No, they don't, for any reasonable reading of that sentence. The physics doesn't allow it (the opposite, in fact).
The *average* current is all that matters. The average also does the heating, so it's a no win to pulse them, other than it's the easier way to dim them.
Note:
Red
Sure dimming LEDs is possible, and actually not all that hard if you start with DC (the unit you linked is battery powered). Doing it from the AC line, cheaply and efficiently, is more challenging. It's easy to do sloppily, but then there is no gain over an incandescent bulb.
OK you have a string of LEDs dropping about 95% of the line voltage and a resistor dropping the rest and limiting current now. How can making that resistance more by adding a rheostat in series be more inefficient?
I still do not get how you arrive at that conclusion. A string of LEDS will draw 15 - 20MA at full brightness and if you increase the size of the current limiting resistor the current will drop from there in a very linear manner.
As usual you are not "completely right". I won't argue and say you are wrong in your assertions - but my UNDERSTANDING is that PEAK current controlls the visible light output, and average current affects lifespan (due to junction heating). It is not totally linear.
My experience is obviously different than yours. As in many other cases, I need to say that just because you haven't seen it, don't make it wrong or impossible. Just means your scope is too narrow.
See:
de quoted text -
LEDs arent dimable in the normal since of the word. The current to them can be pulse width modulated to produce what we percive as a dimming of the light. This is accomplished by switching the diode off and on at a high enough rate that our eyes cant see the flicker. Dimming is controled by changing the ratio of off to on time. You may have noticed that LED Christmas tree light are a good bit dimmer than the typical LED lamp found on an appliance like the computer you are working at now. This is because the computer supplies a constant DC voltage and the LED is on 100% of the time while the Christmas tree lights are powered by a varying AC voltage that will only turn them on for about 40% of the time.
Nope. Current controls the light output. Average current controls the average light (which the eye detects, integrated over the "critical fusion frequency"). Yes, peak current controls the peak light output, if you're detecting peaks, this might be important. It is certainly *not* if you're looking at it. Above the CFF, human eyes average the light intensity. Having high peaks with long spaces does *nothing* to aid perception and in fact reduces efficiency; LEDs are LESS efficient at high currents. Pulsing LEDs is a lose-lose proposition.
No, you're just wrong. It's not the first time.
Did you actually *READ* that app note? An overdrive factor of *6* will produce only 3 times the light (efficiency drops by half).
"The average current must be kept below the current rating for the LED."
IOW, you can't overdrive it for long.
The table "High Power LEDs" indicates that you can drive the LED up to 5x current for 2ms, with a 10% duty cycle. A 5X current you get 2.5X the light or ONE HALF the average light output as you would have gotten if you'd just driven it at 100%, DC. IOW, a loser.
As I've shown with the first article, pulsing LEDs is a loser. You're simply
*WRONG*.
Resistors heat == inefficient
Try it. You'll find that you are *very* wrong. LEDs are *not* in any way linear. You'll also find that the efficiency goes down as you lower the brightness (the resistor takes more of the line voltage).
If you use a current source it works, linearly, but is no more efficient (think of the current source as a non-linear variable resistor).
But the resistor will always be there. You are just making a bigger resistor, the current will drop and the light will dim in a vary linear way. The voltage you drop across your resistor will be the same no matter how big it is. That is not like a rheostat on an incandescent where you are changing the voltage applied to the filament.
Why do you think the voltage across the resistor changes? You have a certain voltage dropped across each junction and the resistor takes the rest. The resistor is a current regulator not a voltage regulator.
The voltage across the resistor *does* change. Also, P=I^2R.
Because it does?
The resistor is a current regulator not a voltage regulator.
te:
Nothing linear about a diode, he should actually try ploting a curve and see how much the resistance of a diode changes as you change the current, even much more so on a series string of the things like a lamp cluster.. Also no big deal to convert to DC to run an LED they are after all diodes they will do it for you. Thats the really cheap way out . Slightly more expesive and only arguably of better design is using a match head size bridge rectifier, cost about as much as a match too.
Jimmie
Jimmie.
No, that would assume an ideal diode. They are not ideal, the I-V curve is not a vertical line. The more diodes you put in series and the lower the voltage across the resistor, the more pronounced this becomes. From some recent work, a blue LED at about 5mA drops about
3V. At 20mA the drop is closer to 3.3V. Now, put thirty of these in a string and the difference is 10V. You only have 20V across the resistor - it's changed 50%. ...and this is quite nonlinear.A resistor regulates nothing. R==V/I. If you say that I is constant because V is constant and V is constant because I is constant, you're getting nowhere. ;-).
Lets get to the bottom line. Are you trying to say a dimmed LED (by increasing the size of the resistor) draws more current than one at full brightness?
You may have a more efficient way of dimming a LED lamp but how many decades would it take to get your money back over just using a rheostat? Sometimes KISS is the best rule.
I like doing experiments like this and I will be back as soon as my high intensity LEDs arrive because this is my plan. We will see. I have no problem admitting when I am wrong but I want to see it.
No, they are diodes, but it's not a good idea to use them as rectifiers. The reverse breakdown of LEDs is pretty poor. As you note, a bridge rectifier is cheap enough. The probelm with running them on DC is storing the energy over the cycle (filtering the DC). That isn't cheap or particularly efficient.
Of course not. I'm saying that it is NOT LINEAR and NOT EFFICIENT.
A reostat is a BAD idea. The worst, in fact. It's easy enough to perform this electronically, but the results aren't good. Doing the job right is somewhat more difficult.
Go for it. I do this stuff all the time, though with indicators, not for illumination.
I just did one where I used current sources instead of resistors to eek out the last couple of tenths of a volt on the supply. It was an application where we were using red and green LEDs but the owner like blue but the product is LiIon powered so we have to operate from 3.6V to 5V (when charging we run off the input). The current source allowed a constant intensity over this range (3.6V is pushing it) while having suffiient drive at the low end.
be very small.
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The string itself is a rectifier. The current through them will be DC, no reason to further rectify it.. In the case of a string of xmas lights they have nearly the full line voltage across them. Works fine.
Jimmie
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