new to car wiring, leds

Hello, I'm new the forums. I've been searching forever for as much information as I can on properly wiring your own LEDs into a vehicle, to prevent shorting anything out or at the worst case scenario, burning it up. I just bought a Nissan pickup a few days ago, and I've decided that I'd like to light up the interior a bit with some 5mm super bright LEDs I bought a few years ago on Ebay. I learned about the whole parallel set up, and found a few calculators online. Unfortunately, I can't be 100% sure as to what the ratings are on the LEDs. I do know that I experimented with them with a 9v battery and some resistors. With pretty much all of the colors I have, I was able to power them one at a time with a 330 ohm resistor and two at a time with a 47 ohm resistor. I'm wanting to run wires directly to the battery terminals in my truck, 12v of course. I'm wanting to put together strips of 5, and would like to have at least 3 different strips in the interior, 1 each for the foot areas and 1 for the center area between the seats, totaling out at 15 5mm LEDs. I took a guestimate that the LEDs must run at least 3.6v forward, although if they were at least 3.2v forward, they would use 330 ohm resistors with a 9v power source, which they do. So I'm a bit confused as to what I should go with, should I assume they're 3.2v or 3.6v forward? Regardless, what I'm really concerned about is the life of the battery in my truck. If I'm running 15 LEDs at at LEAST 3.2v forward and wanting 20mA current, am I taking a risk of draining my battery while I have them on? I'd of course install a switch somewhere and use them only at night, and turn them off when I'm done. Are car batteries able to handle this? Thanks for your help!!

20mA is fairly insignificant to a battery - or it should be. Keep in mind that a single string (ie, series string) of 20mA LEDs is still only 20mA. A 3W bulb is about 250mA - over 10x as much. (P=VI => I=P/V = 3W/12V = 1/4Amp = 250mA) LEDs were traditionally ~1.7V each but recent super-brights etc are double that - ie 3.4V. But ratings like 3.4V are usually "nominal" or based on some characteristic - say 20mA, or Lumens or colour output etc. Whilst LED resistor calculations should be say (12V - 1.7V)/20mA etc, many ignore the 1.7V and just use 12V/20mA or 10V/20mA etc. Can we do that? Well considering most 12V systems are really 12.7V or 14.4V etc, then yes, we can.     Many don't even use resistors - they merely connect (6 to) 8 1.7V LEDs or (3 or) 4 3.4V LEDs across the 12V supply. However, a resistor can be worthwhile as it will "absorb" much of the voltage variation. In short, LEDs are not very critical as to their voltage. They may not be critical in respect to current either. Though some (manufacturer's) sources say "20mA MAXIMUM" etc, that doesn't mean that above 20mA they will blow up, it just means above 20mA, their quoted lifetime of X,000 hours won't be met, at 25mA it may only be (X/2) thousand hours etc. (Just like max operational temperatures - each 10 degC (20F) may mean a halving of their life expectancy.) Some might blow up at 20.001mA - but that is highly doubtful. Alas you have to refer to the LED spec sheets (and decipher them!) Some will argue aspects of the above. I had one such "discussion" with an egotistical ignoramus on another site. (I'm was tempted to write "ignorant loser", but AFAIK he only lost this one argument - I haven't looked at his other threads. However a recent reply suggests.... ) One benefit from the discussion was that the nee-arguer provided a great datasheet for a LED - namely digikey-pdf/Data Sheets/Osram PDFs/LW P4SG.pdf. (643kB download) It shows the variation of current, voltage, luminous output etc as opposed to the "normal" one-line LED data we usually see.    The above isn't an answer to your question.... Let's see. 12V/3.2 = 3.75; say = 4: 4 x 3.2 = 12.8V; 4 x 3.6V = 14.4V (perfect - nominal max voltage in a 12V system) So no resistor... If 3 LEDs in series, 3 x 3.2 = 9.6V; or 3 x 3.6 = 10.8V. Assume 3@10.8 is max voltage allowed in a 14.4V system, hence need to drop 14.4-10.8 = 3.6V@20mA = 180 Ohms (V=IR => R=V/I = 3.6V/20mA = 180; P = VI = 3.6V x .02A = 0.07W => .25 or .5 => 1/4 or 1/2 Watt resistor at 180 Ohms for 3x 3.6ish LEDs on 14.4V. Or try 4 LEDs with the same 180R (180 Ohm) resistor and see what you think. Or 4 LEDs without the resistor.... Or a smaller resistor.... (150, 120, 100, 82, 68, 56 etc. The smaller the resistor, the lesser its effect against voltage fluctuations, but I'd probably use 4 LEDs and no resistor anyhow....)

