# Constant Current vs. PWM dimming Revealed



## NewBie (Dec 28, 2004)

This chart is the CIE color chart with the Constant Current dimming and PWM dimming, with the Luxeon bins shown:








Here is an exploded view of just the data, note the dimming % is listed, % for PWM is the PWM duty cycle, % for Constant Current is the % of the max current:






In the above chart, take note of the percent where the constant current and PWM dimming split, about 50% for the Constant Current. A person could dim with constant current to 50%, then utilize PWM, to minimize color shift, yet pick up the efficiency increase.



Here is the efficiency difference:








Yet that chart only shows half the story, here is the efficiency gain at each step, I was rather surprised here:







Basicially it all boils down to Current Dimming is alot more efficient than PWM dimming (so you get considerably more run time on down to 0.5% current dimming, but PWM dimming results in less color shift. A combination of the two approaches could sure be interesting indeed.

FYI, the current level this Luxeon was ran at is 380.1 mA = 100% Also, the Luxeon emitter slug had a hole drilled into the side and a type-K thermalcouple inserted, the slug temperature variation over the whole test was a maximum of 2.3 degrees Celcius (yes, a large aircooled heatsink was utilized for this). The same emitter is utilized for all sets of data.


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## 4sevens (Dec 28, 2004)

WOW!


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## PeterB (Dec 28, 2004)

Cool! Awesome data!


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## NewBie (Dec 28, 2004)

Thanks fellas.

I hope to get more info at 1 amp current levels today.


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## evan9162 (Dec 28, 2004)

Neat!

Is the CC color shift perceivable by eye?


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## modamag (Dec 28, 2004)

Awsome!

What's circuit size? Will it fit inside Mag AA, C, or D host? /ubbthreads/images/graemlins/naughty.gif


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## PocketBeam (Dec 28, 2004)

Very interesting and surprising too.

I would be interested in knowing how the color changes on a better bin.


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## cy (Dec 28, 2004)

more kudos piled on.. great data


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## MoonRise (Dec 28, 2004)

Dude, /ubbthreads/images/graemlins/wow.gif


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## idleprocess (Dec 28, 2004)

PWM requires power for switching. I'm surprised the efficiency delta isn't just a constant all the way up to 100% power, where they're both just constant-current situations.

Switch power on, switch it off. The hertz should be the same no matter what the pulse width, so the penalty seems like it should be linear.

...but I'm thinking electrically, not optically. I'd think that the optical difference would be negligible with a pulse width that's long enough to reduce LED response time penalty to a negligible function of the pulse.


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## NewBie (Dec 29, 2004)

Right, but the Luxeon emitter becomes significantly more efficient at lower current levels.

Where with straight PWM, at the same current, you are hitting the emitter with full current.

The info is right on the Luxeon datasheet, if you look carefully.

Next to measure the difference utilizing 1 Amp.

I am expecting even a larger efficiency difference, we will see.

The efficiency gain of current dimming of up to 136% was rather surprising, I was not expecting that large of a gain, more like 15%, but the datasheet doesn't go down to 11.4 mA...
http://www.lumileds.com/pdfs/DS45.PDF

It's kinda cool, how the PWM dimming turned out so flat, just like it should.

BTW, each individual measurement point was taken 15 times and the 15 samples were averaged, to increase accuracy. (in otherwords, 270 measurements were taken, thank goodness for computers).

For the technogeeks that wanna be in the know, the frequency that the PWM circuit was ran at was 133 Hz, which means the pulse width at the lowest level (0.05%) was 0.000003759 seconds (or 3.759e-6 seconds).


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## McGizmo (Dec 29, 2004)

Wow! There's detail for you! /ubbthreads/images/graemlins/bowdown.gif


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## NewBie (Dec 29, 2004)

Due to the low light output, and the long integration time for 0.05% duty cycle dimming, the measurement takes 40 minutes for 15 samples.


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## idleprocess (Dec 29, 2004)

[ QUOTE ]
*NewBie said:*
Right, but the Luxeon emitter becomes significantly more efficient at lower current levels.

[/ QUOTE ]

I knew I was missing something.

I wonder if the numbers would change much with a higher pulse rate?


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## NewBie (Dec 29, 2004)

Okay, I ran some numbers at 1140mA, such as you'd have with a Li-Ion, and using a PWM to dim the LED.

First off, at the higher current level, the tint variation increases substantially:








A close up at 1140mA:







And we have the increased light output efficiency of current dimming vs. PWM dimming:







It really adds up alot at the lower dimming levels:







On a side note, I noticed that the curve fit for what is shown on the LumiLEDs datasheet (output vs. current), when measuring a blue emitter, ends up with nearly an exact match , but the curve is more for a white emitter.


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## HarryN (Dec 29, 2004)

Hi Newbie - Great info as usual. Also as usual, I am trying to really grasp some of the more subtle aspects of your data and analysis.

I always try to compare the data and value of an advancement to the most basic way to manage the current of an LED driver (a set of resistors at 2, 10, and 25 ohms in line with 2 x 123 cells.)

Would it be correct to interpret from the information that at dimming near the 10 % x 1100ma point, a resistor setup would be more efficient than PWM ? Of course, color control is something else entirely.

Sorry if this question is entirely missing the point of your post. /ubbthreads/images/graemlins/confused.gif


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## NewBie (Dec 29, 2004)

> *HarryN said:*
> Hi Newbie - Great info as usual. Also as usual, I am trying to really grasp some of the more subtle aspects of your data and analysis.
> 
> I always try to compare the data and value of an advancement to the most basic way to manage the current of an LED driver (a set of resistors at 2, 10, and 25 ohms in line with 2 x 123 cells.)
> ...


 
Well, if you mean 1140mA * 10% = 114mA for current dimming,
vs. a 10% PWM duty cycle with 1140mA pulses, yes, most definitely. If 47.5% of the power to the LED went up in the resistor, you'd be equal for efficiency, with more color tint drift. You'd probably pick up a tad of additional efficiency, since the I^2*R losses in the battery would be lower too.

If you have more than 47% of the power going up in the resistor, the PWM would be more efficient.

