From: firstname.lastname@example.org (Don Klipstein)
Subject: Re: Best way to power array of LEDs?
Date: Sun, 6 Oct 2002 05:03:51 +0000 (UTC)
Organization: MGT Consulting
References: <0001HW.B9C26E7400A7161B165FEAC0@news.covad.net> <email@example.com> <0001HW.B9C33DA500C07DD2165FEAC0@news.covad.net>
NNTP-Posting-Date: Sun, 6 Oct 2002 05:03:51 +0000 (UTC)
User-Agent: slrn/0.9.6.2 (SunOS)
In article <0001HW.B9C33DA500C07DD2165FEAC0@news.covad.net>, DaveC wrote:
>On Fri, 4 Oct 2002 10:51:27 -0700, Lizard Blizzard wrote
>(in message ):
>> Using a lamp to serve as a more or less constant current 'ballast' is a
>> good idea, but if it is run at anywhere near normal brightness, the
>> lamp's lifetime will be short in relation to the LEDs. And of course
>> when the lamp goes open, the LEDs no longer get current. So it would be
>> much wiser to use it at much lower than normal brightness so that the
>> lifetime is extended considerably, comparable to the LEDs.
>But then if it's not running at temperature, you don't get the temp/current
>regulation that is the main reason for using it. Yes?
Tungsten filaments keep their famous nonlinearity down to dim orange-red
glow. Sometimes more so than their usual when down to dimmer orange or
really dim orange-red glow. Current is generally about proportional to
square root of the voltage across the filament, give or take somewhat
depending on the lamp. Some lamps make current more constant than this,
and if they do they generally do that when voltage across them is
somewhere less than the lamp's design voltage.
Of course, the filament will lose its partial regulating effect at
voltages so low that the filament does not get a whole lot past the
ambient temperature. But if you can see it glow in a dark room with dark
adapted eyes with a strong magnifier, the filament will be having at least
close to its usual partial current regulation effect.
What I might worry about is the current surge that flows while the
filament is warming up. Probably not as much a problem with 3 and 5 mm
LEDs as the really high current ones with rated peak current only about
1.4 times their rated continuous current.
- Don Klipstein (firstname.lastname@example.org)