gnupic: Re: [gnupic] LED constant current vs PWM
Subject:
Re: [gnupic] LED constant current vs PWM
From:
Peter Stuge ####@####.####
Date:
22 Jul 2009 19:24:57 -0000
Message-Id: <20090722192454.24579.qmail@stuge.se>
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?
The data sheet for that product also covers white LEDs, for which it
explicitly states "Recommend to drive with constant current." - so I
remembered all wrong.
The white LED module data sheet is a bit brief and just mentions max
1000mA pulse current, characteristics at 700mA and a few diagrams.
I suppose the maximum allowed duty cycle at 1000mA will not be such
that it can perform much better with PWM, so constant current would
be best also for that guy.
Thanks again!
//Peter