If you utilized a switcher that was over 53% efficient, you'd have less losses than a PWM, and most switchers are considerably more efficient than that, even the simple Zetex ones do better than that.

Even at 85% On duty-cycle and below, the NextGen converter that dat2zip did, and setting it for a lower output current level, would do better than a PWM curcuit, due to the efficieny losses emitter when you whack it with 1140mA using a PWM circuit.

So you are going to have to figure out your resistor value, and compute the power you have to burn up as heat, to know for sure.


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## NewBie (Dec 31, 2004)

I just calculated the PWM duty cycle dimming on paper, and the measured results are within 4.17%, excluding 0.1% and 0.05% dimming levels.


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## Doug S (Dec 31, 2004)

Nice work, Doug! I note that the emitter x-y chromacity coordinates fall outside the bin structure of the current Luxeon binning scheme. Do you know the bin of the emitter tested? BTW, On the Nichia datasheets for their high power white LEDs they provide some graphs related to those that you have done. They provide graphs of x-y chromacity as functions of temp and also of drive current.


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## McGizmo (Dec 31, 2004)

Are there any lumen maintenance considerations between a PWM dimmed LED (from a high level of current) as compared to a constant current set up that is driving at a lower current level, comparable in output to the dimmed PWM circuit? If heat is the primary antagonist to the lumen manitneance then I am assuming that the only difference would be due to the delta in efficiencies between the two systems. However is there additional degradation to the life of the LED as a function of the current level beyond the thermal issues?

EDIT: I recall PK stating that there were a number of reasons and merits for going with the various constant current levels on the U2 instead of going with PWM but he did not elaborate. Even though there are the short comings in color shift, SF chose to go with the variationin current level over a duty cycle. /ubbthreads/images/graemlins/icon3.gif /ubbthreads/images/graemlins/thinking.gif


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## Doug S (Dec 31, 2004)

[ QUOTE ]
*McGizmo said:*
However is there additional degradation to the life of the LED as a function of the current level beyond the thermal issues?


[/ QUOTE ]

Don, the Luxeon Application Brief AB25, Luxeon Reliability, seems to suggest so. It would be hard from this AB alone to say for sure whether CC or PWM would be preferred from a lumen maintenance standpoint but if the "all else equal" condition stipulates equal luminous output I would intuitively suspect that the CC method would be preferred.


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## andrewwynn (Dec 31, 2004)

Well. this definitely bodes well for my LDO driver on my upcoming variable nano (which dims down to zero).. i think that it only takes like 80 micro amps to drive my circuit, so when dialed down to low double-digits i should be able to get double-digit hours of runtime with the varinano.

I have a demo online: http://rouse.com/nano click on 'varinano' on the top.

-awr


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## NewBie (Dec 31, 2004)

> *Doug S said:*
> Nice work, Doug! I note that the emitter x-y chromacity coordinates fall outside the bin structure of the current Luxeon binning scheme. Do you know the bin of the emitter tested? BTW, On the Nichia datasheets for their high power white LEDs they provide some graphs related to those that you have done. They provide graphs of x-y chromacity as functions of temp and also of drive current.



I'm aware of the Nichia datasheet, haven't had a chance to look at it and compare. I don't know the bin, this BW01 is before hounding Future to give us the binning data on the emitters. It is a rather yellow emitter, fyi.

When you current dim, the wavelength of a blue LED shifts alot, see below:


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## NewBie (Jan 1, 2005)

FYI, the data there is for the 380mA range.


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## Kiessling (Jan 1, 2005)

This is one of the best threads I read for a while. As I cannot contribute in an adequate way here I simply say "Thanx" to those who actually did the work !
/ubbthreads/images/graemlins/thumbsup.gif /ubbthreads/images/graemlins/bowdown.gif
bernhard


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## NewBie (Jan 1, 2005)

Thanks for the kind words Kiessling.

Here is some more data for everyone.

This is what the blue LED underneath the phosphor is actually doing, as far as wavelength shift, note how it stays put during PWM dimming:









Here is what the spectral output is actually doing, notice how it gets less pure when current dimming:







To help visualize things, here is a visual spectral plot of the blue LED. Notice how the spectral plot for each is not symetrical on each side of the wavelength peak, and more is on the green side. Next, note how the peak has moved, with the different current levels. Finally, check out the width of each plot, and notice how it broadens out at low current levels.






In contrast, with PWM dimming, the dominant and the peak wavelength do not change, and when you dim it the spectrum actually narrows, reverse of what current dimming does. And it doesn't shift in wavelength left or right. See below:


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## McGizmo (Jan 1, 2005)

Well we can certainly see why Lumileds wants PWM dimming VS variable constant current systems! Facinating info Newbie! 

In the name of efficacy, I say bring on the CC and let the color shift be hanged! /ubbthreads/images/graemlins/grinser2.gif Surely it is poetic that the shift is towards the warmer levels for the more intimate settings. /ubbthreads/images/graemlins/nana.gif


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## NewBie (Jan 1, 2005)

McGizmo, you could do a combo of both and take advantages of each...

Anyhow, here is some more info in a different format:


This is the difference in the relative output when you PWM dim, shown in a spectral power plot:







Here is the difference of Current dimming in a spectral power plot:









Here below is the difference of the 0.05% power input levels for current dimming vs. PWM dimming. Take careful note of how with the current dimmed plot, how the blue peak shifted to a longer wavelength towards the red, and how the output of the phosphor (the big hump running from cyan through green to red), increased it's relative output (went higher) in relation to the blue peak at 100%.

Not every emitter has the same wavelength blue LED die in it (think of the luxeon lottery but in blue), and depending on where it's wavelength starts at, in relationship to the peak phophor conversion efficiency wavelength for the blue, emitters in the same batch and binning will do different things. 

I tried to hold the slug temperature as constant as possible. But in a flashlight, it usually isn't possible to do provide such extreme cooling. Additional changes begin to occur once you start changing the temperature.


I hope this has been educational and enlightening for the general CPF crowd.


