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load resistors for LED turnsignal retrofits

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  • load resistors for LED turnsignal retrofits

    I measured the factory current draw of a standard 7440 bulb. The measurement at 12.5V was 1.8amps.

    I built an LED array that draws 600ma at the same voltage. I am still 1200ma short of the draw that the factory bulb does.

    Since I need to draw 1200ma MORE, can't I just use an appropriate resistor in parallel to draw this much more current?

    Every "LED load resistor" I've seen is 6ohms, which will put me well over on the current draw compared to factory.

    According to my math, 12.5V divided by 1.2amps should be achieved with about 10.4 ohms. Can't I just buy a 10 ohm resistor of sufficient wattage? The draw from the 10 ohm resistor, and my 600ma LED array should together put me at the factory bulb current draw levels- preventing "hyperflash".

    Has anyone tried this like this to prevent "excessive" draw?

    I was looking at these: https://www.mouser.com/ProductDetail...iNVzEC1WdQs%3D


  • #2
    For low current draw (like turn signals), a regular resistor may be ok. Another reason why it is in your favor is that turn signals only run intermittently. I did just that for an auxiliary turn signal many years ago; it still works.

    Or, you can get 3 of the 6ohm resistors. Put two in parallel, then 1 in series with them. For the parallel resistors, you get 1/ (1/6 + 1/6) = 3ohms. Then, put the 3rd one in series to get 3 + 6 = 9 ohms. Maybe it will be close enough? Or get 7, put pairs of 3 in parallel (making them effectively 2ohms), put them in series, making the sum 4ohms, and then put a lone resistor in series to get 4+6=10ohms
    Jul 2012 ROTM (3-way quad headlight) ; Sep 2015 ROTM (custom muli-lens 7" fogs)

    3-way quad wiring; foreground limiter; squirrel finder;

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    • #3
      Originally posted by satrya View Post
      For low current draw (like turn signals), a regular resistor may be ok. Another reason why it is in your favor is that turn signals only run intermittently. I did just that for an auxiliary turn signal many years ago; it still works.

      Or, you can get 3 of the 6ohm resistors. Put two in parallel, then 1 in series with them. For the parallel resistors, you get 1/ (1/6 + 1/6) = 3ohms. Then, put the 3rd one in series to get 3 + 6 = 9 ohms. Maybe it will be close enough? Or get 7, put pairs of 3 in parallel (making them effectively 2ohms), put them in series, making the sum 4ohms, and then put a lone resistor in series to get 4+6=10ohms
      The resistor I provided a link for is 10 ohms already, and they aren't that much money. I don't see why it wouldn't be any different but I didn't know if anyone had tried this before.

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      • #4
        My route was always to replace the turn signal relay for one that doesn't care fore the drawn current. Maybe you can find one that fits plug and play.
        Subaru Outback retrofit:
        http://www.hidplanet.com/forums/show...aru-Outback-H6
        License plate lights with dual brightness:
        https://www.hidplanet.com/forums/for...hen-backing-up

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        • #5
          Originally posted by Crossgolf View Post
          My route was always to replace the turn signal relay for one that doesn't care fore the drawn current. Maybe you can find one that fits plug and play.
          this way of doing things has been changed- the "etacs" module controls my signals.

          I did some interesting measurements the other day that your post has reminded me to share.

          I bought a variable resistor, 1-50 ohms. This is a huge 100 watt ceramic unit. I wired it in parallel with my turnsignals with the rear tail lights disconnected. With it set at 25 ohms- the car would not hyperflash. I tried to slowly and carefully increase it until I got into the hyperflash and the threshold is about 25 ohms. This tells me that if I draw about 500ma from the tail lights, my ETACS should not hyperflash. Since my tail lights draw about 600ma for the turnsignals, I am thinking I won't need a load resistor.

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          • #6
            Originally posted by Jp7 View Post

            this way of doing things has been changed- the "etacs" module controls my signals.

            I did some interesting measurements the other day that your post has reminded me to share.

            I bought a variable resistor, 1-50 ohms. This is a huge 100 watt ceramic unit. I wired it in parallel with my turnsignals with the rear tail lights disconnected. With it set at 25 ohms- the car would not hyperflash. I tried to slowly and carefully increase it until I got into the hyperflash and the threshold is about 25 ohms. This tells me that if I draw about 500ma from the tail lights, my ETACS should not hyperflash. Since my tail lights draw about 600ma for the turnsignals, I am thinking I won't need a load resistor.
            This is interesting. I've contemplated using the lowest load possible for a load resistor just like you... Although I never actually got to making LED turn signals.

            That's a pretty good practical way of finding the threshold for your car's hyperflash setting...

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            • #7
              I've also thought of using the LED circuit to trigger a transistor that would control the load resistor... My reasoning is that if an LED burns out, the entire string will be open circuit and cause the load resistor to not turn on also. This would trigger the OEM hyperflash system alerting the driver the turn signal is not working...

              I don't know if this is practical as I haven't dug into the transistor type or how exactly it would be wired up... Just an idea I previously though of.

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              • #8
                I checked and tested this in reality, and my lights DO NOT require a resistor to work OK. My fabricated LED boards draw enough current with the 600ma.

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