Just be careful, that equation assumes the LED requires 20 milli Amps of current, the proper equation is (Vsupply - Vled) = IR, or R = (Vsupply - Vled) / I in this case. V is in Volts, I is in Amps, and R in Ohms.
Originally Posted by Lee Roberts
20 mA = 0.020 Amps.
Some minature LEDs go pop on less than 20mA :)
Technically speaking LEDs require a certain current, not a certain voltage, so it is possible to run a LED from a higher voltage supply without a current limiting resistor, as long as the supply is switched so that the average current is at or below the LEDs forward current.
Last edited by ZeroCount; 16-03-2009 at 11:58 PM.
Reason: Additional information
The more I learn about this hobby, the brighter the light of knowledge grows, the deeper is the pit of ignorance revealed...