gnupic: Re: [gnupic] LED constant current vs PWM
Subject:
Re: [gnupic] LED constant current vs PWM
From:
Mark Rages ####@####.####
Date:
22 Jul 2009 20:01:22 -0000
Message-Id: <74ee72ca0907221301m37cb0877v621d6deaecc5611e@mail.gmail.com>
On Wed, Jul 22, 2009 at 2:24 PM, Peter ####@####.#### wrote:
> Mark Rages wrote:
>> > But in particular the high-power white LEDs really must be driven
>> > with PWM.
>>
>> Please cite a reference for your assertion that PWM delivers higher
>> performance than a current source for a modern LED.
>>
>> Of course PWM has better power efficiency. It's the performance
>> and the "must be" of your statement that interests me.
>
> Thanks for pushing me to look up the data sheets I was thinking of! :)
> You're right.
>
>
> I'm looking at two products, a 5cd@20mA yellow 5mm LED and a
> 480lm@700mA white 18W "LED light source type system".
>
> Attached images show relative luminous intensity vs. forward current
> for both, they're also at http://stuge.se/lumcur_5cd_yel.png and
> http://stuge.se/lumcur_18W_whi.png in case the list filters them.
>
> I could be misunderstanding the human vision but looking at the
> diagram for the yellow LED it seems that the best way to drive it
> would be 40-50mA (where the curve starts to fall off) which needs PWM
> as it's rated at max 20mA constant current. Is your point that the
> eye smoothes that out anyway, so it is perceived the same as half
> intensity at double duty cycle?
>
That's an interesting pair of graphs. You can see how the general
shape of the curve is the same, but the power LED won't benefit from
pulsed driving because it is already "past the hump" at full linear
drive. But the yellow LED will benefit from pulsed power, because
that will push the intensity up the curve a little bit.
It would be easier to interpret if the graph was intensity divided by
current, wouldn't it?
"Don's LED page" lists LEDs in order of lumens per watt:
http://members.misty.com/don/led.html
Regards,
Mark
markrages@gmail
--
Mark Rages, Engineer
Midwest Telecine LLC
####@####.####
