Wiring 4v automotive LED's

[TheNotoriousAG]

I'm working on an automotive project where I need to power six LEDS.

 

Each LED needs 4v, and the fusebox will be providing 12v.

 

So i need to figure out how to wire all of this together with each LED getting the 4v it wants, and then I can handle the relay to have them controlled myself.

 

Thanks for the help, I wish i had more information on this, but the instructions have next to nothing in them in terms of how to do this.

[W-L]

6 minutes ago, TheNotoriousAG said:

I'm working on an automotive project where I need to power six LEDS.

Each LED needs 4v, and the fusebox will be providing 12v.

So i need to figure out how to wire all of this together with each LED getting the 4v it wants, and then I can handle the relay to have them controlled myself.

Thanks for the help, I wish i had more information on this, but the instructions have next to nothing in them in terms of how to do this.

Simply wire 3 of them in series what will give you a total voltage of 12V required to power the 3 LED's. If they must require a 4V individually you can always get a buck converter and have it set at exactly 4V that will also provide a regulated supply to the lights.

[yathis]

Buck converter that downgrades 12V to 4V

buy on ebay, a dime a dozen

[Unimportant]

LED's are current controlled devices, not voltage controlled. Simply wiring 3 in series is not correct, they'll still allow too much current to flow, drastically shortening their livespan. What current are those LED's rated for ?

[mariushm]

LEDs are current driven devices, not voltage driven.

The 4v is the approximate voltage where they'll be the brightest. Some leds would be brightest at 3.8v, others at 4.1v and so on...  This voltage is called "forward voltage"

 

Your car will give you a voltage between around 10v and up to around 14v, depending on your car's battery and the alternator that charges the battery.

If you connect 3 leds in series to get a forward voltage of 12v, the leds may or may not light up if the voltage from the battery is less than 10v.

 

Figure out the current of each led, and then you can use a resistor in series with a number of leds in order to limit the current going through the leds to a specific value.

For example let's say the leds are rated for 100mA or (0.1A) and you'll make series of 2 leds and a resistor.

You have the formula V = I x R  where i = current , r is resistance.

 

So 12v - (2 leds x 4v ) = 0.1 A x R => R = (12 - 8 ) /0.1 = 40 ohm so you want to use a resistor as close as possible to 40 ohm, which would be 39 ohm. The power wasted in the resistor would be P = IxIxR = 0.1 x 0.1 x 39 = 0.39w so you should use a resistor rated for 0.5w or 1w

 

Now with 390 ohm let's take worst and best case scenario ..

10v from battery :  10v - 8v = ? A x 39 = > I = 2/39 = 0.051 A 

12v from battery : 12v - 8v = ? A x 39 => I = 4/39 = 0.102 A

14v from battery : 14v - 8v = ? A x 39 => I = 6/39 = 0.153 A

 

I would probably the next higher resistor value of 47 ohm, which would give you a worst case current of 6/47 = ~ 125 mA through the leds and about 85mA at 12v.

 

If you want the best for your leds, you would put them all in series and you'll use a led driver circuit which will boost the 10v..14v to around 24v at 100mA , which is what your 6 leds will require. The led driver will constantly adjust the voltage to keep the current consumption at the set current amount.

 

[TheNotoriousAG]

I'm going to be upfront with you, that I'm not the most well versed with things like buck converters or LED controllers.  I was under the impression that all I needed was to wires in some resistors in a specific manner and I was good to go, but this is either something that I'm willing to learn, but will need help, but I understand if you recommend I find an automotive electronics installer instead.

 

As far as trying to do this, which I still intend to do, This is all of the instruction they've given me, with the LED's.  Thanks so much!