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## Doug S (Jan 1, 2005)

Interesting data on the blue lux wavelength shift with current. If we take the 380mA as 100% and the 7.6mA as 2%, the shift from 2% to 100% is 5nm. Putting this into perspective, this is the width of a single color bin for the blue emitters. AB21 defines 6 bins for blue emitters, each 5nm wide. There is also a tempco associated with wavelength. The luxeon datasheets give this as 0.04nm/C for the blue emitters.


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## NewBie (Jan 1, 2005)

> *Doug S said:*
> Interesting data on the blue lux wavelength shift with current. If we take the 380mA as 100% and the 7.6mA as 2%, the shift from 2% to 100% is 5nm. Putting this into perspective, this is the width of a single color bin for the blue emitters. AB21 defines 6 bins for blue emitters, each 5nm wide. There is also a tempco associated with wavelength. The luxeon datasheets give this as 0.04nm/C for the blue emitters.



Yes, compare the shift for 100% and 2% for current dimming.

Next, compare the shift for 100% and 2% for PWM dimming.

Think about what is occuring during this time with heat in the die.

Remember, I held the slug itself within a 2.3C range over the 380mA test range. There is a 15 C/W thermal resistance for the 1W emitter to the slug. 3.6V * 380mA = 1.368 Watt
1.368 Watt * 15 C/W = 20.52 C rise in the emitter die. Add in the slug temperature rise of 2.3C (monitored with a K-type thermalcouple embedded in the slug) and we get to a 22.82 C rise.

22.82 C * 0.04 nm/C = 0.9128 nm shift. We are seeing alot more than this when Current dimming...and there is no peak or dominant wavelength shift when PWM dimming (in the blue).

In fact, the PWM blue dominant wavelength and peak wavelength stay put, and match the emitter running 100% duty cycle (on continously at 1140mA), even clear down to a pulse width of 3.67 millionths of a second.

; )


BTW, red emitters when PWMing do change their wavelength.


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## Canuke (Jan 3, 2005)

I always thought that PWM was less efficient, on the grounds that LED's are more efficient at lower currents... but it's cool to see the details.

Great post Newbie.


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## Darell (Jan 3, 2005)

Freaking amazing stuff, Newb!

And now we know what Don's been doing under the blanket with his flashlights all these years...


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## NewBie (Jan 3, 2005)

Canuke, 

There was an un-answered assumption that a PWM'd LED would be more efficient, since it doesn't have power applied all the time, it would be cooler, and thus more efficient. Looking at it from the other side, you are pulsing it, but you are hitting it full tilt, whacking it with full power, during which time it would be less efficient. With the data, it shows that the efficiency of PWM is quite poor, and in one common scenario shown above, showing that current dimming is 230% more efficient. This equates into 2.3x the runtime. Additionally, you loose efficiency in the battery, due to I^2*R losses in the battery. Some day, I hope to put together two circuits that emit the same amount of light, one a dim current circuit, the other a dim PWM circuit, and hook them to a rechargeable cell, and do a runtime comparison.

Again, there should be a Holy Grail combination of both approaches, current dimming and PWM dimming, that gets a person limited color shift, without paying the high efficiency penalty of PWM dimming at low light output levels.

Thanks Darell.

Anyone that hops in bed with several thousand flashlights has to be a cpf'er.


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## McGizmo (Jan 3, 2005)

Newbie,

Perhaps with the nanotubes you brought to our attention, the color shift will be a non issue. The future certainly may be bright and not too wasteful! /ubbthreads/images/graemlins/grinser2.gif


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## NewBie (Jan 4, 2005)

That and silicon nanocrystals to replace phosphor...

Looks like they are attacking it from multiple aspects.


Anyhow, I was testing some red LEDs today, and found out that their efficiency is barely any different from PWM dimming compared to Current dimming. Their color also shifts dang near the same with either method! 

I do notice that it appears the die is bonded directly to the slug. Which is different than a blue/green/white luxeon, where the die is floating on a few solder balls, which connect it to an ESD protection diode, which is then bonded to the slug. I'm guessing the thermal time constants between the red and B/G/White parts are different.

Anyone venture to guess about the other mechanisms that would cause the difference?


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## Canuke (Jan 5, 2005)

I'd wonder about capacitance or some other mechanism that "smooths" PWM enough to make it resemble CC dimming... but I speculate. I know nothing about parasitic capacitances in the different LED architectures.


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## cgpeanut (Jan 5, 2005)

This is awesome work guys thanks for posting, Let me read this thread again give me a chance to digest.


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## andrewwynn (Jan 5, 2005)

Well.. i already built my prototype dimming nano.. http://rouse.com/nano (look for varinano).. and i was definitely NOT unsatisfied by the color shift when i went down as low as 1mA (well.. it doesn't really output any light at that level.. it's just a cute green square).. in any event, once the light is turned up into double digit miliamps.. it might shift around a bit, but i was very happy to see that it's more efficient at lower power levels than PWM which i would definitely normally perfer.

I picked up a few sample TI chips that use PWM for higher power and FSM (frequency shift) for lower levels.. it's only designed to work from like 50-60mA to 400mA, but that's perfect for my little AAA light.. the beauty is that efficiency actually goes up at the lower power levels.. up to 95% at like 70mA.. i'm dying to get a circuit built into one of my prototypes.

Love the details that went into this test, even though i'll be using the more sloppy color-correct way to dim my lights, very nice to see the details. 

-awr


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## NewBie (Jan 6, 2005)

> *Canuke said:*
> I'd wonder about capacitance or some other mechanism that "smooths" PWM enough to make it resemble CC dimming... but I speculate. I know nothing about parasitic capacitances in the different LED architectures.




No, when driven properly, the capacitance doesn't cause a smoothing of the PWM. I've actually made sub microsecond pulses that have a nice square shape to the optical output.

There is only 800pf of capacitance, that changes as the forward voltage changes.

I actually exchanged some emails with Mike Krames of LumiLEDs Advanced Laboratories.

It has nothing to do with the thermal time constants or anything.

