Calculating the Required
The calculation used to find the value of the
series resistor we need to know the diode forward voltage and current
and its connections. This information can be obtained from the
package if you purchased the LED.
In this example it is 2 volts and 20mA (0.02
The cathode lead is the one nearest a "flat" on the body.
Since the voltage across the diode (LED) is 2
volts and the battery voltage is 12 volts, then the voltage across the
resistor is 12-2 = 10 volts.
The diode is in series with the resistor, so the current through then
both is the same, 0.02 amps.
We now know the voltage across, and the current
through the resistor.