"Joe Bleau" wrote in message
...
I have some LEDs that I would like to use on my boat. They currently
operate on 3 AAA batteries or 4 1/2 volts.

Question: What would happen if I wire them in direct to 12 v? My
understanding is that LEDs have a fairly wide operating voltage range.

They don't, it is in fact the opposite. LED's have a very steep U/I curve
just like any diode. They might not draw any current at 1.5V (red LED) and
fry on 2V. LED's operate on current, not voltage. You need to calculate a
series resistor to limit the current to 30mA which is the maximum for normal
LED's.

To do that, take the maximum voltage that you need to apply, i.e. 14.4V for
a 12V system and calculate the resistor as follows: R = (14.4 - Vled) /
0.03. Vled depends on the color, red LEDs are 1.6 to 1.8 and yellow and
green LEDs around 2.1V. Blue leds are around 3V but might depend on the
technology used, the same goes for white LEDs.

You should not parallel LEDs and use one resistor since LEDs might vary a
bit in voltage, resulting in uneven currents through the different LEDs. You
can connect them in series, and add the voltages. So 5 red LEDs would need a
total voltage of 5 x 1.6V = 8V. The added benefit is that there is less
voltage drop across the resistor and the overall efficiency increases.

Meindert