"Efficiency increases with decreasing current for InGaN (blue/green/white) LEDs, to reach a maximum at very low currents (1-2 mA or so for standard small chip used in 5 mm lamps). So, decreasing the dc current level gives you better efficiency, than pulsing at a high current. However, blue shift also depends on the drive current level, so the peak wavelength will change under "current dimming" but not under PWM. This color shift is not due to dimming but is related to some details in the InGaN/GaN electronic bandstructure.

For AlGaInP (red), the blue-shift effects are not present so all you get is red-shifting due to Joule heating, at ~ 0.16 nm/K (which apparently you are avoiding, which is good). Also, for red the efficiency is relative flat with current (it actually *decreases* at very low currents), so you don't observe the effects as with InGaN/GaN. The differences between AlGaInP and InGaN/GaN all relate to bandstructure properties and active region designs, which get into many details."


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## pokkuhlag (Jan 6, 2005)

Free Bump /ubbthreads/images/graemlins/smile.gif Thanks for the info.


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## NewBie (Jan 7, 2005)

I was referred to an article for further details:

On the Bandstructure in GaInN/GaN Heterostructures - Strain, Band Gap and Piezoelectric Effect

....The redshift approximately corresponds to one halfperiod of the Franz-Keldysh oscillation near the band gap. We therefore attribute this redshift to the Franz-Keldysh effect [23]. Spatially indirect transitions across the band gap appear at lower energy due to the tilting of the bands. At the same time the transition probability is reduced exponentially as the area under the triangular tunneling barrier grows without involving impurity or defect states (see schematic in Figure 4): 


(14) 



Within a halfperiod of the first Franz-Keldysh oscillation the transition probability has decayed to 4 % of its value at the DOS band gap [23]. In this framework initial and final states are states of delocalized carriers. The exponential "tail" is not induced by any disorder or impurities but rather by the tunneling process in the tilted bandstructure....


And a whole lot more here:

http://nsr.mij.mrs.org/3/31/text.html


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## 4sevens (Jul 2, 2005)

I still can't believe the staggering losses with PWM especially past 
50% HUGE amounts of energy is lost in the pulsing especially at
higher rates like the lionhearts and lioncubs. LED's are about
efficiency - turning energy into light and not heat. And to believe
the denzens of these LH and LC using LED's like "light bulbs" as
Bulk said in the past. What a waste of LED to use PWM.

Remember theres a HUGE efficiency penalty at higher currents and these
PWM units are always hitting the LED at 1.3A-1.6A !!


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## Doug Owen (Jul 7, 2005)

Gents,

Interesting thread. A couple of thoughts come to mind, first off CC regulators are not 100% efficient, typically far from it. And this is not being factored in. As a guess, this is probably more than the typical 'spread' shown for reasonable current levels above. This means that for most applications, PWM is more efficient. FWIW there's a good reason this technique is so popular.....

Secondly, 4sevens is spot on (again). Part of the potential loss issue is the 'bigger is better' view many have. If you severly overdrive (so 100% is really bright), you automatically impact the efficiency at 'useful levels'. If, however, you keep 'full blast' to reasonable levels, it's gonna be real hard to beat a well designed PWM system for efficiency.

Doug Owen


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## NewBie (Jul 7, 2005)

Reasonable drive is key.

Notice the 253% increase in efficiency of the LED(in current dimming) at 3% PWM vs. 3% Constant Current, when the PWM is hitting the led with 1.140 Amps.

Thats less than a 50% efficiency level there.

It gets worse at higher PWM currents. Go back and compare it to the 350mA PWM chart.

If you go higher for current, with the PWM, the LED efficiency continues to drop when dimmed.

Yes, a well designed PWM, that takes in consideration of the LED, converter/PWM, and the battery.

Lately, Constant Current regulators in flashlights have made leaps and bounds, with efficiencies exceeding 90%.

Another consideration is regulation. Except on high, PWM can also regulate the light by adjusting the duty cycle as the battery depletes, but that isn't done on the mainstream lights which use PWM, like the Photons, LionHeart, LionCub. As commonly implemented it is more like direct drive. Here is an example of the unregulated runtime output on one of the common PWM lights:






Exactly. The whole light needs to be considered for the best solution, not just the converter/PWM circuit alone. Heat generated, thermal paths, temperature, reflector, front lens, battery contacts, converter/PWM, LED, battery, load pulses on the battery, etc.


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## PeterB (Jul 7, 2005)

[ QUOTE ]
*Doug Owen said:*
If, however, you keep 'full blast' to reasonable levels, it's gonna be real hard to beat a well designed PWM system for efficiency.



[/ QUOTE ]
I've a different opinion regarding this statement. Even a linear regulation or a poti in series with the LED is more efficient than PWM (lumen out vs. current draw). 
And how do you reduce the full blast? By a series resistance?


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## NewBie (Jul 7, 2005)

Doug,

Some of the stuff a number of us talked about awhile ago is applicable to some of the points you brought up. It is a long read, but worth it, imho.
http://www.candlepowerforums.com/ubbthreads/showflat.php?Cat=&Board=UBB6&Number=600212


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## Doug Owen (Jul 8, 2005)

[ QUOTE ]
*NewBie said:*
Doug,

Some of the stuff a number of us talked about awhile ago is applicable to some of the points you brought up. It is a long read, but worth it, imho.
http://www.candlepowerforums.com/ubbthreads/showflat.php?Cat=&Board=UBB6&Number=600212 

[/ QUOTE ]

Yup, I recall it. FWIW, I was the 'Doug Owen' taking part....

Thanks for 'the trip down memory lane'.

Doug Owen


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## NewBie (Jul 9, 2005)

No problem Doug.

Thanks for the contribution.


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## cy (Jul 15, 2005)

an excellent old thread!

