Resistor value for LED

this

resistor,

SMPS

Whoa nellie, I didn't suggest the OP do any such thing. ;-) I was only following up on Jim's comment about PICs.

I really wish that every mention of a PIC didn't have to result in this statement. It's sorta becoming the Godwin's law of electronics. It's almost like saying, "everything about electronics is worth learning, no matter how difficult or expensive, as long as it doesn't involve micros because they are too hard and too expensive". At any rate, I didn't suggest using one for the OP's problem, I only mentioned that it could be quite efficient, as a response to Jims comment.

I wasn't looking to beat that dead horse again, I was only hoping that someone could/would (if possible) show me something more efficient using "conventional" components.

Also consider that voltage varies surprisingly little with wavelength/color within a given chip chemistry. For example, less-yellowish green LEDs and red ones of the same chemistry (InGaAlP, or GaP) have surprisingly similar voltage drop. One thing that does usually happen is that efficiency falls bigtime if the wavelength gets shorter to an extent that has the photon energy in electron volts exceeds the voltage drop in volts.

Phosphors don't affect the voltage drop...

But most white LEDs have blue LED chips and phosphor. So white LEDs will have the same voltage drop as related blue ones.

- Don Klipstein ( snipped-for-privacy@misty.com)

job.

at

only

pic

this

no

micros

using

It has _nothing_ to do with "..micros because they are too hard and too expensive". What it has to do with is deception.

Deception. If there were money being exchanged, it could be called something illegal, like fraud. But there's no money being exchanged. The deceiver is advocating the use of something because it's his fervent, almost religious, belief that it's _the_ solution to all of the OP's problems. Which may be true.

But the deceptive part is that the deceiver has not told the OP (who is usually a neophyte and probably knows little about electronics OR pics) that there is a price he will have to pay in up front costs for the hardware and software to program the pic. And there is a learning curve that the OP must go thru to learn how to program and debug the software. It is simply NOT Plug and Play!

I have nothing against using the right tool for the job. But it's deceptive and unfair to tell someone to use something without telling them what they will have to pay and what they will be going thru to complete their project.

It just so happened that microcontrollers were the subject in past discussions. Four decades ago it could have been using a laser as a pointer for presentations. We all know nowadays that the little hand-held ones do an excellent job. Back then, 40 years ago, you would have had to blow your own glass, fill it with helium and neon, and use a high voltage power supply to excite it. If a presenter could do all that, he could then pull out his gatling gun sized pointer and look like the termninator as he pointed to his presentation. But it would be deceptive to not tell him that he would have to jump thru all those hoops to get this cool looking presentation tool.

And there is *nothing* Godwinesque about it. Deception is deception.

\"Watt

5V

color

if

antimony,

Right, sorry if I may have caused any confusion.

I noticed that some white LEDs are now using wavelengths even shorter than blue.

It's

could

that

too

Here we go again. :-(

the

So, suggesting the use of a micro is now tantamount to fraud? The zealot thing is cute too. Not very realistic, but cute.

is

pics)

curve

software.

I don't recall anyone (and especially not me) saying that it was PnP. Of course there is a learning curve, and I believe it is simply another step in someone's education in electronics. As I've said before, if I was recommending one to a newbie, I'd likely be giving him the code and offering to flash the chip for them.

To be clear, I never suggested the OP use a micro. I just wanted to respond to Jim's comment and hopefully spawn some conversation about more efficient ways to drive the LED. It looks like that was simply a pipe dream on my part.

One might also consider it deceptive and unfair for someone to suggest that micros are as difficult and unreachable as you describe.

would

a

like

The primary difference being that your anology is absurd. Learning micros or purchasing the needed equipment is no where near the difficulty you describe. Ten or twenty hours of self study and $75 for a programmer is not quite in the same league as ramping up to make your own LASER tubes.

If there is nothing Godwinesque about it, then why is the same old tired argument used to shout down the micro crowd every time?

