On 16 Apr 2004 22:15:44 -0700, (Jay) wrote:

What is typically done is the LEDs are just put in series with a
current limiting resistor. This forms a circuit akin to a kind of
voltage regulator called a "zener regulator". LEDs have a fixed
forward voltage for the recommended drive current, say for example its
2.8 volts for a green LED. Divide the power supply (e.g. 12V) by the
forward voltage of the LEDs and drop any fraction (12/2.8=4.3 make
that 4 even) Put those in series with a current limiting resistor that
will drop the fraction (.3V).

Your concept is reasonable, but there are some significant problems.
The forward voltage varies with temperature, and the power supply
LIKELY varies over time. As you have it, if the fwd voltage dropped
0.1v, and the supply did not change, you would have 0.7 vs 0.3 across
the resistor, for over 2x the current ... maybe a real problem.

Now, let's have the alternator charging the battery, and have it at
15.5V or so ... now I have 3.8 volts across that current determining
resistor ... 12 times the "design". OUCH.

Now let's have the alternator fail, the battery voltage drop to 10.5V.
Your series string will draw no current and give no light ... and you
are in an emerency situation that is exactly when someone needs to see
you.

What is the solution ...

There are "constant current" devices. I have used them, and they
work allowing operating this string with probably 3 LEDs over the
range at visually constant brightness.

You can put 2 in a string allowing the current determining resistor to
allow a power range varying over about 2:1, 10V to 15V.

You can put 3 in a string like above, power will vary over about 4:1.

You can design a pulse system turning the LED on for (maybe) 0.1ms
then off for maybe 5ms and PROBALBY not overdrive (into damage) the
LED and put "as many" as you want in parallel. The driver will likely
be a FET.

So lets say the recommended current for
the LED is 20mA. Ohms law is R=E/I, so that gives you a resistor
value of .3V/.02A=15 ohms. Check power to make sure it won't over
heat (P=IV) so thats .3V*.02A=.006W so a typicial 1/4 watt resistor is
fine. You must of course have some kind of resistor in series to limit
the current. If you math works out that you need no resistor, put one
less LED in series and then recalculate the limiting resistor. Put
the LEDs and resistor in series (in any sequence) observing the proper
polarity of the LEDS.

I was taught to allow about half the voltage for the resistor, half
for the LED string unless I had current control. In the "old days",
for current control we used an emitter resistor in a common emitter
circuit, 2 or 3 diodes to set bias (single vs darlington), and the
LEDs between collector and V+. There are other (better) ways, but
everyone understood this one.

If you hook up your entire string backwards, no
harm will be done, but if you happen to solder one LED backwards, it
will likely be toasted on power up.

I disagree that there will be damage with any in backwards. The
reverse voltage will almost certainly be higher than the forward
voltage, so there won't be any current drawn. If there is, you still
would have less than correctly wired.

Need more than 4 LEDS? Replicate this circuit in parallel as many
times as you need to get the luminous flux you need.

Of course the numbers (Vf, If) used here are for the older style
single chip LEDS. The parts that are getting everybody excited these
days are the multi-chip variety whose forward voltage and current will
vary alot from my example.

"Dean Head" wrote in message .. .
Jeff,
Would you consider sharing your design for the driver electonics?
Dean
Cozy MK4
BKV FL

"Jeff Peterson" wrote in message
om...
I am building an LED tail light for my Lancair 360. It uses 6 ea 5
watt luxeon LEDs. I can strobe 1 amp through each of these which
should give enough light to satisfy the FAA regs. Runs surprisingly
cool. I have posted photos he

http://w1.lancair.net/pix/album01\

Its not quite finished, but I thought you might like to see my
progress.

cheers,

Jeff
N273CK stilll building.

ps thanks to Eric Jones for his posts on the Lancair Mail List, his
web pages on the subject and many emails and phone calls.