solar cell are not 100% efficient. But unless the gain in energy of the photon can be linked to that inefficiency we have a theoretical problem.

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Rick

Optometrists and ophthalmologists use different conventions. One uses the spherical power PLUS a superimposed cylindrical correction, the other uses sphere MINUS a cylindrical correction. It's confusing.

The values are expressed as the optical powers needed to CORRECT your eye's focal length. So, focusing at 10" is a focal length of 1/.254m = 4 diopters. Your spherical Rx for distance vision would be -4.00 diopters.

The easiest way to peg the angle of your astigmatism is with a starburst-looking chart. You approach it from a distance far enough to blur all the lines, note the angle of the line that comes into focus first, then the distance at which the orthogonal line focuses.

Carefully done, you've got your two focal lengths and their angles, and all you have to do is sort out the optician's nomenclature.

Pretty reasonable. It's a brain thing--some people's hate that, can't ever adapt, others love it. You're one of the lucky ones.

Cheers, James Arthur

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Hi Rick, OK I downloaded some of the referenced articles. There's a bunch of thermo stuff. If you want to send me an email and promise never to share/post the article s then I can send you copies.

Here's a quote from Weinsteins Phys Rev. 119/ 499

(cutting and snipping for a pdf.. it missed some edges... but you get the j ist.) without considering the relationships among the various rates involved in t he process. As soon as we think in terms of rates, it becomes apparent that the Raw in the argument is that it does not take into account the eRect of the lifetime of the excited state. For while it is true that we can, in pr inciple, make the emitted radiation as nearly monochromatic as we please, t he essential point here is that because of the uncertainty principle we mus t, at the same time, make the lifetime of the excited state correspondingly long. The longer the lifetime of the excited state, however, the slower wi ll be the rate at which the entropy of the system decreases, and hence the smaller the rate at which the field needs to carry oG entropy, i.e., the lo nger the lifetime, the more nearly monochromatic the radiation can be and s till satisfy the second law. The object of the present note is to show, by an elementary calculation on Klein's system, that when one takes into accou nt the reciprocal relationship between natural line breadth and lifetime, t he second law is indeed satisfied, and thus to show that the local validity of the second law of thermodynamics for radiative emission processes is es sentially a consequence of the uncertainty principle.

So another paper says the "radiation temperature" of their LED (as defined by Weinstein) was ~1700 K. It's the "temperature" of the radiation that is hot. (Maybe we can get out guru PH to comment.) Not my "absorber theory."

George H.

lationship between natural line breadth and lifetime, the second law is indeed satisfied, and thus to show that the local validity of the second law of thermodynamics for radiative emission processes is essentially a consequence of the uncertainty principle.

The temperature of radiation is *always* hot. That's the point. It was created by some electrical energy and some thermal.

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Rick

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in the process. As soon as we think in terms of rates, it becomes apparent that the Raw in the argument is that it does not take into account the eRec t of the lifetime of the excited state. For while it is true that we can, i n principle, make the emitted radiation as nearly monochromatic as we pleas e, the essential point here is that because of the uncertainty principle we must, at the same time, make the lifetime of the excited state correspondi ngly long. The longer the lifetime of the excited state, however, the slowe r will be the rate at which the entropy of the system decreases, and hence the smaller the rate at which the field needs to carry oG entropy, i.e., th e longer the lifetime, the more nearly monochromatic the radiation can be a nd still satisfy the second law. The object of the present note is to show, by an elementary calculation on Klein's system, that when one takes into a ccount the reciprocal re

ndeed satisfied, and thus to show that the local validity of the second law of thermodynamics for radiative emission processes is essentially a conseq uence of the uncertainty principle.

ned by Weinstein) was ~1700 K. It's the "temperature" of the radiation tha t is hot.

Look Rick, I've offered you articles. I've only skimmed them, so it's hard to pick out the important points. Except that it's a heat engine!* With T_led ~1700 K and T_room = 300 K, and some maximum efficiency of eff < (1-T_cold/T_hot)^-1

(I'm assuming you know about heat engines and the 2nd law.)

George H.

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d in the process. As soon as we think in terms of rates, it becomes apparen t that the Raw in the argument is that it does not take into account the eR ect of the lifetime of the excited state. For while it is true that we can, in principle, make the emitted radiation as nearly monochromatic as we ple ase, the essential point here is that because of the uncertainty principle we must, at the same time, make the lifetime of the excited state correspon dingly long. The longer the lifetime of the excited state, however, the slo wer will be the rate at which the entropy of the system decreases, and henc e the smaller the rate at which the field needs to carry oG entropy, i.e., the longer the lifetime, the more nearly monochromatic the radiation can be and still satisfy the second law. The object of the present note is to sho w, by an elementary calculation on Klein's system, that when one takes into account the reciprocal re

indeed satisfied, and thus to show that the local validity of the second l aw of thermodynamics for radiative emission processes is essentially a cons equence of the uncertainty principle.

fined by Weinstein) was ~1700 K. It's the "temperature" of the radiation t hat is hot.

s

You know... the "entropy" temperature of an led is an interesting question. From one article it changed with current.. from 800K to 1700K. The 800K was at the low current end. (IIRC) Which confused me, I was thinking at low current I'm nipping the tippy top of the thermal tail. (of course not everything written in Phys Rev is correct, but stuff from the 50's-60's seems much more reliable to me.) (A sad comment on the times, quantity over quality. :^(

George H.