So I'm pretty safe to set up a parallel array of 15 LEDs at 3.2v each without killing the battery? It would really suck to flick it on and then have the car die on me out in the country *_* Also, whats the in line fuse that everybody talks about? Do I need to run my wires to the fuse box or something? As I said, I'm new. Thanks!!

It depends how long for, what size/condition battery, and how many parallel strings. 15 x 20mA LEDs in parallel is (15 x 20mA = ) 300mA which is equivalent to a 4W bulb (load) and can flatten you battery within a day of a few. 5 parallel strings of 3 LEDs is only (5 x 20mA = ) 100mA or 3 x less so the battery will last probably over 3 times as long. Find your battery specs and see how long it reckons it will last. Each "string" of LEDs will be 20mA so that's 0.02AH each. 15 strings is 0.3A or 0.3AH. Of course that's for a good/new battery - it doesn't mean your battery won't be flat ... one day (especially that first cold day!). You'd be best to fit a low-voltage cutout, else a dual battery set up - though the 2nd battery should still have an under-voltage cutout to preserve it, and should be a deep-cycle type. Fuses are the sacrifice that blows instead of wiring or batteries if there is a short or fault in the system. If a +12V wire falls off or wears through and shorts to ground (eg - via the body), it will melt and probably cause a fire or other damage unless something limits its current. Hence fuses are placed at the start of most circuits.

I went and used this calculator: http://led.linear1.org/led.wiz 12v Power Source, 3.2v each LED, 20mA forward current and 15 LEDs. Gave me 5 sets of 3 with 120 ohm resistors for each set. Rest of the stats:     * each 120 ohm resistor dissipates 48 mW     * the wizard thinks 1/4W resistors are fine for your application     * together, all resistors dissipate 240 mW     * together, the diodes dissipate 960 mW     * total power dissipated by the array is 1200 mW     * the array draws current of 100 mA from the source. As far as my Dad tells me, while the car is running, the alternator will be replenishing all power, meaning, as long as I only use the lights while the cars running, I won't be able to kill the battery. Remember, I'm new to a lot of this stuff, so even though this may be simple knowledge to some, I'm just now learning it. My final question is about placing a fuse in the line. As far as I know, I run my positive from the battery to fuse box, place a fuse, then run the line on to my LEDs, is this correct? If so, how can I tell what type of fuse I need, since there's different ratings? Anybody have any tips? Thanks again!

With the alternator running - no problems. You could probably run well over 1,000 LEDs without problem. Five 20mA strings = 5x20 = 100mA. Simple. That's 12 x 0.1 = 1.2W at 12V, and up to ~ 1.7W at 14,4V (normal max battery voltage when charging). A 1A fuse is enough to run the LEDs, but that is impracticably small in this case. Just run the power wire from a suitably powered circuit from the fusebox. Match your wire to that fuse - ie, if a 10A fuse, use a 10A or larger wire - ie, 20AWG wire, or 16AWG for 20A etc (the theory being that if the the wire or LEDs short to ground etc, the fuse will blow before the wire melts). Others may have better ideas....

Oldspark, i like how you refer to the fuse as a sacrifice! LOL! commit your way to jehovah and he will act in your behalf. psalms 37:5

Well Mr T+T, what would you call it? I certainly don't call it a donation! I no longer take chances - I sacrifice whatever I must to appease the great Smoke God. Just the other week I learnt what a sacrificial anode is - it's the (smaller) positive lead terminal on a battery. My mate sacrificed it when he jumpered his alleged flattery. Unfortunately his battery wasn't flat.    Unfortunately he connected the jumper battery the wrong way. Unfortunately it was MY battery. That incident was so stupid the Smoke God delegated to the Fire God. But it did get me thinking... what is the sacrificial connection on batteries with solid connectors (ie, not lead)? Maybe internal lead? Forget the Ori. It's those smoke & fire gods that are to be feared.