From: Lizard Blizzard
Subject: Re: Best way to power array of LEDs?
Date: Mon, 07 Oct 2002 18:19:14 -0700
References: <0001HW.B9C26E7400A7161B165FEAC0@news.covad.net> <2Qan9.3129$cS4.firstname.lastname@example.org> <6aa7QNAh$Yn9EwS7@jmwa.demon.co.uk> <95qKYRBfvbo9EwZu@jmwa.demon.co.uk>
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John Woodgate wrote:
> I read in sci.electronics.design that Lizard Blizzard
> wrote (in ) about 'Best way to power array
> of LEDs?', on Mon, 7 Oct 2002:
>>John Woodgate wrote:
>>>A good guide is that the string should
>>>not take up more than half the lowest supply voltage.
>>That will guarantee that the dissipation in the resistors exceeds the
>>LED dissipation. In other words, the circuit is not power efficient.
> In the automotive application, with 12 strings of 3 LEDs, we are talking
> about a TOTAL power of 12 V x 240 mA = 2.9 watt, using the extremely
> simple and cheap series resistor solution. Nine strings of 4 LEDs gives
> 2.2 W, which, considering the OP thought of a 9 x 4 arrangement, could
> be preferable.
> You could save 2 or 3 watts by going 0.1 mph slower. I doubt whether any
> of the other electronic solutions save as much.
> When doing engineering, do NOT suspend common sense. You do that only
> when doing quantum mechanics.(;-)
I agree, I did not suggest that it was a poor idea to do this, but I
correctly observed that the dissipation of the resistors was greater
than the LEDs. Like you say, for an automobile it is inconsequential.
But immaterial of what the use, the resistors, LEDs, and amount of heat
generated by them have to be considered to prevent overheating.
I came across this URL today. It's about making resistors from, well,
read it and you'll see. Go down to the design calculations where he
shows how it's necessary to dissipate the heat built up. Pretty cool.
> The 'good guide' is about whether the crude 'voltage source and
> resistor' is an acceptable 'current source' or not.
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