# 3.5v LED's on 12v battery

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#### felix the cat

##### New Member
I am trying to use 16 - 3.5v LED's on a 12volt system.
What is the best way to drop the voltage to 3.5 ?

##### Banned
The best way is to use an LED driver IC. You should be able to find constant current drivers out there that will supply 3 LED's in series with the right current. Simple resistors will work, but again I'd use 3 LED's in series to avoid having to throw all that power away in resistors completely. With resistors though the lights will get dimmer/brighter depending on weather you're running straight off the battery or if the alternator is running, which assumes this is for a car.

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#### Hero999

##### Banned
A more simple method is to connect three in series and use a current limiting resistor.

For 16 you'll need five strings of three, plus another single LED.

#### felix the cat

##### New Member
I'm confused !
If i got 16 led's, how many drivers would i need ?
Also the 16 LED's will be 25' in the air (wind generator)

Would it be better to just use 12v leds ,and forget about the 16
3.5 leds i have made up .?

#### Mickster

##### Well-Known Member
They don't make 12V LED's (AFAIK just incase...)

It's the current-limiting resistor that allows an LED, or a string of LED's, to be run from different voltage sources.

##### Banned
Use 3 3.5 volt LED's in series. That's 10.5 volts of diode drop from the LED's right there, good place to start. Obviously you'd need to do them in three's so you'd need 18 LED's 3X6, or 15 for 3X5. So you would need either 5 or 6 drivers (you may be able to find a multiple driver output IC)

I hate all these car applications that use 3.5 volt LEDs and a resistor to drop the rest, the entire point of LED's is they're efficient, if you add a big series resistor to control current you're throwing the heat away yourself, might as well just use a normal incandescent bulb!

#### felix the cat

##### New Member
Thanks very much to everyone.
I just went ahead and added 470 ohm resisters to each. for the meanwhile.
They look very bright. will leave them hooked up overnight and see how they do.

This way I'll only need two wires to run up the anemometer pole

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##### Banned
You did ask for the 'best' way not the easiest =) If your have free air cooling your resistors will likely work forever if they're rated properly. Just keep mind mind how much power you're throwing away. If you're okay with that, the the circuit is fine the way it is.

#### Hero999

##### Banned
It isn't even the easiest or cheapest.

If you ask me connecting the LEDs in series is just as easier and less resistors is cheaper.