Car LED tail lights strobe rate too slow

Hello all, On subject of Led's and strobes, I intend to build a high brightness white Led strobe, but due to the required duty cycle of 20 uS, the white Led wont do because of 'phosphor persistence' . Unless someone knows of a super bright white that doesnt have this problem, i'll have to use banks of the brightest Red, green and blue. I have some questions on which transistor would be best suited to drive strings/bunches of Leds at up to an amp, at

1khz, 20uS duty at 12vdc. With the low duty cycle I should be able to whack 200mA through a single Led (correct me) but at high freq. things would get hot. My strobe will mainly be used between 0-100hz. So whats the best choice of Led's and Tranny driver?

Thanks for any help, Mark Kelepouris

There are currently two and a half white LED basic technologies on the market!. Blue with phosphor, is the most common, then RGB, has multiple LED's and produces 'white', by mixing the colours. There are also some new 'variants' of the phosphor technology, with slightly different characteristics. Get rid of the phosphor based units, and try units like:

or for higher power: Several of the ultra high output white LEDs from other manufacturers also use this type of technology. Talk to an LED supplier, about RGB ultra bright LEDs. They should be able to help.

Best Wishes

There may be some flicker or strobing present, I'm not sure, but I suspect most of this is aliasing.

This phenomenon also exists on LED displays run on pure DC.

When you move your head, or chew potato chips, or play very low notes on a bass trombone your eyeballs jiggle. Each time the eyes move you lose vision for a small amount of time. This has the effect of chopping the light into flashes even though the light may be steady. It is not often noticable, but at night when the light source is small it can be.

That's my theory, anyway. I'd go play my trombone now but it's all full of potato chips.

Really? I did a project a few years back using PWM for exactly that purpose. Indeed the LED manufacturers data sheet included precise specifications for such use as well.

Pete C.

I would agree there might be specific applications for using PWM to boost output, but for automotive tail lamps it didn't seem to benefit. The LEDs used in the stop/tail lamps were predominantly the lumileds snapled 75 and snapled 150's early on and they have since moved towards the luxeons and K2's. Also used in stop/tail lamps were the Osram power topleds. The snapled 150's usually had to be driven near their maximum rated current (150 ma) to get the desired output for stop mode. The composition of the red and yellow dies made it so that the light output fall off due to junction temperture rise is really quite bad. The LED loses about 60% of it's light output at the rated junction temperature of 25=B0C when you drive it at any reasonable current. So pulsing didn't seem to get any output back because the duty cycle had to be so long, that there was still significant die heating.

Which LED's were you using the PWM to boost output with? I haven't had a chance to try that.

Which LED? What was the pulse rate?

--
Vic Roberts
http://www.RobertsResearchInc.com
To reply via e-mail: 
replace xxx with vdr in the Reply to: address
or use e-mail address listed at the Web site.

This information is provided for educational purposes only. 
It may not be used in any publication or posted on any Web 
site without written permission.

State your source. Note that this thread has been linked to sci.engr.lighting. We lighting experts know what you're saying is incorrect. If all you're doing is throwing back information that LED sales people have given you, then you and they are mistaken.

By what meter do you judge this? I see all manner of lights from continuous sources to PWMs at 20 kHz or more. Once the pulse rate increases beyond 5 Hz, it becomes very difficult to estimate a flash rate by eye alone. I can't believe you're taking a light detector hooked to an oscilloscope to determine a PWM pulse rate of an LED lamp on a truck.

The point is that the eye is an integrator - it sees only the average light output of sources pulsing at the frequencies we're discussing. Same principle that allows people to see an image on the TV. Your comment regarding strobe tubes incinerating is totally irrelevant to the subject at hand. We're trying to educate you - you're trying to bully your point through.

Peak intensity is only relevant for slow flash rates - below the '20 Hz' you stated above. Warning lamps flash in the 1 - 4 Hz range where the peak intensity does increase conspicuity (not visibility as is often misused in this case as you are doing). Once all those flashes merge into a continuous beam, conspicuity decreases.

