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Subject: Re: Best way to power array of LEDs?
References: <0001HW.B9C26E7400A7161B165FEAC0@news.covad.net> <firstname.lastname@example.org>
Date: Sun, 06 Oct 2002 23:34:09 GMT
NNTP-Posting-Date: Sun, 06 Oct 2002 19:34:09 EDT
> On Thu, 3 Oct 2002 22:00:20 -0700, DaveC wrote:
> >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?
> Here's another idea:
> 1) Wire LEDS in 9 groups of 4 series connected leds. Then all 9
> groups wil conduct with a total voltage of about 9V, and can be
> connected in parrallel.
> 2) Use a LM317T voltage regulator as a "Current regulator" This will
> limit the current to what ever amount you decide. The current can be
> adjusted by changing the resistor value. The circuit looks like this:
> (+12~13.8V)--------[ LM317T ]------- (output pin)
> [ ] >
> | < 5.6 ohm 1W
> | > Resistor
> | <
> ------------------ (adjust pin)
> [ LED]
> [ LED] Parrallel
> [ LED] x 9
> [ LED]
> So if you use a drive current of 25mA per LED it will be 225mA in
> total. Othwerwise you can calculated. The LM317 will dissipate just
> over 1W when the input volage is maximum. So it will need a small
> heatsink, or if you reduce the current to about 100-150mA you probably
> dont need any heatsink, although it will still get quite warm.
> You can calculate the resistor value by:
> R = 1.25 / (Current in Amps)
> Hope this helps,
The 317 in this configuration serves no useful purpose.
Limiting current to the 9 strings in parallel does not
limit the current to individual strings. One string can
"hog" as much as it is capable of, up to the limit established
by the LM317. You're looking to keep the current through
each LED to 25 ma - your circuit won't do that.