I am building an 5x5 array of 25 LED lights, assorted white and blue:
 http://www.superbrightleds.com/leds/b2_specs.htm
 http://www.superbrightleds.com/leds/b_specs.htm
 http://www.superbrightleds.com/leds/w4_specs.htm

I have a very basic understanding of Ohms law, etc, but not enough to
determine how to connect these with the appropriate power supply,
resistors, etc so that each LED receives the appropriate voltage and
current.

I would like a description of how to wire and power this circuit,
giving information both for powering it from batteries and from a
power supply.

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Clarification of Question by placain-ga on 21 Oct 2002 15:26 PDT

It seems that all three types of LEDs require the same current (20mA)
and exhibit the same voltage drop (3.5V ± 0.1V), so what
combination of them I use should not matter.

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Request for Question Clarification by sgtcory-ga on 21 Oct 2002 16:20 PDT

Hello placain,

Did you have a preference as to how you wanted to wire this? In series or parallel?

Thanks for the clarification -

SgtCory

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Clarification of Question by placain-ga on 21 Oct 2002 17:30 PDT

That's precisely the sort of thing I want *you* to give advice about :)

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Request for Question Clarification by alienintelligence-ga on 21 Oct 2002 22:43 PDT

Hi placain...

You don't want 'series' wiring
unless your idea specifically 
calls for it. The end result of
series wiring is, the entire 
25 LEDs lit will cause a dimming
effect. A parallel voltage signal
out to each individual LED will
provide the optimum light output
and ease of switching. 

What type of detail do you want
for your answer? A schematic
taking into account the single
circuit for 1 LED or do we take
into account all 25 LEDs and the
method of switching? Do you have
a method in mind for the on/off
circuit? Do we represent that as
a SPST (single pole single throw)
switch controlling some arbitrary
power supply?

The power supply will have to
stated by you. What application
will this be used in? Home? Will
we need to show a transformer from
AC to DC? Auto? Will a voltage
regulator be enough? Batteries?
The exact voltage but short life.
We need that info.

-----
btw, you do realize that white LED
you posted the link for is rated 
at 10000mcd vs the blue(s) at
2400mcd and 5500mcd? Do we take
that into account with our circuits?
-----

thanks
-AI

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Clarification of Question by placain-ga on 22 Oct 2002 05:53 PDT

This should be powered from a wall-adapter transformer - it's not
necessary to actually show the transformer, just specify the required
power (V and mA rating). I realize now how quickly 25*20mA would drain
batteries...

I realize the whites are much brighter, but I was under the impression
that they were electrically identical to the others. If this is not
the case, let me know.

In any case, the entire thing should be controlled by one on-off
switch. (Actually, two would be even better - one for the whites and
one for the blues - but I'm confident that once i see how this thing
is laid out, I can figure that out for myself.)

The calculation for the LED series resistor is

R = (Vs - Vf) / If

Vs is the supply voltage (volts)
Vf is the forward voltage drop across the LED (volts)
If is the forward current across the LED (amps - divide mA by 1000)
R is the series resister (ohms)

So for a typical LED (I am following a worked example and NOT your
figures above) If = 20mA, Vf = 2.5V powered by 12V supply Vs:

R = (12 - 2.5) / 0.02 = 475 ohms

The nearest preferred value would be 470 ohms.

For AC operation connect a diode in inverse parallel with the LED and
halve the value of the resistor determined by the above formula.
Ideally you want each LED to have its own resistor, connect each
(LED+res) in parallel across the supply. If you wish to cut down on
the number of resistors you could parallel say 5 LEDs on one resistor;
that resistor serving 5 LEDS would then be 1/5th of the value
calculated.

If you are using LEDs with a 3.5V Vf then the next convenient
"standard" wall-wart type PSU will probably be 6V and 25 x 20mA is
500mA, so you will do the appropriate calculations on the resistors
for Vs=6V and to allow a little spare capacity, especially if the
display is going to be on for sustained periods of time I'd suggest a
*regulated* PSU at about 600-750mA.

HTH

Owain