Re: [Amc-list] Blue LED tamed

[Tom Jennings <tomj@xxxxxxx>]

On Sat, 3 Mar 2007, Widiker, John D wrote:

> Subject: [Amc-list] Blue LED tamed

> Tell the person at radio shack you want a 15K (1/4 watt) power limiting
> resister,

15K is too big!

The resistor is to limit the current through the LED. You need
to know the rated "forward current" for the LED, but nearly all
small-ish LEDs want 20 to 40 milliamps max. More than that they
may get brighter, but their life will drop DRAMATICALLY!! Use
more LEDs if you want more light.

When on, blue LEDs drop about 3 volts across them (this is a
constant).  For a typical small blue LED, 20 mils of current,
you would use a 470 ohm resistor; for a 40 milliamp LED, half
that; 220 ohms. (Resistors come only in standard values, you
won't normally find a 500 ohm resistor.)

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