= (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
= (12-3.4)/. 010 which is 860 ohms. Since this is not a standard value I would use an 820-ohm resistor
.Aug 19, 2012
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
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.