An LED (Light Emitting Diode), is a device that normally runs on a voltage as low as 2 Volts.
You cannot run an LED on AC voltage, as this will reverse bias the diode and destroy it.
The series dropper resistor is required to lower the voltage supplied to the LED, as most of the voltage, is "dropped" across this resistor.
A formula to find the value of this resistor,would go like this:
R= (E-1.7)x 1000/I
Where E= Supply Voltage(DC)
I=LED forward current in milliamperes.
With this calc, you shouldn't go wrong!.
The 1.7 in the formula, is the forward bias voltage of the LED, itself, please make changes, according to your particular LED.
[This message has been edited by Trumpy (edited 12-22-2002).]