PWM is used in order to exceed the maximum continuous output available from the LED. LEDs can be overdriven past their continuous operating currents and generate higher peak light outputs (within some limits) by operating them at a reduced duty cycle to hold the average current and therefore chip heating within the continuous operating limits.
Pete C.
I've never experienced that. Do you wear glasses or contacts?
I'n not sure there is a problem to be corrected.
I just hope I don't wreck on the way home tonight. Now I have to look at the Led's more than I'm allready doing.
I just wished they would mandate being able to see both left and right front turn signals on vehicles like they used to. Putting them on the side mirrors in front, would solve that problem. When I see a stopped vehicle and don't see their turn signals, I don't give them any slack. To me they are there and don't know what their doing, but I have to still watch out in case they shoot out in front of me.
greg
What you describe works only if the LEDs are being flashed slower than the flicker fusion frequency, which is not the case here. And even then, only the pulse output is higher than the maximum continuous rated output. The average output is not.
These LEDs are being pulsed fast enough that the eye-brain system sees the average light output while the LED chip temperature responds to the average power. I don't see how there is any increase in available output due to pulsing.
The strobe effect that people are complaining about could be eliminated by filtering the power delivered to the LED after the PWM control circuit.
-- 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.
I have seen a lot that were in the 15Hz to 20Hz range, including the new Cadillacs.
--scott
-- "C\'est un Nagra. C\'est suisse, et tres, tres precis."
In message , snipped-for-privacy@yahoo.com writes
Agreed. The new buses with LED lights here in the UK have a fairly low PWM rate.
-- Clive Mitchell http://www.bigclive.com
For sheer stupidity, Volvo come a pretty close second. 'Designed' by salesmen, 'engineered' by work-experience schoolkids (or so it appears).
-- ~ Adrian Tuddenham ~ (Remove the ".invalid"s and add ".co.uk" to reply) www.poppyrecords.co.uk
Allthough I have not noticed the effect prior to reding about it here, night driving and high speed lane switching might take on a new perspective.
greg
Try looking at an LED tailight, then shift your eyes a little up/down or left/right. As the light pulses fall on different parts of the retina, instead of seeing a streak of light, you'll see dashes.
Dave
It was exactly the 'trick' used by the strip systems, that have a line of LED's on a strip, that you can wave up and down, to generate text/pictures. Instead of the strip moving, you move your eyes (though the effect can also be seen when a car passes you on the pavement, if you are looking at something stationary, as it passes).
Best Wishes
What advantage do you have in mind for peak exceeding the average? In most high brightness LEDs, for a given average current close to maximum there is little to nothing to gain from making the peak higher except in cases where you want flashing or visible flicker.
Old digital displays, however, often had each die receiving a couple mA average while the die had higher efficiency at higher instantaneous current. That was a secondary benefit of "strobing" digital displays - the primary purpose of strobing them was to have a multiplexing system to reduce the number of wires going to the display. But many people believed that the increased luminous efficacy was mainly from a quirk in human vision rather than a nonlinearity in the LED.
Meanwhile, some but not all taillights are dimmed by means of PWM rather than changing the instantaneous current either because it's easier or because the LEDs have efficiency at low current that varies too much from one unit to another or is too unreliable.
- Don Klipstein ( snipped-for-privacy@misty.com)
Well, if you're going to resort to road rage, it's better to use a revolver, unless you're in one of those drive-on-the-wrong-side countries, because hot brass in your lap is not fun at highway speeds.
Good Luck! Rich
Huh? You've never run an LED at 10 mA? 5 mA? 0.1 mA?
It's almost linear, albeit I haven't tested any over 20 mA.
Cheers! Rich
The reason PWM dimming is used on LED taillights is because there is a ratio requirement between the photometrics of the stop lamp and the tail lamp of at least 5:1 at some of the test points. If you are going to do the tail lamp by just dimming the stop, then to just barely comply you have to be running at 20% of the stop current. With a reasonable design margin, you proably want to have on the order of
10:1. Another consideration is that there is a maximum limit on tail lamp output that could be exceeded even if you have the right ratio between stop and tail which would necessitate using more than the 5:1 drop. If you read the manufacturers specs on the LEDs used in these tail lamps, the performance of the led at the low currents is usually not spec'd or is not gauranteed because the behavior of the diode is significantly different in that region. So running the tail at a lower current isn't an option. Most of the times, the PWM is a lot more difficult/costly to implement than a simple current draw down because the PWM creates EMI that has to be dealt with or you might find weird hums on your radio when your taillights are on.PWM really can't be used to increase the light output in these applications as a previous poster implied, because you would need to overdrive the chip significantly and for a long enough duty cycle that the heat generated would negate any advantage and you might try to gain from PWM.
cars
Note I said higher peak output. The average output would be the same as the continuous rating, but the peaks are much higher resulting in greater and further visibility. It's the same concept as strobe lights (and now the LED version), the peak light pulses are vastly higher than the average power output.
Pete C.
Hmm. Subliminal messages in the taillights?
-- Paul Hovnanian mailto:Paul@Hovnanian.com ------------------------------------------------------------------ Only through suffering comes wisdom. -- Zeus
Thanks for writing this post. I thought I was the only one that saw it. New Cadillac's and Infiniti FX 35 & 45 are the worst! I first noticed it going down a long dark curving hill, with a normal car between me and the FX35. The Infiniti looked like a strobe light moving left and right in front of the other car.
Kevin
Stop blinking your eyes ;-)
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
Only the Infiniti strobed, the other cars were OK.
Kevin
cars
The visibility is higher only if the pulse rate is lower than the flicker fusion frequency - which is not the case for the automotive applications being discussed.
Strobe lights operate at a pulse rate lower than the flicker fusion frequency. If they did not, humans would see only a continuous light output.
-- 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.
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