LED wiring

[FunkZ]

I'd like to do some LED wiring. If I buy a bunch of LED's that are 2.8v min, 3.8v max, and string them in series (longer, positive leg of one to negative of another, and so on) using 4 LED's per series, that should give each one roughly 3v when I connect it to a +12v source, correct?

When a LED is prewired is the white wire the ground and the colored wire the positive?

Also, will a 3mm LED fit through a standard case screw hole or would I have to ream them out a little? The screws are #6-32 which translates to about a 3.5mm thread but that's the outer edge of the threads.

[silenze]

This site is a good reference, has a lot of examples: http://www.quickar.com/ledbasics.htm

Says the longer lead is generally the positive lead.

I think first you would have to wire up one or two LEDs to a circuit and then measure the current before and after each one to determine the loss at each LED, at a certain point the voltage required at the source to get enough volts to power the last LED in your circuit may be impractical...

Should read up on that site, good stuff...

I would drill the holes out a tiny tiny bit at a time until they slide in without enough force to worry about breaking them while install/removing and probably stick them in place with a drop of hot glue or something...

As for white and colored wire.... I believe most LEDs are sensitive to the polarity of the current and won't light up when wired reversed, so that should be easy to figure out..

[FunkZ]

So that site calculator says, if I wire 4 3v LED's in series (as I mentioned doing) and supply them with +12v from the power supply, then the resistance needed is 0 Ohm. So in this case I wouldn't need to fool with a resistor, correct?

When the manufacturer gives a forward voltage rating of 2.8min, 3.3typ, 3.8max, does that mean each LED needs at least 2.8v to turn on, and should get no more than 3.8v or it will burn out? (and voltages in between will regulate how bright or dim the LED is)

Or does that mean the variance between each LED's forward voltage can be between 2.8 and 3.8 and will be different for every LED? If this is the case then each and every single LED will need to be measured for voltage drop and then figured into the calculations to determine the proper resistor for each series?

I would think the former is true rather than the latter. If calculating the resistance needed to run a series of LED's was so difficult then how would it be possible to power most LED case fans? Using a fan speed controller you can vary the voltage to the fan from 7v to 12v and the fan LED's simply get dimmer or brighter as you adjust the voltage.

Oh BTW I tried a 3mm LED in a standard case hole and it will not fit without drilling out the hole slightly. You could probably get the LED to thread into the hole by turning it with a pair of needlenose but this would carve up the exterior of the LED jacket.

*edit* Here's a quote from another site on LED basics:

The other arrangement is to run the LEDs in serial, i.e. daisy chained off one another. In this arrangement you run the risk of the old Christmas light problem that if one goes out they all go out. It however will allow you avoid the use of resistors. To do this simply add the number of LEDs until the voltage they use is greater than or equal to the pack voltage.

So according to that if I'm using 4 blue LED's in series because typical voltage for the LED is 3.3 and I'll be supplying it with the +12v from the power supply (which could actually be 12.1 or even 12.2) then I don't need a resistor.

[silenze]

"So that site calculator says, if I wire 4 3v LED's in series (as I mentioned doing) and supply them with +12v from the power supply, then the resistance needed is 0 Ohm. So in this case I wouldn't need to fool with a resistor, correct?"

-- Nah you shouldn't need one for that setup, generally they're used for fault isolation, an anchor point on a PCB board, or for programming purposes.

"When the manufacturer gives a forward voltage rating of 2.8min, 3.3typ, 3.8max, does that mean each LED needs at least 2.8v to turn on, and should get no more than 3.8v or it will burn out? (and voltages in between will regulate how bright or dim the LED is)"

-- Correct. I believe the 2.8v rating for the LED would be the minimum voltage required for the LED to be lit up but not at it's brightest, and 3.8v the max voltage/brightness they recommend for extended use as not to burn it out.

"Or does that mean the variance between each LED's forward voltage can be between 2.8 and 3.8 and will be different for every LED? If this is the case then each and every single LED will need to be measured for voltage drop and then figured into the calculations to determine the proper resistor for each series?"

-- Your other train of thought is correct, but it wouldn't be a bad idea to measure those points regardless of what the manufacturer says, just to make sure that the characteristics are all similar -- but that would be pretty obvious when you put power to the circuit, varying brightness levels between them..

"Oh BTW I tried a 3mm LED in a standard case hole and it will not fit without drilling out the hole slightly. You could probably get the LED to thread into the hole by turning it with a pair of needlenose but this would carve up the exterior of the LED jacket."

Yeah, fragile little things... I'd rather have to put a layer of black electrical tape around them for a snug fit..

[FunkZ]

Excellent. I will plan on running 4 3.3v LED's per series then and it should be a snap to wire up to the +12v.

Now I just need to disassemble the case in order to drill out the holes. :(

[T-shirt]

what the resistor is limiting is current (amperage) while the calc show it as a balanced equation (i.e. the "natural" resistance is = to the load) you may find (due to a low or high rail, or parts(the Led's) normal variation from spec) that it doesn't light, or(worse) bursts into flame)
mocking up your sequence (breadboarding) is a cheap way to check it out