14 3v LED's powered by 12-15vdc

[alaskanvegi]

I'm putting togather a control panel for my truck and I have a packet of 3v LED's that I want to use to illuminate the switches, guages and labels. The LED's came with 470 ohm limiting resisters. I need to use 14 LED's. How can I connect these LED's to the vehicles 12-15vdc electrical system so that they don't burn out?

[audioguru]

Hi Alaskan,
Welcome to our forum. ;D
If your LEDs are a normal size then their max current rating is probably 30mA. When the vehicle's electrical voltage is 15V then the current in each LED is limited to 25.5mA by its 470 ohm resistor so should be safe without burning out. The 470 ohm resistors should be 1/2W because 1/4W resistors will get too hot.

[alaskanvegi]

These are a 5mm diameter, instructions on their package reads, "A 3 volt power source is recommended. 470ohm limiting resistors are included to prevent burnout. Hook up in series with LED for up to 15VDC." I hooked one direct to a 12vdc power source that read 15vdc on my multimeter and it flashed once. The package held 18 LED's and 18 470ohm resistors. Do I have to use all 18 resistors to get 15vdc down to 3vdc? Do the LED's drop voltage themselves when wired in series? What is the simplest way to use 14 of these LED's?

[audioguru]

The simplest way to connect 14 of your LEDs is to connect a 470 ohm resistor in series with each one. Then the combinations of the LEDs with resistors connect to the electrical.

You mentioned series connections. Three LEDs can be wired in series then in series with a different resistor. The brightness will change a lot when the electrical voltage changes and the wiring will be much more complicated.

[alaskanvegi]

Thanks, I think I got it figured out.

[windoze killa]

You probably don't want to connect them in series. In this configuration if one fails none of them wil remain lit. This will also mean you will have to check each one to find the dead one. If you have them in parallel then only the one that dies will be unlit. Although these LEDs have a built in 470ohm resistor it maybe wise as already stated to include another. I would use a potentiometer on one so you can adjust it to an intensity you like and then measure the resistance and then replace it with a resistor that is close to that value. You would put this resistor into each LED leg.