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Pre heating incandescent light bulbs

  1. Oct 2, 2013 #1
    I want to be able to make my headlight bulbs last longer. I know the main reason they burn out is the massive current surge when you turn them on when cold.

    I was wondering if you put a thermistor in parallel, then the current would be split through the thermistor reducing the surge. Eventually increasing current to the bulb once the thermistor is hot.

    Will this work? Is there such a thing as a pre-heater for incandescent light bulbs?
  2. jcsd
  3. Oct 2, 2013 #2
  4. Oct 2, 2013 #3


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    I don't think you will achieve what you want with a passive component because the lamp filaments require the full supply volts on them for full brightness. Some active circuitry would be necessary - probably solid state.
    Components in parallel are not a good idea as they do not affect the volts on a car battery until you are taking many many amps (and, in any case, a thermistor resistance goes down as it heats up). A suitable resistor in series, which is switched out after a brief time, will reduce the thermal shock effectively. A pulse width modulated switch would be a good dimmer control, to bring the temperature up slowly, over a second or two.

    Dimmers work very well for stage lights, which can dissipate hundreds of Watts. These (mains voltage) bulbs can last for ages longer than domestic light bulbs and I think it's largely because they are faded up and down rather than hard - switched.

    Car headlamp bulbs are usually pretty long lasting. Are you sure there is not some vibration causing the early demise of yours?
  5. Oct 2, 2013 #4

    jim hardy

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    Do you notice your battery losing water? What does your car's voltmeter read?

    Incandescent lamp life versus voltage is surprising , like 1/ (volts to twelfth power).

    However - if you're one of those misanthropes with those awful Xenon headlamps - I can muster no sympathy. :frown:
  6. Oct 2, 2013 #5


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    As Jim said, checking your car's voltage regulator could be a good idea.

    I don't know if modern cars have a "soft start" built into the light circuits, but with my previous car I used to blow a headlight bulb about every 6 months (and that wasn't too untypical compared with the cars I had before it), but my current car still has all the original bulbs working after more than 4 years.

    I once did some cine photography using hired "professional" lights, and those high powered bulbs (i.e. 1000W or 2000W per bulb) required warm-up and cool-down times of about 20 minutes running on reduced voltage, before and after use.
  7. Oct 3, 2013 #6


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    The same mechanical shock occurs when you switch from dipped to high beam, I expect.
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