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[Kuu]

When you start working with LEDs, invariably you ask... what size resistor do I need for X?

For folks like me that never memorized the equation ...

http://led.linear1.org/led.wiz

So for example, if you have a 9v battery... and you want 3 LEDs in your "whatever" ... what size resistor do you need?

Say you have some LEDs that are 20 mA (milliamp - your packaging on the LED or seller will tell you - if you don't know for sure assume 20 mA for most LEDs) and have a voltage of between 2.0 and 2.4 volts. Since I like to build safety into the system, I will use the halfway point and say these are 2.2 volt LEDs.

So into the above wizard, I punch in

9 = as my 'Source voltage'
2.2 = as my 'diode forward voltage'
20 = as my 'diode forward current (mA)'
3 = as the 'number of LEDs in your array'

Then I select 'Wiring Diagram' and check 'Help with Resistor Color Codes' then click 'design my array'

It spits back that I need a 120 ohm resistor  (R = 120 ohms). The "wiring diagram" will even show you how to hook everything up.

... Just remember the "long" pole on a LED is positive (anode). The rest should be straightforward.

+9V R = 120 ohms

[Dresdon Acacin]

The wizard won't do the math for me because it claims the idea of hooking up a 1.7V LED to 650V three-phase is unpractical XD

Seems to be fine for most stuff, even goes up to 120V but it warns that the current may be too much for the resistor to cope with

I usually just use have a set of resistors for certain applications and then if anything out of the ordinary is being done I bust out the maths or pm some experts