In the real world I find PWM VS Constant Current regulation only matters if you are limited to battery choices. 

if you are limited to or just prefer primary cells. then constant current regulation would be prefered. 

with availibity of cheap hi-cap li-ion cells. like many on cpf, I'm using li-ion as first choice in almost all my lights. 

effiency with li-ion/PWM matters little when you are running basically for free. 

with PWM you are esentially running at direct drive on high. VF matters very much as that's what determines current going across luxeon. 

with PWM first choice is Kbin (vf closely matches li-ion's voltage under load) which limits current to aprox. 1.3 amps. factoring in further VF shift downwards to Jbin, which will further increase current at lux to aprox. 1.5amps. Hbin will produce higher yet current draws of aprox. 1.75 amps. which is undesirable. 

with CC regulation inverse is true. lower VF the better to increase runtimes from primary cells. 

two different sets of requirements are emerging in PWM VS CC reguation camps.


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## NewBie (Jul 15, 2005)

Thats not entirely true. 

With a buck-boost, you can still run CC (constant current) off a single Li-Ion.

With a buck, you can run off two or more Li-Ion.

Efficiency is still essential, as it determines your runtime, and many would prefer not to replace cells in the field. On top of higher efficiencies when dimmed with CC for the LED, you also get higher efficiency out of the battery with CC. With CC, since you get more light out of the LED for the same amount of power when dimmed, you need less power to make a given amount of light, which means the runtime is increased. Now add on the higher efficiency of the battery with CC when dimmed, lower power draw, less heat, and things really start adding up fast.

And with the lower Vf LEDs, since power = Voltage 
* Current, you are consuming less power for a given amount of light, yet again.

PWM for LEDs came out before 1975, when watch and calculators makers started PWMing to reduce power consumption, as at the time, the only other technique was resistor/linear regulation, which was very wasteful. At that time, switching power supply technology had not came of age for small portable items. Thats all changed now, most especially with cell phones over the past few years, greatly extending runtimes, laptops, MP3 players, portable games, GPS systems, etc.


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## cy (Jul 15, 2005)

back to the real world again,

most usage of lights, PMW included if given a choice is at low levels. with high only used selectively. 

this is especially true with todays overachieving luxeons, where high beams is esentially too bright for most closeup activities. 

Just got back from a two week backpacking trip, where Ti lioncub was my primary light. Used that light constantly everynight without worrying about conserving the R123 li-ion cell. first R123 cell lasted for 12 days under extreme use conditions. naturally I changed to my backup cell. 

My backup light hanging on my neck was Li14430/CR2 also li-ion cell, which made it thru the trip with original cell. 

tent light was ARC AAA 2003 cpf edition which made trip with original cell. 

yes a buck/boost can use a single li-ion cell, but is not very good at low level modes. besides most of my regulated lights are CC not buck/boost. I've purposly not purchased any wizard equiped lights due to reports of low level issues. which is of course 98+% of my light usage. 

Wayne's new two stage CC board looks real promising. I'm looking forward to get one of these. don't have the details yet, but if it's CC only. it too would be running at essentially DD if VIN exceeds VF with li-ion.


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## cy (Jul 15, 2005)

newbie, forgot to add, excellent point about PWM being used in early LED aplications. 

here's an example of early LED's controlled by PWM, introduced in 1974 by National Semiconductor. Mathbox 650 a simple RPN calculator pictured with Ti Cub and Li14430/CR2


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## NewBie (Jul 19, 2005)

Yes, for those who have been around long enough, they will remember the simple mod for many of these, adding a speaker in series with the battery, which causes the speaker to emit lots of neat beeps and buzzes as you operate it.


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## NewBie (Feb 8, 2006)

CY,

Did you ever try putting a speaker in series with the batteries on that one?


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## shiftd (Sep 21, 2006)

wow, totally missed the thread development. 

plenty of useful info, need to catch up on the reading .

Never liked PWM, but i guess it has its own application


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## cy (Sep 21, 2006)

WOW... a blast from the past! 

certainly one of Newbie's best contribution to cpf and always worth another read.


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## NewBie (Sep 22, 2006)

Thanks for the comments.

I had a lot of fun doing it!

BTW, did you ever try the speaker thing I mentioned before?


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## NewBie (Sep 23, 2006)

I was looking thru some old pictures and realized I never posted this one:


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## jsr (Dec 13, 2006)

Sorry for my lack of understanding here, but I'm wondering how everyone's distinguishing PWM from Constant Current. Here's my understanding...

A CC power supply varies the output voltage via a sense resistor to keep the current (the voltage at the sense resistor) the same. The way to vary the output voltage is via a PWM (or PFM) signal that adjusts the on-time (and thus average voltage and average current) at the output. The output is then fed through a filter (cap typically, and the inductor in a buck or boost (or sepic) topology acts as a filter also) so the output is not on or off, but a rippled voltage. That's the only method I know of on how to make a CC PS (by using PWM or PFM). Any dimming would just be a change in the feedback signal, reference voltage, or pulse width (or a combo thereof).

Unless there's another way to control the reduction of voltage/current, I don't see how PWM and CC is being distinguished. And I also don't see how people could be seeing flicker from PWM/SMPSs unless the output's not being filtered at all (but that's just a sloppy design if there's no output filter, unless space is really so constrained, but even a small cap would reduce the ripple to better than just on and off).

TIA for the education.


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## evan9162 (Dec 13, 2006)

The PWM we're describing does not have anything on the output to smooth the signal - so no cap, inductor, etc. Each pulse is a full current pulse to the LED, followed by some off time where the LED emits no light at all.

Adding a cap/inductor and some feedback gives you a switching DC-DC converter which gives you constant current on the output.


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## CM (Dec 13, 2006)

THAT is great data!

I remember a patent that was granted for using PWM for tint management in dimming LED's. Can't seem to find the patent now :thinking:


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## jsr (Dec 14, 2006)

evan - thanks for the clarification. All PS designs I've ever seen have output filters (not for flashlights tho). I'm surprised filters aren't used for flashlight drivers...the cost of a cap is minimal.


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## Kinnza (Dec 14, 2006)

This is a great thread. Thanks for bumping it 

But i think a basic point is missed in all this analysis. Of course, PWM is less efficient than CC dimming talking in radiometric units.

But it not necessarily mean it is less efficient in the whole visual effect.

Think you are in front of the flashlight, and at 100% power, you receive and perceive 50 cd. With CC dimming at 50%, you receive and perceive, say for example, 30cd (due to increase efficiency).

