Date:         Wed, 19 Sep 2007 16:00:39 +0200
Reply-To:     Jaap Nauta <bus.mail@IAE.NL>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Jaap Nauta <bus.mail@IAE.NL>
Subject:      How to use LED's (a bit long)
In-Reply-To:  <20070919115207.DCA371165C3@hamburg.alientech.net>
Content-Type: text/plain; charset=iso-8859-1

I get the impression a few things get mixed up here.

In general it is a good idea to use LED's in series connection, to have
equal illumination, and less loss in regulation. This is however only the
case if you have proper current regulation. Then you will still have the
correct current when one of the LED's fails (short).

BUT if you only use a resistor as current regulator for a string of LED's,
and the majority of the voltage drops over the LED's, failure of 1 LED
will certainly lead to failure of the whole string (in case of shorting).

An example: you want to use 5 LED's with a recommended current of 50mA at
typical 2V drop, on a 12V car supply.

For series connection 5x2=10V, so you would need a 40 ohm resistor. When 1
LED fails and would create a short, the current will go up, and due to the
non-linear behavior of the LED's will lead to (estimated) 2.1V per LED,
3.6V for the resistor and current 90mA, 80% more!
By the way, the same effect will appear if you start your engine, and
voltage goes up to 14V. And what to think of the spikes of up to 18V
caused by the ignition?

When using a simple (dump) current regulator the effect of l LED failing
(or supply voltage going up) will be that current is still 50mA, but power
dissipation in the regulator will raise (0.2W, still no big deal). And the
other LED's will not be affected. LM317 is a cheap electrical component
you can use to make this kind of regulator.

When using high power LED's such as Luxeon's I would always use so-called
buck regulators, they actually convert the input voltage into the desired
current with high efficiency. For example a 1 Watt luxeon would need 350mA
at 3V, with regulator only drawing around 100mA from 12V. Or 3 luxeon's in
series needing 350mA at 9V, the regulator then drawing around 300mA from
the 12V. You can get this kind of regulators for less than $10.

By the way, another kind of regulator is the boost regulator, to operate
high power LED's from low voltage sources (e.g. torches)

So my advice (for automotive environment):
- Use resistors only for small indicator LED's
- For illumination purposes use at least a dump current regulator, but
preferably buck regulators

Jaap
'87 Caravelle diesel



(Indeed, an electrical engineer :-)