DRL LED Bulbs....why 6ohm resistors and not 10ohms? (EE degrees, please chime in)

[advan24r]

It seems most LED sellers recommend the 6ohm but if you do the math, for a 21W OEM bulb that is powered by a standard 12V, won't a 10ohm resistor be closer to actual amperage draw of the car?

[FraggyA4]

V=IR, P=VI 21=12*I..So I=1.75 12=1.75*R....So R=6.85 Ohms so the suggested 6Ohms is correct

[Schweini]

It seems most LED sellers recommend the 6ohm but if you do the math, for a 21W OEM bulb that is powered by a standard 12V, won't a 10ohm resistor be closer to actual amperage draw of the car?

If your battery is measuring 12v, especially once the cars started, you have more problems than not knowing the proper resistance for your circuit in which I've already beaten you over the head with

[advan24r]

This was my math..

Power (Watts) = I (current Amperage) * Voltage (V); 21W (OEM BULB) = I * 12V; I = 1.75A

Most LED Bulb Draw; atleast based on my research (i.e. superbrightLEDs.com) is 250-290mA

Voltage (V) = I (current Amperage) * R (resistance); 12V (standard car output) = .29A * R = 41.38 ohms

Wiring in Parallel:

(R1 * R2)/(R1+R2) = R3

R1 = Resistance of the LED
R2 = would be the Ohm of your resistor (we will use 6 ohms and 10 ohms to compare)

(41.37*6)/(41.37+6) = 5.24 Ohms

V = IR

12V = I*5.24 Ohms ; I = 2.29 A vs. 1.75 A (OEM draw)

now using a 10 Ohm to calculate
(41.37*10)/(41.37+10) = 8.05 Ohms

V=IR
12V = I*8.05 Ohms; I= 1.49 A vs. 1.75 A (OEM draw)

So can you see where I was coming from, the difference in the 10 Ohm resistor is much closer to the OEM vs. the 6 Ohm. Ideally, if I can use a 7 or 8 Ohm it would be EVEN BETTER.

Let me know if I am calculating it correctly.

[Schweini]

math's fine, values are wrong. LED's carry next to no resistance values, you don't need to calculate them in.

if your battery is measuring at 12v your car's not running

in all seriousness, resistors cost no more than $5. Get 2, or 3 different ones and see which works better if you still aren't sure about it.


edit: BTW, you're putting the resistors into a series circuit, taking the place of the load (the OEM lamp). Solving for a parallel circuit is not the case here

[advan24r]

math's fine, values are wrong. LED's carry next to no resistance values, you don't need to calculate them in.

if your battery is measuring at 12v your car's not running

in all seriousness, resistors cost no more than $5. Get 2, or 3 different ones and see which works better if you still aren't sure about it.


edit: BTW, you're putting the resistors into a series circuit, taking the place of the load (the OEM lamp). Solving for a parallel circuit is not the case here

LOL, am I driving you nuts w/all this now =P I do apologize.

Once a bulb out message pops up, do I need to take it to the dealer to get it reprogram or does it just disappear if everything is fine?

Another thing is, if the OEM bulb metal bayonet states 21W, can I assume that the DRL OEM bulb is 21W?

[Schweini]

LOL, am I driving you nuts w/all this now =P I do apologize.

Once a bulb out message pops up, do I need to take it to the dealer to get it reprogram or does it just disappear if everything is fine?

Another thing is, if the OEM bulb metal bayonet states 21W, can I assume that the DRL OEM bulb is 21W?

Nah, I'm actually enjoying this. It's rare I get to help out others on this forum because my car theory isn't strong. When it comes to electricity, being an electrician helps and I can finally contribute some worth to the boards.

your bulb out error will go away once it's corrected (via: putting the oem lamp back in, or turning off the lights)

Another thing is, if the OEM bulb metal bayonet states 21W, can I assume that the DRL OEM bulb is 21W?

No. Everything has it's own power rating depending on manufacturer specs. Remember when you asked me if its okay to put a larger wattage rating resistor in a circuit? Well if the resistor was rated for 5w and the circuit only drew 2w (which is totally fine), would you assume the circuit drew 5w since the resistor you are looking at says so?

[FraggyA4]

The resistance of an LED is extremely low so low it's negligible. Different resistances will cause your LED to be brighter but also may cause it to burn out faster. If you really want to know about this PM Phil he did multiple test with LED's for his lights.

[plokm]

i been running 6 ohm resistors over 3 years now with no problem

[advan24r]

like what i posted in the other thread...the majority wins...will try out the 6 ohms. If not, I'll be more strict w/my calculations and go w/that. The idea of splicing my stock harness actually played a bigger role and also laziness on building my own. I figured the cost to build one is similar to the price of this. the built in load resistor according to the store is 6Ohm/25W

http://www.ledlight.com/1156_ba15s_i...equalizer.aspx

[jayulzvern]

^ I love those adapters, such a clean way to do a retrofit, I also love how they brand those boxes as some sort of complicated in this case "decoder" when its just a resistor in line.

Here's my problem LED's are better for 2 reasons, first they use less power and second they have a longer life. The longer life will never off set the higher cost of the bulb and what ever required adapters, so that benefit is off the table. The second reason to switch to LED's is they use less power but if we have to put a resistor in line so the car is happy then the resistor is eating up any power savings. LED's are great and have ideal applications but I don't think for our cars it makes any sense unless you like the color. IMHO

[advan24r]

Got to revive this again....if an led says 24watts...do I need to review over the resistor that I used for my previous led that was used? The current resistor I am running for my DRL is 8ohms/25w. I am looking to change the led to a 24w Led bulb now.

[Schweini]

Got to revive this again....if an led says 24watts...do I need to review over the resistor that I used for my previous led that was used? The current resistor I am running for my DRL is 8ohms/25w. I am looking to change the led to a 24w Led bulb now.

not even an issue at all