But, if you dim at 50% by PWM and the frecuency is enough high, you receive in average, 25cd (using the corresponding 100% power current in the light pulse). But you still notice 50cd of light, with flickering (depending of the frecuency used). This is why PWM was so widely used in the past, specialy with red leds, wich increase light output in a near linear way with increasing current, so the efficiency loss is only due to higher voltage applied during the pulse.

So a well designed PWM may be more efficient, in terms of human perception, than CC dimming.

Hope you understand what i want to mean with my reduced english.


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## evan9162 (Dec 14, 2006)

> But, if you dim at 50% by PWM and the frecuency is enough high, you receive in average, 25cd (using the corresponding 100% power current in the light pulse). But you still notice 50cd of light, with flickering (depending of the frecuency used).



No, this is not true. You still percieve 25cd. Above a certian frequency your eyes integrate the total output into a non flickering beam equal in brightness to the average of the pulses and off times. Everyone keeps saying that your eye see the maximum brightness of the pulses, but it is simply not true.

Think about this - if at lower duty cycles, PWM still appeared as bright as a single pulse, *just how would PWM achieve dimming at all?* Ponder that question and you will realize that your eyes average everything out.


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## Kinnza (Dec 14, 2006)

Good reasoning, *evan9162,* you convinced me.

As you said, everybody says it, and its wrong. I read it so many times i believed it without questioning.

Its possible its one of the false things with, due to repetition, sounds true.

Or maybe it depends of the frecuency used?

(im going to research in this topic a bit more, as im not sure human eye averages out all, in my understanding, eye response isnt always linear)​


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## NewBie (Dec 19, 2006)

The effect of CC dimming is even more dramatic if you go on down towards 3% dimming, where you get 240% more light, if the LED I used was driven at 1140mA (data from that example on page 1).

So, if we assume you had 50 candelas at 1140mA
-PWM would give you 1.5 candelas
-CC would give you 3.6 candelas

For the same power consumed.

Or another way to do it is to dim both down to 1.5 candelas, and consume 58% less power.


Another piece of the puzzle is the effect on battery efficiencies when you take a full blown current pulse out of the battery (PWM), vs. sipping ever so lightly on the cell. The battery will deliver quite a bit more energy if you just pull a light current off of it, instead of yanking a full 1140mA pulse out of it.


Now, a typical switcher starts getting less efficient down at those levels. Most chips these days have burst modes/pulse skipping, which will greatly increase the efficiencies, and if you have a proper sized capacitor, you can keep the ripple down at 0.8%. Another technique is to use two phase switchers, and just switch off one phase. If you think a little deeper, you can put heavy MOSFETs (higher gate charge, lower on resistance) on one phase, for the higher currents, and put light MOSFETs (low gate charge, higher on resistance), and keep the efficiency of the switcher up. You can also lower the frequency, to reduce losses due to gate charge and gate drivers at light loads. Often you will save additional energy on the very light load side, if you replace the one of the MOSFETs (depends on buck or boost), with a schottky diode.

As you start going up in PWM frequencies, you start taking additional losses due to the 700-800pf of capacitance in the LED. Adding just a capacitor on the output of the PWM, without an inductor, will greatly increase the losses in the transistor.

CM-

The paper on the PWM thing was done by Henry of HDS, where he used PFM to get around the PWM LED flashlight dimming patent.


Another technique is to dim with CC, on the upper end, where efficiency is important (high power draws), and then switch to PWM/PFM of the CC mode, where you convert down to lower the current, but then pulse that.


One note about dimming of LEDs, is that if you use something like an X1/WF bin, the green tint will become pretty obvious and re-inforced at the lower currents. Of course, this also depends on where in the X1/WF bin it is actually at- within the X1/WF bin.

Both PWM and CC have tint shift issues when dimming, and there is a chart on page one demonstrating the effect. PWM has a bit less shift, but notice how when CC dimming, it started shifting back towards the cyan direction at the very low levels.


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## Kinnza (May 12, 2008)

I did some research about the possible enhancement of photometric effect using PWM, but i was unable to find any solid info at the time this thread was active.

But today ive read this article:

"A research group at Ehime University developed a pulse drive control method to make LEDs look twice as bright by leveraging the properties of how people perceive brightness. The group was led by Masafumi Jinno, an associate professor of Dept of Electrical and Electronic Engineering at Graduate School of Science and Engineering of Ehime University. 
When a short-cycle pulse voltage with a frequency of approximately 60Hz is applied to an LED at a duty ratio of about 5%, the LED looks about twice brighter to human eyes than that driven by a direct voltage, the research group said. 
Based on an evaluation test using subjects, the group reported that a blue LED looks 1.5-1.9 times brighter while green and red LEDs look 2.0-2.2 and 1.0-1.3 times brighter, respectively. 
"With this method, the brightness of LED with a luminance efficiency of 100lm/W can be simulated by using a 50lm/W LED," Jinno said. 
The test result was unveiled at the "New Technology Presentation Meetings by Four Universities in Shikoku Region" sponsored by Japan Science and Technology Agency (JST). 
There are two principles, the Broca-Sulzer effect and the Talbot-Plateau effect, involved in how human eyes perceive brightness. The Broca-Sulzer effect refers to a phenomenon in which light looks several times brighter to the eye than it actually is when exposed to a spark of light, such as a camera flash. 
In addition, the Talbot-Plateau effect is a principle where human eyes repeatedly see flashes and sense the average brightness of the repeated lights. Thus far, "it has been believed that, due to the Talbot-Plateau effect, the brightness perceived by human eyes would not change even if an LED is pulse driven," Jinno said. 
"The Talbot-Plateau effect is a principle found in the days when fluorescent mercury lamps and other light sources driven by a power supply with a longer voltage cycle of about several hundred milliseconds were used," Jinno said. 
Thus, the group decided to drive the LEDs using a power supply with a shorter voltage cycle of about several hundred microseconds. As a result, the group discovered that, when a pulse voltage with a frequency of approximately 60Hz is applied at a duty ratio of about 5%, the impact by the Broca-Sulzer effect becomes greater than that of the Talbot-Plateau effect so that the light emitted from the LED looks brighter to human eyes. 
The LEDs in three colors used in the evaluation test were all manufactured by Nichia Corp. The model number of the 464nm blue LED is NSPB500S, the number of the 520nm green LED is NSPG510S, and that of the 633nm red LED is NSPR510CS."