One final time, I wasn't wishing to debate this topic (as it's like talking religion or politics to some people), I was looking for some clever analog or digital based current source that would be more efficient. Any ideas? ;-)

[snip]

telling

I didn't describe "how unreachable" they are.

I did say that it's deceptive to portray them as Plug and Play when they simply are not. When a person asks for a simple answer to a simple problem that is low cost, giving him an answer that isn't (and saying that your answer is the best) isn't telling the whole truth.

for

your

Maybe my analogy flew over the top of your head. What I'm trying to say is that "Ten or twenty hours of self study and $75.." is *not* the appropriate answer to a question that should cost $5 and take a half hour using skills that are already learned.

But what's more to the point, telling the asker that it is the answer, and can do all these wonderful things, and leaving out the "Ten or twenty hours of self study and $75.." part is deceptive.

deception.

tired

It has nothing to do with micros. It has to do with deception.

Exactly. Some people consider Pics to be a religion, and give that answer to those problems that could be solved with a simple 555 timer. And I (and others) are only trying to dispel the myth that a Pic is the answer to all your problems, even the smallest.

[snip]

"Watson A.Name - "Watt Sun, the Dark Remover"" wrote

As I have now said multiple times, I do not wish to have this "religious" debate.

Very well, I take it that you wish not to contribute anything useful to the subthread about driving an LED with extreme power efficiency.

Don ...

I've noticed this effect, also. Yet a hallowed source none other than HP has a full chapter in AN-1005 describing how a pulsed drive increases the optical output of the lamp. I either don't understand completely what HP is telling me or my eyes are deceiving me, which is entirely possible.

Let's take a relatively trivial example. How about we generate a square wave to drive our lamp. Let's not worry for the moment about how we generate it, let's simply say that we can have either a square wave or a DC signal for drive. Let's also make the math easy and say our LED has a forward voltage of 2 volts at 20 mA and 2.5 volts at 40 mA (real data for HLMP-1340, typical high brightness red LED).

So in the DC case, we choose a resistor value of 150 ohms, and in the square wave case we choose a resistor value of 62 ohms. The battery doesn't care. It is pumping out an average power of 100 mW in either case.

However, in the DC case, the (peak) power delivered to the LED is 40 mW and in the square wave case the (peak) power delivered to the LED is 100 mW. And yet, to my admittedly untrained eye, they are either the same brightness or perhaps the square wave is just the littlest bit DIMMER than the DC drive. What's happening? My understanding of optics is that the eye is pretty much a peak detector when the PRF is greater than the flicker rate of

24 Hz. or so, but that isn't the case in the experimental lab. Comments?

Oh, and to the feller that wanted to use a PIC and an inductor at a cost of $5 or so, you can achieve the same thing with a single CMOS quad NOR gate, two sections as oscillator and one section as monostable multi plus a single PNP transistor in saturation driving either the LED directly through a much smaller resistor (the cheap method) or throught the inductor and a catch diode (the expensive method) for less than two bits worth of parts (the cheap method) or a buck's worth (the expensive method).

Jim

I played with some white LEDs awhile ago. I made a 10 LED flashlite out of a VCR hand remote. 2 AA's, 1 hand made inductor and 1 2222 or NTE123. I can dimly lite up the front of the house about

20 to 25 ft. away. I used 50/50 narrow and wide beam white LEDs, you want a pic? I'll try to get it. I took a plastic straw wrapped it w/ fiberglass and epoxy. Cut this in 1/2. That's my 1/2 cylinder reflector. I put aluminum foil in the straw as a better reflector. I wasn't impressed with LED flashlites. I made one w/ a charge pump from Micrel, using 6 LEDs and 2 cells. ahh. I had to make everything SMD to fit in the back of the flashlite head.

is

DC

square

care.

and

brightness

of

of

single

much

5

achieves

using

Sure. A single cell lead acid battery.

Ed

they

1.8

color

a

wonder?

They're making LED chips with a whole new and different chemistry.

