Re: LED-lights flickering (Was: Re: Who can supply correct LEDs?)

[Jan van de Wouw <delorean@xxxx>]

On Tue, 20 Jan 2004 01:39:18 +0000, d Martin Gutkowski wrote:

> Hmm. I'm not sure about this but I think that's down to the power
> supply and not the LEDs. Certainly LEDs do not have a lifespan in
> the conventional sense. Put simply, they *should* last forever.
[snip]
> Of course, I could be talking nonsense - can anyone clarify?

What's done regularly (like on the new BMW 7 series) is putting
a block wave signal onto the LEDs with a less than 50% duty cycle.
LEDs can only have a certain amount of current, but do have some
"spare" capability for a short time before overheating.

The trick is that by actually overloading the LED for a VERY small
amount of time and then giving the LED time to recouperate in the
"off-time" of the blockwave you can get -what looks like- more light
out of the same LED and the AVERAGE used current is less.
This same principle is also used on the DeLorean, where the rear
window defogger is switched on and off about every 5 minutes,
effectvely cutting the AVERAGE power consumption in half.

For this purpose you now get things like "LED-driver" ICs,
which can also let you run a 3 or more Volts LED on a
1,5V battery (this setup also needs a coil and a capacitor,
but the idea's the same...)

On the taillights by altering either dutycycle or the frequency
you can also alter the brightness of the lights.
BMW also uses this system on their adaptive brakelights; these
light up brighter if you hit the brakes harder and/or faster.
I agree it can be annoying driving behind a car with LED tails,
especially when going over a sequence of bumps. I've been behind
a BMW once that looked like it had 7 taillights on top of each
other because of that, boy was I glad when I could pass it...

Hope this clarifies things.

JAN van de Wouw

Thinking Different... Using a Mac...
Living the Dream... Driving a DeLorean...

#05141 "Dagger" since Sept. 2000
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