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From: Kevin McMurtrie
Subject: Re: Best way to power array of LEDs?
User-Agent: MT-NewsWatcher/3.2 (PPC Mac OS X)
Date: Fri, 04 Oct 2002 07:56:23 GMT
NNTP-Posting-Date: Fri, 04 Oct 2002 00:56:23 PDT
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 power
>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 press
>the brake pedal) power source?
>Other options for powering this array?
Automotive DC power varies from 11V to 15V with spikes of +/- 20V or
worse. You'll want a voltage difference of about 6V so you can use
cheap resistors to limit current with resonable accuracy and heat
generation. You could do this with groups of 3 or 4 LEDs in series and
a resistor for each of the 12 or 9 groups.
I recommend a little power filtering, especially for the negative
voltage spikes that some LEDs don't like. Put a 3.3 Ohm .5W resistor in
series with the main power and a 47uF 16V capacitor between the output
of the resistor and ground.
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