But I thought the LED voltage corresponded to the energy level of the photons... don't have the equation handy but can look it up. Hence my question about whether the internal resistance is higher now. Maybe it is.

As a person with software background since 19 years, that sentence reminded me of a quote regarding regular expressions (regexps):

"Some people, when confronted with a problem, think 'I know, I'll use regular expressions.' Now they have two problems."

- Jamie Zawinski

--
Rikard Bosnjakovic                         http://bos.hack.org/cv/

Anyone sending unwanted advertising e-mail to my address will be
charged $250 for network traffic and computing time. By extracting
address from this message or its header, you agree to these terms.

Voltage drop across the LED is often but not always close to that of the "electron volts" of the emitted photons.

One thing that can happen is that when an electron "drops" from the conduction band back to the valence band, it may have an intermediate stop on the way down, causing the descent to have more than one step - one of which radiates a photon that has less energy than that required to push an electron from the valence band to the conduction band. I suspect this is what happens in InGaN green LEDs, with voltage drop well above the "electron volts" of their emitted photons even at 10% of the current they were designed for.

Another thing that can happen is that factors besides bandgap energy affect the wavelength of the emitted light. For example, the chip may have layers of such thickness that have interference effects that reinforce a particular wavelength. Of course, efficiency may be reduced if this is a wavelength other than that at which the chip would otherwise emit, especially if the wavelength has photon energy greater than the bandgap energy.

Still another thing that could happen is that an electron has more than one mode of descending from the conduction band to the valence band - a radiating mode and a non-radiating one. If the radiating one has average photon energy greater than the bandgap energy, what happens is that most photons take the non-radiating path. Thermal agitation gives a few electrons the above-average energy needed to take the radiating route, but thermal agitation is actually detremental as a net by causing what I believe is "thermal quenching" - kicking electrons onto the non-radiating route. At lower temperature, fewer electrons take the non-radiating route, and the voltage drop is higher. Example: GaP and InGaP green LEDs. A really efficient one with wavelength in the upper 560's or around 570 nm - the usual "chartreuse" color - is about 1.5-2% efficient. The "pure green" varieties of these chemistries (wavelength around 550-555 nm, still "lime green" as in less yellowish but still yellowish) are much less efficient - and have average photon energy around 2.25 eV and voltage drop maybe usually 2.1 V.

- Don Klipstein ( snipped-for-privacy@misty.com)

In article , mc wrote in part:

Actually, Radio Shack has 150 ohm resistors all by themselves in

5-packs, both in 1/4 watt (271-1312) and 1/2 watt (271-1109). Probably because, as you said, this is one of the values that 20% resistors came in.

They also have 180 ohms (a value that 10% resistors came in) in 1/2 watt (271-1110).

Assuming none of these were discontinued since they printed the catalog that I just pulled out (the 2002 one).

- Don Klipstein ( snipped-for-privacy@misty.com)

This business of the eye being a peak detector even when the pulse rate is above that necessary to avoid visible flicker (more like 50-60 Hz rather than 24 Hz if duty cycle is near or under 50%) is largely not true. This is a myth (at least mostly) that started due to an LED chemistry commonly used in digital displays appearing to produce more light at high instantaneous current (10's of mA per chip) than at low instantaneous current (a couple to a few mA per chip) even with the same average current. The truth is that the reason they looked brighter when pulsed was because average light output actually increased from pulsing them even with the same average current, because the LED chips were nonlinear, with higher efficiency at higher current. This was fortunate, since the circuitry got easier with multiplexing that had only one segment in each digit (or sometimes even in the whole display) on at a time.

- Don Klipstein ( snipped-for-privacy@misty.com)

my

it is.

the

stop

of

push an

drop

of

reduced

otherwise

than

a

bandgap

needed to

detremental as

electrons

take

"chartreuse"

these

less

average

Thanks for an explanation to a noticeable difference between older and newer LEDs. I measured some yellow ultrabright LEDs yesterday, and they were 2.3V at 20mA. That's less than I expected, since some of the red and orange high brightness LEDs are almost that high.