Need help! (V=IR)

[Tree]

Hi,

I'm doing a school project, and I was a little ambitious and chose to build a miniPOV3 (MiniPOV3 - Unending POV).

I now have show why I used 100 ohm resistors and I'm a little confused why the 100 ohm resistors worked, instead of 47 ohm resistors. (There is a guide on that website, and it says that BOTH 47 and 100 ohm resistors work, but the V=IR equation only works for the 47 ohm resistors).

Anyway, to make this more clear, here is the schematic (the zeners and the 4.7k ohm resistors are for the serial port, ignore them for now):

https://www.ladyada.net/images/minipov3/minipov3schem.png

Here's a part list:

MiniPOV3 - Parts

Now...I'm using 1.9V-2.0V Red LEDS, with a forward drop of 2V (I don't really understand forward drop, please explain ? They are also @ .02amps.

I'm using two 1.5V batteries, so it's 3V. You subtract the 2V forward drop so you get 1V

This means that 1V/.02amps = 50 ohms.

HOWEVER, I need it to equal 100 ohms! What am I doing wrong ?

This project is due tomorrow, and my entire write-up is complete, beside this equation...I don't understand what I'm missing.

 

[Mikebits]

You are just running the diode at half its current rating. Your making the assumption that the current through the diode is .02A, but if you rearrange the formula E= IR to solve for I you will see that you have I=E/R which is 1V across your 100 ohm resistor equals .01A, so if you place another 100 across one of your 100 ohm resistors you will notice that the LED gets brighter.
The way yoou have it is just fine as long as you can see the leds, and is probably better to conserve your battery power. That could be your justification for halving the LED current.