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I recently installed an H7 Led Headlight kit 12V/25W in place of a standard halogen 55w bulb. Lights are great, but they flash periodically during can-bus Cold/Warm checks to see if a bulb is out.

I was advised to install a 55Watt 6Ω Resistor that will prevent the LEDs from flickering during checks, but work normally when switched on.

Now from my basic electronic knowledge I calculated using Ohm's-law the resistor needed as:

for the halo I1 = 55W / 12V = 4.58A and and the LEDs I2 = 25W / 12V = 2.08A

so the difference of 30W is 2.5Amps that translates into:

R = V/I = 12 / 2.5 = 4.8 Ω

My question is (provided my calculations are correct :P) will the 6 Ω resistance cut off light output from the LEDs or was it indeed correctly recommended or I should look for a ~5 Ohm ?

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  • \$\begingroup\$ Are you sure that the check is done with full 12V and up to the max current? I have seen cars that check with 20mA constant current and 6V compliance just for continuity. Absorbing that into a cap that slowly bleeds was much nicer. \$\endgroup\$
    – PlasmaHH
    Dec 7, 2016 at 13:03
  • \$\begingroup\$ You are right the check is only 2-5V for continuity, but don't know about the current though. So if that is the case what size of cap would be needed? \$\endgroup\$
    – Mike D.
    Dec 7, 2016 at 14:32
  • \$\begingroup\$ Measure and experiment, there are so many ways they do it. Sometimes a 2000µF with a 10k resistor was working, sometimes just a 1k resistor in parallel with the lights. There is no standard to do these checks, the only thing that they all need to do is not to light up incandescent lights, so it can't be that much energy. \$\endgroup\$
    – PlasmaHH
    Dec 7, 2016 at 14:39

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The LED light has some electronics in it and that confuses the headlight-OK-current detector. The resistor sets a certain minimum current to fool that detector.

That 6 ohm resistor will actually conduct 12 V/6 ohm = 2 A and dissipate 12 V * 2 A = 24 W, so not 55 W !

It depends on your car's headlight-OK-detection-current if that 6 ohm will do the job. It could be that 1 A through a resistor is already enough, then you could use a 12 ohm resistor. But it can also be that it needs that suggested 6 ohm.

I doubt if going as low as 4.8 ohm is really needed, that also wastes a lot of power.

The thing to do is to try it out and see what works.

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  • \$\begingroup\$ Ok, probably will have to start with 6ohm and see if that is enough to stop the LEDs from flashing, as suggested. \$\endgroup\$
    – Mike D.
    Dec 7, 2016 at 14:39

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