So finally, the enhancement of photometric effect has been proved. Human eye is an imperfect integrator of light. From the article, i take the conclusion the effect is more noticiable when using short pulses, below 1ms.



The problem then deciding what is more efficient photometrically, if PWM or CC, depend on if the enhancement is higher than the radiometric losses, wich have been well caracterized along the thread (we miss you, Newbie ). As the article report different enhancements depending of the wavelenghts tested, i think its going very difficult to do accurate maths about it, almost impossible.



It would be required to do a function wich takes into account frecuency, pulses duration, lux level of the pulse (perfectly integrated) and wavelenght. Too difficult, IMO.



Ive found too a graph wich explain brightness enhancement perception for different lux levels and pulses duration:









It seems 60 ms is the limit of pulses duration to get noticiable effects and that the effect is way higher as higher is the flux level. And from the previous article, that human eye integrates better as longer the wavelenght (so strong bluish spectrum benefit more than reddish).


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## MrAl (May 12, 2008)

Hi there,


First off, this is interesting. It was known that the eye integrates light but
it was always suspected that there may be some combination of pulse and
frequency that could upset this characteristic. Apparently, if what they say
is true, we have something new now.

There are some problems however.
The first one i notice is that 5 percent is a pretty low duty cycle.
The second one is that the effect is different for different wavelengths.

The difference with wavelenghts is going to cause color shift, which will
shift white into the sky-blue color. This isnt good, unless the white can
be first shifted into pink, but that would require different LEDs than
the white that we have now. I guess it's doable though.

The duty cycle problem is a little harder to get around. With a 5% duty
cycle and a pulse 20 times as high as normal, there will be great efficiency
problems that will reduce the light anyway, so even if there is a twofold
increase at low currents that doesnt mean the same at high currents.
Also, a pulse 20 times as high as normal means the manufacturers specs
will be exceeded for max pulse amplitude.
Ok, so say we drive it with a pulse 5 times as high as normal to stay within
(some LED) specs. At 5 percent duty cycle that means we're down to 
1/20th of the normal light output, then increased it my 5 times times the
eff factor, increased by the magic factor (of the article). This leads to
a rough equation like this:
E2=E1/20*5*e5*m
where
E1 is the light output of a white Nichia at 20ma,
E2 is the new light output
e5 is the efficiency decrease factor for driving at 5x nominal current
m is the magic factor, a vector really.

To get an idea in percent, we can set E1=1.
e5 would be around 0.75
m is 2 for blue and green, and 1 for red (approximately)

This gives us:
E2=1/4*e5*(2+2+1)/3
so
E2=1/4*0.75*5/3
so the result is:
E2=0.3125
which means the max light output is 31 percent of what an LED is when
run at nominal 20ma, and the color is sky blue.

Now if we tint the LED plastic we also reduce light output, so that's a
problem too. Thus, we would end up with white light, but even less
than 31 percent of the full brightness.

The average current, however, would only be 5ma. This is 1/4 of the
nominal current. The new efficiency over the old eff could be expressed
as:
Eff2=Eff1*4*0.31
so after normalizing the old efficiency to 100 percent (1) we get:
Eff2=1*4*0.31
so
Eff2=1.24, which means 124 percent.
Thus, we have gotten a little more light out of it, at that light level.

The final analysis:

The max light level is 31 percent of normal brightness (about 1/3 of normal).
The color is tinted sky blue.
The efficiency is better, meaning about 20 percent less energy is expended
in lighting the LED, at 1/3 brightness.
The small Nichia LEDs could be driven like this, but the larger high powered ones
are not rated for over 2x nominal current so technically speaking, they cant be
powered in this way.
The LED can not be lit at full brightness without going way over manufacturers specs,
and this may not even be possible due to serious efficiency issues at *much* higher
peak currents.


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## uk_caver (May 12, 2008)

Wouldn't a light with a pulse frequency of 60Hz actually be potentially flickery, especially for low-level lighting where some dark/peripheral vision could be expected to be operating?

In the study, I wonder if the people were looking _at_ the LEDs, or using the LEDs to actually light up a scene.
Personally, looking at coloured 5mm LEDs, I've found it very hard to make meaningful brightness comparisons. Having identical LEDs side-by side running at significantly different currents, apparent differences (if they exist at all) are much smaller than the differences in drive currents. The LED is the brightest thing around of its colour, and the end result is that it seems to practically saturate a spot on the retina +/or brain.

I'd wonder if pulse vs. constant brightness results from looking at (relatively very bright) LEDs would differ from looking at (relatively much darker) scenes lit by LEDs?


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## MrAl (May 12, 2008)

Hi again,

In addition to my rough analysis above i'd like to add the following...

60Hz is pretty fast. It's not super fast, but it's usually fast enough to fool
the human eye, or at least mine (chuckle) because i have built white LED
lights that run from half wave rectified waves and they seem ok to me
(although i prefer 120Hz for other reasons). If you turn your head fast though
you can see the pulsing, so i guess it depend on how you are using the light,
if it will work for you or not. Even displays are sometimes run at 50Hz,
and some monitors operate the vertical sweep at 60Hz, although i dont like
them because you do notice some slight blink effect of some kind. But then
i dont know the phosphor decay properties anyway.
I'd also be very surprised if they were looking right at the LED, as that would
not do much for anybody, although i dont know for sure.


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## Kinnza (May 13, 2008)

Very good point, MrAl. I think you nailed up the main problem: as the higher enhancement happen as higher is the light pulse (higher the surge current), the highest, and probably, the most noticiable effect, when using short duty cycles and huge surge currents.

But high power leds have a low marging respect to max pulsed current, usually up to 2A. For a 350mA rated led, its about 6 times higher than the nominal continous current. Meaning we can use a 20% duty cycle as shorter. And when using such high pulse currents, the radiometric losses are way higher, too.

