Resistor Values?

[jeeep]

I am using this great tool found here to calculate the value of resistor needed. For example with 12Vdc supply voltage, 2 forward voltage (per led), 20mA of forward current, 5 leds it recommends a 120ohm 1/8 resistor. Can I use a 1/4 watt resistor? Would it have any effects on these calculations? Many thanks

 

[LEDependent]

If these LEDs are in series, then you add the voltages of each LED (2V sounds low, unless they're red/orange/amber). 2*5 = 10V across the LEDs. Since your power supply is 12V, you have to absorb those extra 2 volts when running 20mA through the circuit. Using the equation Voltage = Current * Resistance, you use 2 = 0.020 * R. R = 100 ohms.

The calculator incorrectly instructs you to use a 120 ohm resistor (it goes for the NEXT highest resistor, even if the calculated resistor matches exactly).

A 1/8 watt resistor just means that it can handle the heat from 1/8 of a watt passing through it. There is no harm in using a higher-wattage resistor.