--
Douglas Cummins
Calcoast - ITL

Drive higher = brighter = more visible

I've spent quite a bit of time in the film / video world so I'm a bit sensitized to things in the 24-25-29.97 Hz range.

The phosphors in the CRT are what does the integrating for the most part, or in newer technologies it's the drive logic that is holding the pixel data.

Care to explain exactly what's wrong with higher drive = higher output = brighter / more visible instead of claiming I'm wrong with nothing to back that claim?

Peak intensity translates to visibility and low flash rates translate to conspicuity. If it's brighter you can see it from further away and if it's flashing slow enough to see the flash rate it grabs your attention. They are not the same thing

Pete C.

I don't recall which ones, it was a while back and someone else's project I worked on. It was pre-Luxeon days as well.

Pete C.

I don't recall, it was a while back and someone else's project I worked on. It was pre-Luxeon days as well or we probably wouldn't have had to bother.

Pete C.

You must publish your results of your project 'from a few years back' ASAP since your more modern work appears to contradict the body understanding of held for over half a century on the human optical responses and it is important that the truth of the matter be revealed for the benefit of mankind.

It has long been suspected that the huge wealth of evidence corroborating currently advocated principles of vision is totally wrong and is, in fact, a conspiracy by every single one of those (other than yourself) that have studied such phenomena in depth.

Of course it is ackowledged that waggling a leds supply around at speed improves all led efficacies - the faster the better - as well as improving perception.

The power companies (amongst others) suppress this information to try to sell more electric.

"Douglas G. Cummins" wrote in message news:sRTog.10$ snipped-for-privacy@newsread2.news.pas.earthlink.net...

My favourite stroboscopic effect was when motorcycle shops used to sell those novelty neon spark plug caps, if the bike goes by at a reasonably good speed persistence of vision makes the flashes in the plug caps appear as a line of red dashes. There's quite a difference between the flicker rate of a fixed lighting installation and one mounted on a moving vehicle.

Thanks Douglas. Your explanation is better than mine.

--
Vic Roberts
http://www.RobertsResearchInc.com
To reply via e-mail: 
replace xxx with vdr in the Reply to: address
or use e-mail address listed at the Web site.

This information is provided for educational purposes only. 
It may not be used in any publication or posted on any Web 
site without written permission.

No, I do not see this with incandescent lights and I do not see it when the LED tail lights are on full bright when the brakes are on. The effect is seen only on the LED tail-lights when they are being dimmed (brakes off but tail light on) using a slow PWM.

Mark

newer cars

has

Are you claiming that with pulse rate higher than the flicker fusion frequency, the visibility is affected by the peak even when the average is the same and the lamp appears to be glowing steadily?

That part I agree with, except the pulse frequency for a lamp visibly strobing only when moving being that low. I have played around with these things enough, and it takes about 50 Hz or a little more for a pulsing lamp to appear non-flickering. Sometimes a lamp will visibly flicker at 60 Hz even if it is not moving.

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

Not true - ever roll your eyes while looking at a CRT monitor, a color TV or a B&W TV?

Then why do LED tail/brake lights in "brake" mode and LED traffic lights either operate continuously or with peak not a whole lot above the average?

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

See if it is actually that bad. The amount of energy the phosphor over an LED chip stores is so low and it emits at such a high rate (when the LED is glowing at least) that I suspect it won't glow much for more than a few microseconds after the LED turns off.

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

It's (or was back then) common practice with the digital displays of the

1980's, since the LEDs there were often run with just a couple milliamps per die, and the particular dice in most of those had a nonlinearity where they were much more efficient at 40 or 50 mA than at 2 mA or whatever.

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

Plenty of times. As I said "for the most part". Phosphor persistence has a lot to do with it. And of course now the pixel data is latched anyway on the LCDs and whatnot.

Dunno, I don't have any on hand to test or verify.

Pete C.