Subject: Re: Best way to power array of LEDs?
Date: Fri, 4 Oct 2002 12:53:37 -0700
References: <0001HW.B9C26E7400A7161B165FEAC0@news.covad.net> <firstname.lastname@example.org>
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On Fri, 4 Oct 2002 3:43:38 -0700, Ian Buckner wrote
(in message <email@example.com>):
> "DaveC" wrote in message
>> Have assembled an array of LEDs for use as a 3rd brake light in my
>> 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
>> 2) Wire each row in parallel and connect one resistor based on
>> current for 9 LEDs
>> 3) Install voltage regulator to output voltage (ie, 1.3 volts?) to
>> 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?
>> Note that my return address is corrupted in an attempt to reduce
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>> Dave Carpenter
>> Sound Logic
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> This was discussed a while back on this newsgroup.
> I believe the conclusion was to have groups of series connected
> LEDs, so the the failure of one would only take out a portion
> of the display.
> To decide how many in each series group, you need to find out
> what the forward voltage drop is, and decide how you will drive
> them (1.6V has been quoted, but there are other red LEDs with
> typically double that).
> The simple approach with series resistors suffers from voltage
> drop tolerance and temperature coefficient (-2mV/deg C,
> automotive temperature range including self heating is probably
> over 100 degrees), and also efficiency issues (if the resistors are
> next to the LEDs watch out for the extra heating). The large
> variation of battery voltage is also a problem.
> National Semiconductor do a couple of switcher driver chips,
> such as the LM2704, LM2794. It looks to me that you could
> easily adapt many single chip switchers to do a similar job.
> That approach fixes battery variations and helps with efficiency.
> Your forward voltage drop variations could be dealt with by
> selecting which string the devices go in. A flyback driver allows
> you to use any number of LEDs in series as convenient.
I'd need one of these for every 4 LEDs:
< http://www.national.com/ds/LM/LM2794.pdf >
Elegant solution, but I think I want to go with something less complex. With
all the support passives, it's a bit much for this small box.
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