E46 Fanatics Forum banner
  • Hey everyone! Enter your ride HERE to be a part of October's Ride of the Month Challenge!
1 - 13 of 13 Posts

·
Registered
Joined
·
307 Posts
Discussion Starter · #1 ·
Just curious if anyone has attempted to tackle this. I am figuring it isn't that hard. I'm an EE and hoping to figure something out soon. I will try to experiment with different reistors and such. My main problem is the heat build up as just wrapping a simple resistor in electrical tape wont work :). I beleive u just need to get a 12v rated 6 ohm resistor and wire it in parallel with the led light.. i.e. have one end on the positive and one end on the negative. I"ve never actually played around with car resistors and such so if anyone has any experience it would be appreciated.
 

·
Registered
Joined
·
210 Posts
Use power resistor bro. They work. My car is a lilving example of this.
 

·
Registered
Joined
·
77 Posts
I think he means a 1/2 or 1 Watt resistor. Resistors are rated both in ohms and power dissipation. Bigger the resistor, more power it can dissipate.
Why do you want to wire one in parallel? The resistor goes in series with the LED as a current limiter, no?
 

·
Registered
Joined
·
1,962 Posts
Roose said:
... Why do you want to wire one in parallel? The resistor goes in series with the LED as a current limiter, no?
No, LEDs have higher resistence and draw less current compared to incandescent bulbs. To make the car computer think the bulb is still there you need to lower the resistence by wiring the resistor in parallel.
 

·
Registered
Joined
·
77 Posts
Jspeed said:

No, LEDs have higher resistence and draw less current compared to incandescent bulbs. To make the car computer think the bulb is still there you need to lower the resistence by wiring the resistor in parallel.
Ah, so how are we wiring the LED in? Why not just tap off a source of 12V somewhere, then through the current limiting resistor on to the LED? Which bulb is being replaced by the LED?
I guess I'm missing a piece of information here. :)
 

·
Registered
Joined
·
950 Posts
Roose said:


Ah, so how are we wiring the LED in? Why not just tap off a source of 12V somewhere, then through the current limiting resistor on to the LED? Which bulb is being replaced by the LED?
I guess I'm missing a piece of information here. :)

The resistor is not the current limiter for an LED. LED = very high resistance. Incandescent bulb = small resistance. The car thinks the bulb is burnt out because the resistance is infinite when the filament breaks in a regular bulb. The resistor has to be in parallel to allow the total resistance of the wiring to be comparable to a regular bulb. It's a simple resistance formula.
1/Parallel resistance = 1/r1 + 1/r2
Series would just combine like so: R = r1 + rLED making an even higher resistance and no change to the problem.
:thumbup:
 

·
Registered
Joined
·
77 Posts
coolio4433 said:



The resistor is not the current limiter for an LED. LED = very high resistance. Incandescent bulb = small resistance. The car thinks the bulb is burnt out because the resistance is infinite when the filament breaks in a regular bulb. The resistor has to be in parallel to allow the total resistance of the wiring to be comparable to a regular bulb. It's a simple resistance formula.
1/Parallel resistance = 1/r1 + 1/r2
Series would just combine like so: R = r1 + rLED making an even higher resistance and no change to the problem.
:thumbup:
I understand. My question is - which "bulb" does the car think is burned out? How is the LED wired into the electrical system?

I would wire in a LED like this:
+12V power source -------> Resistor ------->LED --------->12V return
The car doesn't care nor know about any circuit at all.

This is the way you govern how much current flows through the LED so as to not go over it's rated value.
LEDs are not typically high resistance devices since they are forward biased. The resistance is just not low enough to signal the computer that a "bulb" is indeed in place and working.
 

·
Registered
Joined
·
950 Posts
Roose said:


I understand. My question is - which "bulb" does the car think is burned out? How is the LED wired into the electrical system?

I would wire in a LED like this:
+12V power source -------> Resistor ------->LED --------->12V return
The car doesn't care nor know about any circuit at all.

This is the way you govern how much current flows through the LED so as to not go over it's rated value.
LEDs are not typically high resistance devices since they are forward biased. The resistance is just not low enough to signal the computer that a "bulb" is indeed in place and working.
I think he's referring to LED turn signals. Without a parallel resistor the car thinks the flasher is burnt. Correct me if I'm wrong here.
 

·
Registered
Joined
·
77 Posts
coolio4433 said:


I think he's referring to LED turn signals. Without a parallel resistor the car thinks the flasher is burnt. Correct me if I'm wrong here.
That's it! Now I understand. I've been thinking this was for the Angel Eyes mod that people have been doing.

Yes, you will need power resistors for this type of application. They look like little gold metal guys - maybe 1.5" to 2"
Let's see what a ballpark rating would be:
12V / 6 ohms = 2 amps
(2 amps)squared times 6 ohms = 24 watts. Wow!
Ok, so a 25 watt will do. They are called wirewound, metal clad resistors and they look like this:
 
1 - 13 of 13 Posts
Top