From: "Ian Buckner"
Subject: Re: Best way to power array of LEDs?
Date: Fri, 4 Oct 2002 11:43:38 +0100
Organization: Agilent Technologies
NNTP-Posting-Date: Fri, 4 Oct 2002 10:43:40 +0000 (UTC)
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"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
> 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
spam. If you
> choose to e-mail me, please correct my address as described below.
> Dave Carpenter
> Sound Logic
> Remove the numbers to reply via e-mail
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.