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From: Watson 'Atto Parsec' Name
Subject: Re: Best way to power array of LEDs?
References: <0001HW.B9C26E7400A7161B165FEAC0@news.covad.net> <firstname.lastname@example.org>
X-Newsreader: MicroPlanet Gravity v2.60
Date: Sun, 6 Oct 2002 07:02:46 -0700
NNTP-Posting-Date: Sun, 06 Oct 2002 06:54:49 PDT
Organization: InReach Internet
In article ,
JoeBloe@thebarattheendoftheuniverse.org talked about...
> On Fri, 04 Oct 2002 10:51:27 -0700, Lizard Blizzard
> Gave us:
> >Keith Wootten wrote:
> >> In message <email@example.com>, "Wouter van Ooijen
> >> (www.voti.nl)" writes
> >>>> Have assembled an array of LEDs for use as a 3rd brake light in my car,
> >>>> configured as 4 rows of 9 jumbo LEDs, as of yet unwired. Of course
> >>>> the power
> >>>> supply will be 12 volts nominal (13.8 volts, maximum).
> >>>> What is the best method to power these from an intermittent (whenever
> >>>> I press
> >>>> the brake pedal) power source?
> >>> The crucial point is that when you put a resistor and N LEDs in
> >>> series, the LEDs will take roughly a constant voltage so all voltage
> >>> variation from your battery will be over the resistor, thus causing
> >>> current variation.
> >> You can mitigate this to a useful extent by using a small filament bulb
> >> instead of a resistor. The positive temperature coefficient of the bulb
> >> will help to keep the current variation reasonable as the supply voltage
> >> changes. The bulb should be selected to run at more or less normal
> >> brightness at maximum supply voltage while passing maximum LED current.
> >> My bicycle rear light is an array of 50 LEDs from RS components which
> >> are arranged in a series/parallel configuration with a nominal Vf of 9V5
> >> IIRC. Using two small paralleled 5V lamps (can't remember the type -
> >> selected from a junk box) the current swings 2:1 for a 14V-10V supply.
> >> In my case, it's also a useful low battery warning - the filament lamp
> >> goes out long before the LEDs.
> >> Cheers
> >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.
> Not only that, but they emit IR like wild when they are at "normal
> brightness"! Doh! Does nobody care that the enclosure is going to
> pull a Three Mile Island here?
See my previous post re your lame statement.
My email address is whitelisted. *All* email sent to it
goes directly to the trash unless you put NOSPAM in the
Subject: line. alondra101 hotmail.com
Don't be ripped off by the big book dealers. Go to the URL
that will give you a choice and save you money(up to half).
http://www.everybookstore.com You'll be glad you did!
Just when you thought you had all this figured out, the gov't
changed it: http://physics.nist.gov/cuu/Units/binary.html
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