Its not a problem for low current leds, though. And maybe further improvements on high power leds may do viable very high pulse currents. Im thinking on increasing die sizes specifically. The way the Osram choosed with the Diamon Dragon, wich use a 2 sq mm chips (double than actual high power dies), wich can by runned up to 4,5A (manufacturer rating, we know very often we can surpase it). This first device using a 2 sq mm chip havent a top end efficiency at all, but having doubled area, doubling current only increase current density at the die level by half, wich mean half the radiometric losses, given a good cooling (the DD has only 2,5 K/W of thermal resistance due the large die).

So probably, in the practice currently is going to be difficult to take advantage of the Broca-Sulzer effect, but its an effect we must keep in mind when deciding between PWM and CC for an application.

Of course, any application where flickering are a problem is a candidate for CC drive. But there are many applications where flickering isnt a relevant issue.

Personally, i hate flickering, gives me headache. I need 100hz frecuency to avoid it.



uk_caver said:


> In the study, I wonder if the people were looking _at_ the LEDs, or using the LEDs to actually light up a scene.
> Personally, looking at coloured 5mm LEDs, I've found it very hard to make meaningful brightness comparisons. Having identical LEDs side-by side running at significantly different currents, apparent differences (if they exist at all) are much smaller than the differences in drive currents. The LED is the brightest thing around of its colour, and the end result is that it seems to practically saturate a spot on the retina +/or brain.



I wondered the same when read it. Hopefully, somebody will get the original study and could clarify it.

Think than 660nm leds have a LER (lm produced by 1 optical watt) below 60 lm/w, and mostly, actual efficiencies around 20 lm/w (input watts), so they cant produce high lux levels, so the Broca-Sulzer effect is almost unnoticiable.

But orange-red leds have LERs in excess of 200lm/w, wich arnt so far from white leds, mostly below 300lm/w actually. The effect should be still noticiable when using them.

All i wanted when i posted the article is add it to this excelent reference thread, so people wanting to decide between PWM and CC drive keep in mind all the factors wich may affect its relative perfomance.

And be aware that luxometers dont measure the Broca-Sulzer effect at all!


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## uk_caver (May 13, 2008)

The LEDs in the study weren't exactly high-power - green and red were somewhere in the 4000-8000 mcd range at 20mA, with a peak drive current of 200mA for the red and 100mA for the green, so at a 5% duty cycle, the red would be at roughly half the 20mA brightness, and the green at a quarter.

At those light outputs, to actually light up any kind of target, you'd think it'd have to be a close target in a room with subdued lighting.


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## MrAl (May 14, 2008)

Hello again,



For a comparative light meter that i think will work
even with the Broca-Sulzer effect see this page:

http://hometown.aol.com/xaxo/page1.html

Kinnza:
You bring up some good points...
First, many people dont want any flicker at all and i think
the amount of flicker noticed varies by person...for example,
i like at least 75Hz myself.
Second, i dont know of any light meters that take the B-S
effect into account either, and we dont know how this
varies from person to person either, or if it varies
with age even. What if people over 60 years old dont
notice anything? I wonder about if they took that into
account with the study.


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## VidPro (May 15, 2008)

uk_caver said:


> The LEDs in the study weren't exactly high-power - green and red were somewhere in the 4000-8000 mcd range at 20mA, with a peak drive current of 200mA for the red and 100mA for the green, so at a 5% duty cycle, the red would be at roughly half the 20mA brightness, and the green at a quarter.
> 
> At those light outputs, to actually light up any kind of target, you'd think it'd have to be a close target in a room with subdued lighting.


 
What Caver said twice now, the study doesnt mean much to OUR purposes, cause 60Htz is horrible light, 
and Green leds have no Phosphors, so there is no phosphor persistance used in the equasion, or phosphor averaging occuring. so ya phosphor based LEDs like white and quantum dots would be different again.

besides if they start using that on signal lights, and set off some epeleptic into a fit, that aint good. heck 60htz leds already give me a fit


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## 2xTrinity (May 15, 2008)

VidPro said:


> What Caver said twice now, the study doesnt mean much to OUR purposes, cause 60Htz is horrible light,
> and Green leds have no Phosphors, so there is no phosphor persistance used in the equasion, or phosphor averaging occuring. so ya phosphor based LEDs like white and quantum dots would be different again


At 60hz pulses, phosphor persistence is utterly irrelevant. I was able to look at my LF2x (7.8kHz, as low as 0.2% duty cycle) output using a photodiode and oscilliscope , and did not see any trailing edge due to phosphor persistence. And on the lowest setting the pulses are only 250 nanoseconds.

The biggest problem with this effect though is that it only works at 60Hz, 5% duty cycle, and probably really low illuminance. I couldn't tolerate my Fenix L0D at 100Hz/15% duty cycle. A shorter duty cycle and lower frequency would be horrible.

I noticed they were using pretty weak LEDs. They probably also had all other lights off -- whcih would imply extremely low illuminance. I highly doubt it would work the same way as illuminances high enough for general purpose room lighting, or even outputs on the order of existing high power LEDs, where driving at duty cycles like that would be less efficient than simply CC at low current due to power supply reasons, anyway.


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## VidPro (May 15, 2008)

2xTrinity said:


> At 60hz pulses, phosphor persistence is utterly irrelevant. I was able to look at my LF2x (7.8kHz, as low as 0.2% duty cycle) output using a photodiode and oscilliscope , and did not see any trailing edge due to phosphor persistence. And on the lowest setting the pulses are only 250 nanoseconds.
> 
> .



right, even phosphor based leds have very fast responce times, but testing without phosphor based leds, and forming a conclusion FOR phosphor based leds , is how we got other totally eronious information and specs and data.


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## MrAl (May 15, 2008)

Hi again,

Some very good points being brought up in this thread.
It's starting to look like the applications, at least for now, will be limited
to either very special purpose (cockpit illumination) or simply indicator lamps.
It may also help in emergency situations where there is no other lighting available
and the available power source (battery) is a very valued resource, such as in
emergency caving.


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