PS, It's also a huge waste to have all this nice science hidden behind pay walls. We could write government contracts such that after 20-30 years everything published was released to the public, That would seem to be a huge win for everyone, but the publishers, charging much to much... Maybe throw 'em a government "bone".

It is still the same. You do what astronomers and submariners do or rather did which is use shielded red light to illuminate your clip board and as little of it as you can get away with. These days people use faint white light but red LEDs back then were way better than red glass.

It also makes reading much easier. Red light has a hit on chromatic aberration that makes it harder to read fine detail.

I once had a young deer bump into me at night whilst observing in Zion Canyon USA. The ground close to woods was very dark but the sky bright.

But you can still use Exactly the same techniques astronomers use to stay dark adapted - a patch over the good eye and a clipboard with well shielded just bright enough illumination of the page on it.

These days we are talking white LED with

You don't lose dark adaption by using just enough task lighting to do what you need to do - especially with red light although you will need glasses matched to your eyes working in red light.

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Regards, 
Martin Brown

Aren't you just a tiny bit curious about how it works?

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Regards, 
Martin Brown

Not enough to get into the solid-state physics. A qualitative feel for the physics and some quantitative data on black-box device performance is enough for me. I don't actually understand how a diode works, and I really don't need to. I can measure things quickly when I need to.

Electronic design is complex. Some people can do it and some can't. Physicists are generally mediocre at it and chemists - who should understand the devices - are generally terrible. Circuit design is not science, and need not apologize for not being science.

It's more fun than science anyhow.

You aren't thinking about it the right way. The LED is operating as a heat engine pumping out visible light photons and using energy to do it.

When i*V is large the component being taken from the crystal lattice is tiny by comparison so that the i^2r dissipation dominates. When the current is sufficiently low the i^2r dissipation drops below the more or less fixed contribution of the thermal energy in the crystal lattice providing energy to photons and so the LED is cooled slightly by the emission of photons stimulated by the applied voltage.

ISTR that modern high efficiency LEDs use a much more rigid lattice than early ones. It used to be a party trick to dip an old red LED in LN2 and watch the quantum efficiency at constant current rocket up.

Power dissipation in a resistor scales as i^2 whereas photons emitted scales roughly as i.

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Regards, 
Martin Brown

So what is to prevent you from using the photons to generate electricity which powers the LED resulting in > 100% efficiency?

According to wikipedia, "In December 2014, a solar cell achieved a new laboratory record with 46 percent efficiency in a French-German collaboration.[31]" The LED in the MIT experiment produced 230% efficiency. That's 105.8% loop efficiency. So it's more than just theoretical... We need to get these two groups together. "Hey, you got chocolate in my peanut butter!"

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Rick

The LED is cooling in the process and so it cannot emit more than a tiny number of photons. It becomes self limiting even if you could convert the emitted photons with 100% efficiency which I doubt.

Monochromatic light sources appear hot to the outside world even when they are cold since the photon carries away a lot more energy than their surface temperature would seem to imply.

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Regards, 
Martin Brown

That doesn't make sense. The heat can come from anywhere. Once the LED is cooled heat will rush in from the surroundings. That is the point of the second law of thermo. You can't push heat uphill without expending energy. In this case we are sucking heat out without expending energy which must have some limitation.

Only if you are in the path of the photons.

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Rick

You are pumping it with power i*V and losing i^2r as heat but gaining energy into the emitted photons from the crystal lattice which has to be replaced from the environment. As i -> 0 the number of photons decreases linearly but losses decrease as the square of the current.

Obviously.

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Regards, 
Martin Brown

Your air conditioner emits more power as heat than it takes in from the mains. Once power is applied, you aren't in thermodynamic equilibrium anymore, and a lot of theorems don't apply.

Cheers

Phil Hobbs

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

160 North State Road #203 
Briarcliff Manor NY 10510 

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

Thermo I and II *always* apply. The AC or heat pump push heat uphill by turning highly focused energy into heat, increasing entropy overall. Using electricity to produce highly ordered photons of light is not a large change in entropy if any at all. Sucking the heat from the warmest molecules to increase the energy of the light photons clearly lowers entropy in violation of Thermo II. There must be some effect which balances this lowering of entropy or the extent must be limited to be less than some inherent production of heat or other increase in entropy.

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Rick

You are expending power i*V and wasting i^2*r and pinching a bit out of the crystal which has to be replaced from outside the LED by conduction.

So far we are all agreed - the best you can hope for is to break even when the entire closed system is properly accounted for.

Now I see your problem. It isn't that energy is being taken off the warmest molecules (although it could be - that is how natural OH masers like W3OH work when pumped by UV bright young stars).

The energy is being robbed from lattice vibrations aka phonons which in modern stiff LED materials play a part in the emission process and are the initial absorbers of waste heat. What happens isn't really any more paradoxical than the development of delta-T across a TEC diode junction.

You are not getting something for nothing but in the low current limit you are getting very close to 100% thermodynamic efficiency.

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Regards, 
Martin Brown

of the photon. They can be large or they can be small. We can make these losses as small as possible without changing the impact on the effect.

The fact that there are other heat sources in the universe has nothing to do with this as well.

No, that is energy and the 1st law of thermodynamics, no free lunch.

2nd law is entropy always increases which means you can't even break even.

I never said anything about getting something for nothing, again, that is the 1st law. Your link above doesn't appear to have any relevant content.

Perhaps you can read up on the 2nd law and entropy?

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Rick