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References: <0001HW.B9C26E7400A7161B165FEAC0@news.covad.net> <firstname.lastname@example.org>
Subject: Re: Best way to power array of LEDs?
X-Newsreader: Microsoft Outlook Express 6.00.2600.0000
Date: Thu, 10 Oct 2002 11:32:39 -0500
NNTP-Posting-Date: Thu, 10 Oct 2002 12:17:51 EDT
Since one of the biggest advantages of LED's over incandescents is the
almost unlimited operating life, putting a bulb in series with them
(especially when running the bubble at nearly full voltage) would seem to
entirely negate the reason why one would sub in LED's in the first
"Keith Wootten" wrote in message
> 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
> >>supply will be 12 volts nominal (13.8 volts, maximum).
> >>What is the best method to power these from an intermittent (whenever I
> >>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.
> Keith Wootten
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