From: firstname.lastname@example.org (Bob Wilson)
Subject: Re: Best way to power array of LEDs?
Date: Sat, 05 Oct 2002 03:32:55 -0000
Organization: Your Organization
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In article <0001HW.B9C26E7400A7161B165FEAC0@news.covad.net>,
>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).
>My options, as I see them, are:
>1) Wire individual resistors to each LED based on its operating current
>2) Wire each row in parallel and connect one resistor based on operating
>current for 9 LEDs
>3) Install voltage regulator to output voltage (ie, 1.3 volts?) to operate
>all LEDs in parallel.
>What is the best method to power these from an intermittent (whenever I
>the brake pedal) power source?
>Other options for powering this array?
>Note that my return address is corrupted in an attempt to reduce spam. If
>choose to e-mail me, please correct my address as described below.
Not really rocket science.
Point 2 and 3 (above) are electrically almost identical and won't work at
all, unless all your LEDs are EXACTLY identical, and have the idential
forward voltage over temperature. Not very likely.
Point 1 will work just fine, but is hopelessly inefficient since
approximately 90% of the power consumed will obviously end up as heat in the
A simple solution would be to do a series-parallel arrangement and have each
series string controlled by a single resistor. The aim is to have the
forward voltage of each series string end up at around 10V or so, so that
the majority of the power is actually dissipated by the LEDs, and not the
As a side issue, one might well ask why bother to go to all this trouble to
make something like a 3rd brakelight which is pointless as a safety device
anyway. If one is actually close enough that the center light is more
visible than the normal ones, you're gonna hit the guy ahead anyway. The
only so-called research that was done to justify these things was done
before car makers were forced to install them, and they were a novelty that
got noticed. Now that they are on every car, no one takes notice of them.
There is no statistical evidence that has shown that these things have
reduced rear-enders, that I'm aware of. Well, I suppose there are those that
claim that the fast response of LEDs (a mere 50 to 100 mS faster than normal
brakelights) will prevent accidents, but then there are also those that
claim to cows can fly, as well.