From: Lizard Blizzard
Subject: Re: Best way to power array of LEDs?
Date: Fri, 04 Oct 2002 10:51:27 -0700
References: <0001HW.B9C26E7400A7161B165FEAC0@news.covad.net> <email@example.com>
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Keith Wootten wrote:
> In message <firstname.lastname@example.org>, "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.
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.
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