Resistor = (Battery Voltage \u2013 LED voltage) / desired LED current. So assuming a 12-volt power source and a white LED with the desired current of 10 mA; The
formula becomes
Resistor = (12-3.4)/. 010 which is 860 ohms. Since this is not a standard value I would use an 820-ohm
resistor.
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LEDs typically require 10 to 20mA, the datasheet for the LED will detail this along with the forward voltage drop. For example an ultra bright blue LED with a 9V battery has a forward voltage of 3.2V and typical current of 20mA. So the resistor needs to be 290 ohms or as close as is available.Jan 29, 2014
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To reduce voltage in half, we simply form a voltage divider circuit between 2 resistors of equal value (for example, 2 10K\u03a9) resistors. To divide voltage in half, all you must do is place any 2 resistors of equal value in series and then place a jumper wire in between the resistors.
