Homebrew PCBs

Even LEDs from the same batch cannot be trusted to
have the same brightness for the same current.  After
they age, it is even less likely that they will match. If you
want them to stay matched you would have to have some
kind of feedback or periodic re-calibration of each LED.
I think that is probably overkill.

I recommend that you use a current limiting resistor and
put 10 or 20 or more in a string.  This guarantees that every LED
in that string will get the same current.

For example, if you had 20 LEDs and each one had 1.7V
forward voltage drop, you would have a total forward voltage
drop of 34 volts.  If you use a 36 VDC supply you can tweak the  
resistance
to get the current you want.  To get 20ma you would divide 2 volts
by .02 A to get a resistance of 100 ohms.  The 2 volts comes from
subtracting 34V from 36V.

This approach can be modified to match whatever power supply
you have laying around to the LEDs you have.

Gus


On Feb 14, 2008, at 10:19 PM, javaguy11111 wrote:

> I went ahead and placed an order for some as well.
>
> I am wondering if just doing simple current limiting resistors is
> sufficient or if a proper LED driver chip would be the better way to
> go. From what I have read, at least for visible LEDs, you want to
> match the current in the LEDs to ensure that they all shine evenly.



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