Load resistors for converting incandecant bulbs to LEDs

Discussion in 'Automotive Electronics' started by Vince Janeczko, Mar 22, 2016.

  1. Vince Janeczko

    Thread Starter New Member

    Mar 22, 2016
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    Noob here;

    I purchased a kit to change out my regular brake light bulbs in my car to replace them with LEDs, which came with 50w 6ohm resistors to be wired in parallel. The instructions indicate that the resistors should not touch plastic to avoid melting and they should be mounted to metal to dissipate heat. What I'm wondering is this; if they're going to generate that much heat, does that mean this resistor is providing too much resistance?

    I measured the resistance of the old bulb and it was about 1.5 ohms The LEDs showed zero resistance, so wouldn't that suggest that the resistor could be as little as 1.5 or 2 ohms, or one-third the resistance they sold with the kit? I'm replacing a 10 watt 12v bulb so the power consumption is comparatively small and I'm thinking the resistor is overkill - which is causing the heat. If it were a lower resistance, wouldn't it generate less heat?

    I also understand that if a resister with too small a resistance is installed it could cause a fire so maybe the kits are sold with these 'heavy duty' resistors to ensure there's no safety issue.

    I'm not looking to redesign anything, just want to wrap my head around the limited knowledge I have of electricity.
     
  2. KJ6EAD

    Senior Member

    Apr 30, 2011
    1,425
    363
    Light bulbs and LEDs don't yield all of their secrets to simple Ohmmeter tests. The effective resistance of your original bulb when lit is 14.4Ω. The resistor included in your kit is necessary to insure correct function of your flashers and turn signals.
     
  3. Sensacell

    Well-Known Member

    Jun 19, 2012
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    The new LED units consume so much less power than the incandescent lamps they replace that 'upstream' circuits won't even know they are there.

    The resistor is supposed to "ghost load" the circuit so it draws enough current to trigger the blinkers / flasher circuits.
    It's a simple but wasteful way to be backward compatible.
     
  4. WBahn

    Moderator

    Mar 31, 2012
    17,757
    4,800

    The resistance of an incandescent light bulb at room temperature is significantly lower than when it is at temperature.

    The power dissipated by a resistance in a DC circuit is V²/R. So if you cut the resistance in half you dissipate twice the power and make your problems worse.

    Heatwise, 10 W is a lot more power than you think.

    Why do you want to replace your brake light bulbs with LEDs? If it's because you want to save energy, you are seeing that it isn't so simple since your car was designed with the nominal power consumption in mind and is relying on it to operate your flashers, possibly among other things.
     
  5. eetech00

    Active Member

    Jun 8, 2013
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    Hummm....I've purchased 1157 LED direct replacement 12v bulbs. They don't have external resistors.
     
  6. Vince Janeczko

    Thread Starter New Member

    Mar 22, 2016
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    0
    Thanks for the info. I'm not looking to save energy, just going for the 'cool look' :)
     
  7. Vince Janeczko

    Thread Starter New Member

    Mar 22, 2016
    3
    0
    The bulbs I bought off of Amazon didn't come with them either. The seller had a separate package for the resistors. Here's the link if you're interested:
    http://www.amazon.com/4Pcs-Aaron-6ohm-Load-Resistors/dp/B00L4V9ECY
     
  8. WBahn

    Moderator

    Mar 31, 2012
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    Your application may or may not need them. Traditional flasher units (most/many of them) rely on the heat produced by the bulb current flowing through a bimetallic strip to operate. Reduce that current below a certain amount and the flasher units can't work. Newer cars control the flashing via programming and so don't care about the current.
     
  9. WBahn

    Moderator

    Mar 31, 2012
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    That's fine, I guess. Actually, nothing wrong with it at all.

    I envision a time in ten years or so that people will be replacing the stock LEDs in their cars with bulbs to go for that 'cool (retro) look'.

    I remember when PCs were new and the rage and people would print out their resumes on dot matrix printers because that showed that they were hip with technology (since most resumes at the time were hand-typed). A few years later it was certain death for your job search if you used a dot matrix printer because it was "unprofessional" -- they had to be laser printed.
     
  10. eetech00

    Active Member

    Jun 8, 2013
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    I don't know about anywhere else, but in USA 1157 bulbs are common auto brake/stop/tail light bulbs. The 1157LED is a direct replacement for the 1157 bulb. It doesn't need an eternal resistor.

    Oh....and it made by Phillips.
     
  11. WBahn

    Moderator

    Mar 31, 2012
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    As you look at reviews of the bulb there is no shortage of people that have either had to add external resistors or replace their flashers with LED-compatible relay units (probably the better option).
     
  12. k7elp60

    Senior Member

    Nov 4, 2008
    478
    69
    To amplify a previous poster, with some automobile LED replacement turn signal bulbs the external resistance is required to prevent the flash rate from really speeding up. The next paragraph I tell some facts about incandescent bulbs.

    The hot resistance of an incandescent is about 10 times the cold resistance. It takes less than a second after power is applied to the bulb when is a full brilliance and drawing the least amount of current. Over 10 years ago I developed a circuit that control the surge current on
    incandescent bulbs and really extend their life. The circuit was mostly for 12V RV lamps. I have RV lamp fixtures on my headboard in the bedroom. The bulbs have been there over 10 years, they are #1141. As a added benefit, the on/off switches are that old too, as the limited surge current does not burn the contacts of the switches.
     
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