Connect with us

CAR LED headlight - load resistor (aka "CANBUS")

Discussion in 'LEDs and Optoelectronics' started by Dulus, Oct 7, 2019.

  1. Dulus

    Dulus

    4
    0
    Jul 15, 2014
    Hello,
    short story:
    I have a car light harness where i have added an 8Ohm wirewound 200W resistor parallel to the LEDs. When testing and i input 12V (from table power supply) on input of the wire harness i measure the voltage on the output of the harness (where the LEDs would be connected) is ~10V(voltage drop across the resistor ?, and as the LED is in parallel, it receives the same voltage, correct ?). When installed in the car, when i turn on the lights without starting the motor (volatage ~12V), the LEDs are ok, no problem, but when i start the car (~14V) the LEDs are flickering, sometimes only one, sometimes both , sometimes very short, sometimes for few minutes, but after a while they stop flickering. What is causing this and what should i change to end the flickering, i suspect the resistor values have to be changed, but i would like some opinions for what value.


    Long story:
    i am currently in the process of changing the lights in my car from halogen to LED, because the better halogens i was buying (Philips RacingVision 150% H7) were only lasting very short time. I now bought Philips X-treme Ultinon 200% H7. When i fitted them in my car they were performing very well, no flashes from oncoming traffic, good cutout etc, but were giving an error in the car, that the bulbs are burned down (obviously because the LEDs are drawing much less power than halogens). My car has sensitive "bulb failure detection".
    So obviously next step is to add some resistors to "fool" the car the halogens are still in place.
    I have seen some already made harnesses selling on some shops, but the cabling was thin, without any protection, and the resistors were very small, and surely would get very hot.
    So i created a wire harness myself.
    I bought H7 sockets, cables, cable sleeves etc., big 200W 8R wirewound resistors, glue heatshrinks etc.
    I created a harness, where the resistor is parallel to the LED bulb. As showed on the diagram.

    When testing and i input 12V (from table power supply) on input of the wire harness i measure the voltage on the output of the harness (where the LEDs would be connected) is ~10V (voltage drop across the resistor ?, and as the LED is in parallel, it receives the same voltage, correct ?). When installed in the car, when i turn on the lights without starting the motor (voltage ~12V), the LEDs are ok, no problem, shining bright, no flickering but when i start the car (~14V) the LEDs are flickering, sometimes only one, sometimes both , sometimes very short, sometimes for few minutes, but after a while they stop flickering.
    I was thinking it might be a voltage drop on resistor, so the LEDs are receiving toow low voltage to function properly, but why they are not flashing when light are on without motor/alternator running, but when motor is running they flicker.
    They were functioning ok when connected directly without the resistor harness.
    What is causing this and what should i change to end the flickering, i suspect the resistor values have to be changed, but i would like some opinions for what value.
     

    Attached Files:

  2. duke37

    duke37

    5,231
    718
    Jan 9, 2011
    I know very little about powerful leds.
    When running from a battery the supply will be steady. When the engine is started, the alternator will switch on and off as the battery is charged giving a variable supply. When the battery is fully charged, then the charge rate will be very low and the supply will be stable.
     
  3. Bluejets

    Bluejets

    3,489
    706
    Oct 5, 2014
    Calculations do not make any sense.
    8R to begin with is 1.5A whereas an original H7 lamp at 55W *2 = 110W = (roughly on 12V) 9.2A so if the ECU or whatever is monitoring the system, this resistor certainly would not fit the bill.
    Again 8R on 12V is 18W so do not seen why the 200W resistor.
    I think you are going to have to dig deeper into why the LED's are flashing as it would certainly not be voltage drop as your current load is well below standard.
    Try doing some meaningful tests rather than guessing.

    At the cost of these light replacements ( some showed AU$275.00/ pair) I'd be reconsidering possible move to a pair of aux spots, possibly with the same lamp if you must but with wiring that is isolated from the current monitoring system.

    One can easily trigger an aux relay from the existing system and run a separate fused supply directly from the battery, extra dash switch if required in the relay coil circuit.
    Just be aware before digging into the existing circuit that earth return systems can be tricky, especially those with a constant feed line to the headlamps.
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-