Tom=E1s =D3 h=C9ilidhe wrote:
> I have bi-colour LED's on my board that have about 2.2 volts across
> them when there's 25 mA flowing through them.
>
> ...
>
> 1) Can I go straight from the 1.5 V battery into the LED's without a
> current-limiting resistor?

Think about it.  You have already measure the LED voltage to be 2.2V, at
least at 25mA.  Will the LED draw more or less current at 1.5V?  You could
use your measuring setup to decrease the current until the voltage drops to
1.5V and see what happens.  Can you even see any light from the LED at 1.5V
in normal office illumination?

In fact, here is a good exercise.  Measure the LED voltage at various
currents, like every 5mA going down from 25mA.  (are you sure you LED is
really rated for 25mA, not 20mA?)  Also include 2mA and 1mA.  Then plot
current as a function of voltage and show us the result.  For extra credit
make some rough notes about how readily visible the light was in office
illumination, daylight, and in a dark room.

Seriously, you should really do this.  You will learn a lot about LEDs and
get some intution that is nearly impossible to get from any datasheet.


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