HOWTO - Calculate LED resistance

Ok let me explain how to calculate the optimal settings for your circuit. First of all you have to look at the settings of your LEDs: Each of it needs 2V and 20mA. For two LEDs in series, it means the two need 4V (Voltage is added because at each LED there is a drop of 2V) and 20mA current. The current stays for both because it flows through both without loosing current ;) So now you know your Powersource has to bring at least 4V! ;) Next is a bit specific knowlege: The Powersource is in AC mode, so you should consider that 4V would just be the peak of the amplitude and just for a tiny tiny little time there is actually 4V... therefore the LEDs would just light very shortly. To get around this we define how long during a positive wave the LED should light: We can define e.g. when the wave reaches a quarter of the full amplitude, therefore our peak voltage should be 4V x 4 = 16V. Now comes the last but most important part: LEDs need a resistor in series to limit the current to the specified current the LED needs. You ALWAYS have to use a resistor or otherwise the LED bursts! ;) Calculating the resistor is easy when we have DC, but what about AC? Well you just use the peak voltage. Because LEDs are just in positive Voltages active there are exactly 2 LEDs in series active at any time. The current needed is therefore always 20mA. Now calculating: U = R x I --> R = U/I = 16V/20mA = 800 Ohm. Congratulations! ;)

published 11 years ago