A note about LED's and power supplies. You need to match the correct resistor to the LED on the positive side of the LED (should be a longer lead) Use this simple formula to figure out the resistance.
R=V/I
Lets say our LED is 3.8v and 30mA. That's all the info you need, other than the voltage coming out of your DC power source. My Rivergate 30amp puts out 13.5V so that's what I use in my formula.
So plug in the values to the following equation.
(DC power supply voltage) - (LED Voltage)/ LED Amperage (In mA) *1000
So using the example above -
(13.5V-3.8v/30mA)*1000 = Resistor value needed to buy
This comes to 323.3 ohms. So search out a resistor that is close to 323.3 ohms (most likely it will be a 330 ohm resistor) LED's doesn't matter if they're 1/4 or 1/2 watt resistors.
That's my electrical lesson for the day
John