Ok, so now that my CCFL Angel Eyes project is complete, I started looking into the LED bulbs mods for the signals...so here's my take on this:

I've noticed that most people use 6 ohms/50W resistors, but complain that they get very hot...I thought that a higher ohm value would solve the heating problem, but I haven't seen anyone use 25/50/100 ohms @ 25W...everyone seems to be using the 6 ohms resistors...

I could be wrong, but here's how I see things:

The LED bulbs + resistors are supposed to draw less amps than the stock 21W bulbs...based on the Power law equation (P=V*I), a quick calculation reveals that when signaling, the stock bulbs draw a max 21W/12V = 1.75A...while at night (when on all the time) they'd draw a constant ~600-700 mA (since they operate @ aprox. 30-40% nominal power).

So, if the Resistor goes in parallel, based on Kirchhoff's current law ( I = I1 + I2 ), I would have to look at ensuring that the total Amp draw: I1 (LED) + I2 (Resistor) < 1.75 Amps? So now, if I go with the 6 ohms/50W, Amp2 > 2A, so no good...if I use a 100 ohm/25W resistor, then I2 = 125mA, which is OK...so, basically, I1 (LED) would have to draw a max of 1.625A, which I think they don't... so wouldn't the LEDs actually put less stress on the LCM if we use a Resistor with a higher value (not to mention solve the heat problems)?

I'm still debating on this one...any electricians around?

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Well... without all of the calculations, I can tell you that a parallel resistor may work, but you will never know when you have a bad bulb even if its a factory bulb. YOu have to put something that works that way the car works, and parallel is not the way the car works.

THe check is done with inline ( series ) resistance so in my mind, I would simply ohm out a normal bulb, get a resistor that has the same rating +/- about 5%, minus the resistance of the new LED, then put that sucker in SERIES with your bulb circuit at each connection. This will ensure notification if the led failed.

 

The LCM expects to find good working bulbs of specific wattage. 5W on the pilot lamps and side markers (when not factory LED), 22W in the corners, 55W on the high beams and foglamps (and low beams on non-HID cars). @12vdc, that gives you a specific current thanks to Mr. Ohm (V = IR). Any significant deviation is interpreted as a failure, which the LCM deals with in two ways: a "check lamps" warning on the console and a much faster blinking rate on the turn signal, both designed to draw your attention to the problem.

When retrofitting with LEDs, that draw a minuscule amount of current in comparison to regular bulbs, the load resistors are used to trick the LCM into believing that there is still a regular bulb in there. 6 ohms is used cause it is roughly the resistance a corner bulb has (P = VI = V(V/R) = V^2/R => R = V^2/P = 12^2/22 = 6.6 ohms). Can you use a bigger value, for a lower current draw and power dissipation? Yes, but will it still trick the LCM into thinking that the "bulb" is still good? You will have to experiment to determine where the go/no-go threshold lies.

Oh, yeah, and I'm still interested in learning where's the best place to screw the load resistors to avoid a blaze of glory...

 

Oh, yeah, and I'm still interested in learning where's the best place to screw the load resistors to avoid a blaze of glory...

I mounted mine on the fender wells away from wires and plastic, and have had no issues...I shot it down with one tec screw...

 

You could use a carbon pile resistor bank, like those used to test batteries or alternators. It is basically a variable resistor capable of low ohms and high power dissipation. So, you set it to 6 ohms, connect it in paralell to your LED bulb and then increase the ohms till you get a "bulb out" warning. Back it down a little bit for a safety margin and you've got your threshold.

Beware, beware, don't go below 6 ohms, cause then you're simulating a short and you'll blow a fuse.

 

Yes, that's a good idea... I was actually looking at some rheostats but they're a bit expensive

Here's also a link with some LED specs that I found: http://www.superbrightleds.com/specs/115x-x12x19.htm

 

No sense in buying one just for the sake of science. If you have access to a high school or college physics lab, they probably have one they may lend.

Even the newest Luxeon or C4 high power LEDs in cluster configuration are going to be leagues away from what the measliest light bulb sucks in terms of current.

Wonder if the LCM comes with the threshold "bulb out" current hardcoded, or whether it can be reprogrammed? Maybe a toggle switch? Or does BMW have a zilliion different models of LCM's depending on what lighting configuration each Bimmer is built with? Could someone with access to a trashed Bimmer from a junkyard get ahold of the LCM and tear it apart to see how it is built? It sure would be nice to do away with having to deal with the load resistors.

 

What about LED Flashers like these ?