From: Keith Wootten
Subject: Re: Best way to power array of LEDs?
User-Agent: Turnpike/6.02-U (<7r9OMDU2ooRJCQmjnYJxyjtHfm>)
Date: Fri, 4 Oct 2002 15:22:09 +0100
NNTP-Posting-Date: Fri, 04 Oct 2002 15:24:15 BST
Organization: ntl Cablemodem News Service
In message , John Woodgate
>I read in sci.electronics.design that Keith Wootten
> wrote (in ) about
>'Best way to power array of LEDs?', on Fri, 4 Oct 2002:
>>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.
>A good idea IF you can find a suitable lamp. The lowest current lamps
>readily available in UK (excluding 28 V ones) are 6 V 40 mA. You *could*
>use one in series with two parallel strings of three LEDs.
In fact the 50 LED array I use has only two connections, so must be ten
parallel strings of 5 series LEDs. It's potted, but appears to be 50
individual 5mm LEDs. I suppose the Vf for each LED is reasonably
similar. Mine draws about 100mA at 10V to about 200mA at 14V using the
(two paralleled) bulbs.