Car LED tail lights strobe rate too slow

[Pete C.]

Well, no, you can have the LED produce any amount of light, between
none and its maximum, by varying the amount of current flowing through
it. Of course this requires dropping the rest of your supply voltage
across some other circuit element and so will use more power. But I
think the real reason that auto manufacturers use PWM is that
transistors for just on-off switching are cheap.

To join the chorus, I find them annoying also and really wish they
would use a higher frequency.

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.

 

[John S.]

I've never experienced that. Do you wear glasses or contacts?

I find it very annoying and

I'n not sure there is a problem to be corrected.

 

[GregS]

PWM is used in order to exceed the maximum continuous output available

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

 

[Victor Roberts]

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.

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
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[Scott Dorsey]

I've been behind a few I can say were no where near 500Hz.

I have seen a lot that were in the 15Hz to 20Hz range, including the
new Cadillacs.
--scott

 

[Clive Mitchell]

I've been behind a few I can say were no where near 500Hz.

Agreed. The new buses with LED lights here in the UK have a fairly low
PWM rate.

 

[Adrian Tuddenham]

10 years ago, the engineers of BMW used modern technology and
significant effort to serve the aggressive drivers of their cars
to annoy the drivers of any other manufacturers car,
by installing xenon front lights with damn increased light output,
ignoring that on wet streets light is shed directly into the eyes
of the driver of an oncoming car, because the water on the streets
acts like a perfect mirror.
It took another 10 years for the engineers of BMW to come up with
a solution to annoy even the drivers of cars following up. It was
difficult, because just like the front lights, the back lights must
also conform to law. Again the newest technology was used to screw
up everyone driving behind a BMW by using far too slowly pulsing
LEDs.
That's BMW, worlds stupidest engineers.

For sheer stupidity, Volvo come a pretty close second. 'Designed' by
salesmen, 'engineered' by work-experience schoolkids (or so it appears).

 

[GregS]

Agreed. The new buses with LED lights here in the UK have a fairly low
PWM rate.

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

 

[[email protected]]

I've never experienced that. Do you wear glasses or contacts?

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

 

[Roger Hamlett]

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

 

[Don Klipstein]

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.

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 ([email protected])

 

[Richard The Dreaded Libertarian]

Oh yeah! I see individual LEDs with blurry arced paths as I move my
head. It bothers me to the point of dizziness; I can't drive with
such a car in front of me. And no, it isn't the acid.

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

 

[Rich Grise]

LEDs only light up at 'one' brightness level...

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

 

[Boxman]

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.

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.

 

[Pete C.]

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.

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.

 

[Paul Hovnanian P.E.]

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).

Hmm. Subliminal messages in the taillights?

 

[Kevin Graf]

Anybody else notice that the strobe rate bothers them on the newer cars
with LED taillights.

At night, when the tail lights are dimmed, (brakes not on) and you
look at them and blink or turn your head, the after image is very
strange, you see dashes of light. I find it very annoying and
distracting and I hope that this problem is recognized and corrected
before every car on the road is like this.

Mark

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

 

[Jim Thompson]

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

 

[Kevin Graf]

Stop blinking your eyes ;-)

...Jim Thompson

Only the Infiniti strobed, the other cars were OK.

Kevin

 

[Victor Roberts]

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.

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.

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.

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.