Re: (ET) Ralph's LEDs

["RJ Kanary" <rjkanary nauticom net>]

    I have no scientific evidence to back up  the idea of whether or not 
LED
signaling devices use PWM power technology. What I can tell you is that 
here
in the Commonwealth of Pennsylvania, LED signaling is taking root. All new
installs are of that variety, and most PennDot owned fixtures are being
updated.
     I have noticed a perceptible flicker, in some units. I am blessed with
what computer geeks call 'Fast Eyes'. I have to have a refresh rate of 
75Hz.
or higher on my monitor, or I can't stand to be in front of it . So, that
being the case, if there is a flicker there, indicating PWM operation, I'm
the person that would be likely to see it.
    The signals I have noticed flicker on also appear to have a higher
number of dead elements in the arrays.
 Your thoughts ?
----- Original Message ----- 
From: "William Korthof" <wkorthof earthlink net>
To: "David Roden (Akron OH USA)" <roden ald net>;
<elec-trak cosmos phy tufts edu>
Sent: Sunday, November 23, 2003 8:20 PM
Subject: Re: (ET) Ralph's LEDs


> David,
>
> I'm friends with the owner of one of the most established LED traffic
> signal retrofit manufacturers, and I've been following progress with
> them for 10 years. Every production unit I've seen uses some form
> of current limited power supply designed to approximates a
> constant current supply (with varying effectiveness). Where did you
> get the information that a pulsed power supply is common?
>
> /wk
>
> At 09:53 AM 11/22/2003, David Roden (Akron OH USA) wrote:
> >I recently read something fascinating about LEDs.  Have you noticed how
> >intense the new LED traffic signals appear?  (At night some are so 
> >bright
> >that the green ones actually are almost painful to view.)
> >
> >This is how it's done.  The LEDs are driven by a pulsed waveform.  The 
> >on
> >current is TEN TIMES their rating, but the duty cycle is only 10% - that
is,
> >on (for example) on 1 picosecond, off 9 picoseconds, repeat.  Thanks to
> >persistence of vision, the human eye sees the peak intensity as
continuous
> >high intensity; but the LED "feels" the average current, which is just
its
> >normal rated current.
> >
> >I'm sure the same thing could be done with automotive signals (if it
isn't
> >already).
> >
> >Clever, no?
> >
> >
> >David Roden - Akron, Ohio, USA
> >1991 Solectria Force 144vac
> >1991 Ford Escort Green/EV 128vdc
> >1970 GE Elec-trak E15 36vdc
> >1974 Avco New Idea 36vdc
> >= = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =
> >Cutting the space budget really restores my faith in humanity.  It
> >eliminates dreams, goals, and ideals and lets us get straight to the
> >business of hate, debauchery, and self-annihilation.
> >
> >                                -- Johnny Hart
> >= = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =
> >
> >
> >
> >
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