Which one of these three solutions is the best for the best light intensity?

http://img329.imageshack.us/img329/5950/solucijenw2.jpg

More info:
source voltage - 12V
diode forward voltage - 3.7V
diode forward current - 20mA
number of LEDs - 23 (blue, 470nm)

On Sun, 05 Aug 2007 01:18:55 +0200, Vlad wrote:

> Which one of these three solutions is the best for the best light intensity?
>
> http://img329.imageshack.us/img329/5950/solucijenw2.jpg
>
> More info:
> source voltage - 12V
> diode forward voltage - 3.7V
> diode forward current - 20mA
> number of LEDs - 23 (blue, 470nm)

The light output from each LED depends only on the current through
that LED, so I would just minimize the number of series strings to get the
parts count down. The light intensity from the whole array will depend on
the physical arrangement of the LEDs and what kind of optics you put
around them.

In article <f931h3$qi6$1@sunce.iskon.hr>, Vlad wrote:
>Which one of these three solutions is the best for the best light intensity?
>
>http://img329.imageshack.us/img329/5950/solucijenw2.jpg
>
>More info:
>source voltage - 12V
>diode forward voltage - 3.7V
>diode forward current - 20mA
>number of LEDs - 23 (blue, 470nm)

I prefer the one with 12 series strings, 11 of them having 2 LEDs.

The third solution using 8 series strings, mostly having 3 LEDs, I
consider likely problematic since LED current will vary greatly with small
variations or changes in LED cvharacteristics and with small variation in
supply voltage.

- Don Klipstein (don@misty.com)

One mistake; second SOL.1 should be SOL.2... SOL.2 draws 240mA.

I also didn't mentioned that SOL.1 array draws current of 460mA, SOL.1 240mA
and SOL.3 160mA.

So if intensity of all three is the same then it's the best to pick one that
will minimize the current draw, am I right?

Probably second solution is the best because Stephen J. Rush mentioned that
the third one could be problematic.

On Aug 4, 7:18 pm, "Vlad" <vladana...@gmail.com> wrote:
> Which one of these three solutions is the best for the best light intensity?
>
> http://img329.imageshack.us/img329/5950/solucijenw2.jpg
>
> More info:
> source voltage - 12V
> diode forward voltage - 3.7V
> diode forward current - 20mA
> number of LEDs - 23 (blue, 470nm)

To answer your question, they all provide about the same light
intensity, since each diode draws 18 +/- 1 mA. Other considerations
are:

Sol.1 provides the best current regulation for changes in supply or
diode voltage, but uses the most power.

Sol.3 uses the least power, but provides the worst current regulation
for changes in voltage.

Sol. 2 is a compromise between the other two circuits.

Deciding which to use is a tradeoff, depending on your application and
specifically on what you use for the 12V supply. Does your voltage
supply deviate very much from the 12V spec? Does the voltage vary
with time, as with a battery?

Regards,

Mark

>Does your voltage supply deviate very much from the 12V spec? Does the
> >voltage vary with time, as with a battery?
>
> Regards,
>
> Mark

Thanks Mark, I use wall adapter that give 12V/800mA... i got it for free,
but it said "For Information Technology Equipment use only", khm... is it
ok? Btw the adapter is not dead, broken or something.

On Aug 5, 1:15 pm, "Vlad" <vladana...@gmail.com> wrote:
> >Does your voltage supply deviate very much from the 12V spec? Does the
> > >voltage vary with time, as with a battery?
>
> > Regards,
>
> > Mark
>
> Thanks Mark, I use wall adapter that give 12V/800mA... i got it for free,
> but it said "For Information Technology Equipment use only", khm... is it
> ok? Btw the adapter is not dead, broken or something.

Okay, since it's a fairly steady voltage, you don't need to worry
about voltage fluctuations changing the current. Also, power
consumption isn't really an issue either; for example the electrical
cost of sol.3 would cost about 1/4 cent (in the U.S.) per 10 hours of
operation.

I guess I'd go with sol.3, just because there are fewer parts to wire
together.

However, I would measure the actual voltages of the supply and diodes
under the expected current load (for sol.3, 160 mA total for the
supply, 20 mA each LED), and then use resistor values based on the
measured voltages.

For example, if the supply voltage were really 12.5 V, sol.3 would
produce 30 mA through each LED, not 20 mA.

Good luck,

Mark

In article <f950kd$gtq$1@sunce.iskon.hr>, Vlad wrote:
>>Does your voltage supply deviate very much from the 12V spec? Does the
>> >voltage vary with time, as with a battery?
>>
>> Regards,
>>
>> Mark
>
>Thanks Mark, I use wall adapter that give 12V/800mA... i got it for free,
>but it said "For Information Technology Equipment use only", khm... is it
>ok? Btw the adapter is not dead, broken or something.

In my experience, those things often produce more than rated voltage
when less than max load current is demanded from them.

I suspect option #3 would work well enough there - but plan for more
like 14 possibly 15 volts with load current less than 250 mA, so make thye
dropping resistors accordingly higher. I would measure output voltage to
a 47 or 50 ohm power resistor.

- Don Klipstein (don@misty.com)