Don, do you have any actual measurements on this?
Best regards, Spehro Pefhany
-- "it\'s the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
One point that hasn't been made is how the current can be modulated. In a PWM you can do it through different ways. You can have a fixed pulse width and modulate the off time (or the inverse). You can can have a fixed frequency and modulate the ratio between on and off.
Further you can work at different current levels, specially if you are working at short pulse widths. Since you can work at a much higher current level.
My guess it that maybe the switching frequency isn't to high, but when you have a long off period it could appear as a low frequency strobe. Does any one have a scope?
I think it's a deliberate choice to make it like that. If they ran it at a higher frequency, you wouldn't notice that the car has "New LED Taillights" and the marketing value would be lost.
Chris
I do, and I have tested hundreds of automotive LED rear combination lamps. If the LEDs were modulated, it was by PWM (pulse width modulation) with a constant peak intensity at a frequency usually greater than 200 Hz (can't remember them all, but it seems that they were never near the flicker frequency range). The pulse width depended on the function being activated (Stop versus Tail) and, for the more advanced ones, the input voltage so that no matter the input voltage the output light was the same. This is very important in trucking applications where the voltage at the cab may be around 13.5V but only
12.0V at the end of a couple of l> One point that hasn't been made is how the current can be modulated. In-- Douglas Cummins Calcoast - ITL
I have rolled my eyes and looked at plenty of CRT monitors and TV sets, and consistently see quite a lack of phosphor persistence - downright dropping something like 90% well within a millisecond.
(about to shoot photos with my digital camera with a fast exposure time)
(Shoot a 1/250 second exposure of my CRT color TV and my Dell Trinitron monitor catching roughly the middle of the screen being scanned)
Results are at until July 15 2006:
Both show all three color phosphors largely coming on and dying out within a millisecond, with the green and blue phosphors doing so much faster and red having perhaps so much as about .4-.5 millisecond halflife. Overall brightness fades a majority of the way as fast as just a couple lines get scanned.
- Don Klipstein ( snipped-for-privacy@misty.com)
In message , Don Klipstein writes
Try looking at a traditional CRT monitor with it turned upside down or looking backwards with your head between you legs. (Well _I_ can do it...)
We've adjusted to ignore the top to bottom scan flicker, but not in the opposite direction.
-- Clive Mitchell http://www.bigclive.com
in an NTSC display..the interlaced scanning reduces the visability of the flicker .. even so, if you look at a TV with your periphrial vision, it is easy to see the flicker.
Mark
On West Midlands Travel buses, they also seem to have one LED within the cluster that stays on longer than the rest -- just noticeable.
What's the deal with that?
[And yes, they do flicker when dimmed]-- --------------------------------------+------------------------------------ Mike Brown: mjb[at]pootle.demon.co.uk | http://www.pootle.demon.co.uk/
In message , Mike writes
If the cluster of LEDs doesn't have an exact multiple of the series circuits number, then the odd LED/s out will have their own circuit with a higher value resistor, but will also have a lower forward voltage than the rest of the circuits. As the power supply to the light runs down after disconnection the last remaining light lit will be that circuit. Because there's only a single circuit lit at that point it may trail off very slowly as the capacitor discharges.
-- Clive Mitchell http://www.bigclive.com
That makes sense, but it's definitely just one LED. Sounds like it would've been simpler to just use one less LED :)
Off by one errors, don't you love 'em?
-- --------------------------------------+------------------------------------ Mike Brown: mjb[at]pootle.demon.co.uk | http://www.pootle.demon.co.uk/
Do you really mean that the multiplexing frequency was intentionally "too low" in order to make them more efficient? I've always thought that it would be just a "bug" that "visibly low" freq is used, just like in the case of these flickering PWM'd taillights...
Isn't multiplexing used anymore? Or did you mean that higher freq is used nowadays? (you said "old digital displays"). At least my friend has a Panasonic microwave with a LED-7seg display, bought in 2001, and its display has very visible multiplexing flicker.
No, I was talking about why digital displays were strobed rather than having each LED die being fed steady DC. I made no mention of what frequency to strobe them at.
Most digital displays are now LCD and I don't know how those work and they may be multiplexed. In any case, LCD responds very slowly and I suspect it will smooth out any multiplexing if multiplexing is used.
Meanwhile, unlike the old digital displays, modern LEDs in tail/brake lamps (used as brake lamps), traffic signals and flashlights will generally have little or no or downright negative gain in efficiency if they are pulsed with the same average current or the same average power at which they are operated steadily. Many but not all taillights use PWM because the efficiency of the red LEDs there will be either reduced or unreliable (varying excessively from one production run to another) with steady low current.
- Don Klipstein ( snipped-for-privacy@misty.com)
In message , Simoc writes
Multiplexing flicker can sometimes increase dramatically in old equipment due to the increased ESR of the PSU capacitors creating enough ripple to cause a beat effect in conjunction with the multiplexing frequency.
Ask if it's always flickered so strongly.
Then again, the older processors ran slower and multiplexing speed was compromised by other processing tasks.
-- Clive Mitchell http://www.bigclive.com
Like, "Back off, idiot"? ;-)
-- Service to my country? Been there, Done that, and I've got my DD214 to prove it. Member of DAV #85.
Michael A. Terrell Central Florida
Fair Radio used to have new surplus strobe tubes made for aircraft wing tip markers. Think what a dozen of them would do, mounted in the grill.
-- Service to my country? Been there, Done that, and I\'ve got my DD214 to prove it. Member of DAV #85. Michael A. Terrell Central Florida
[snip]
Goshawk1, Are you having a stuttering problem, that's four identical non-content "retorts" now, or are you just a disgruntled Beamer ?:-)
...Jim Thompson
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