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Halogen light bulb filament thicknesses?

  1. Jul 22, 2015 #1
    Hello, new to the forum, just a average guy, not too much more informed on physics then the average person, so this will be a pretty simple question, anyway, to my question:

    with halogen bulbs, does higher wattage always mean thicker filament? Because i am trying to work out if tricky Chinese are making -30 Watt bulbs and selling them as h3 12v 100W.

    Thanks!
    MRunknown
     
  2. jcsd
  3. Jul 22, 2015 #2
    I am not an expert in light-bulb manufacturing, but I can share what I do know. Filament based light bulbs take advantage of the resistivity of certain metals. When a current passes through this wire heat is generated and, in turn, light is emitted. The intensity of the light depends on the temperature of the wire. The key idea, resistivity, depends on the properties of the metal AND it's cross-section. The thicker the wire, the LOWER the resistance. A thin wire has more resistance if they are made of the same materials. Having said that, you must also take into account several other factors. Heat from the filament causes the wire to literally evaporate and also increases the rate of oxidation, which is why they are usually under vacuum or inert gases. This is different in a halogen bulb, from what I've read a halogen lamp induces a chemical reaction in which the thrown-off metal of the wire can redeposit itself back onto the wire. So, taking all these factors into account I don't have a direct answer for you. Had the bulb been a non-halogen light bulb they would have to almost certainly use a thicker wire or it would blow too fast, but the halogen chain cycle makes things different.
     
  4. Jul 22, 2015 #3

    sophiecentaur

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    The basics of filament design are the same for all bulbs. The resistance determines the wattage and the physical dimensions (and its environment) determine how hot the filament gets. You can get the same resistance from a short, thin filament or a longer, fatter one. Things don't scale, though and that's the relevant thing, here. The smaller one will operate at a higher temperature (to get rid of the same amount of heat as the large one). All bulbs have an oxygen free gas (argon in regular ones) so the filament will not oxidise but it still ablates due to the high temperature and the metal molecules will condense on a glass envelope - making it darker. The Halogen bulbs have a quartz envelope and contain a halogen gas. The halogen reduces the rate that a hot filament will allows the filament to ablate and the quartz envelope can run so hot that the tungsten molecules will not condense on it (stays clear) but they condense back on the filament.
    So a halogen bulb can operate at a higher temperature, giving off more visible light and it will last longer. Clever, eh?
     
  5. Jul 22, 2015 #4
    Ok thanks for your answers guys! so if my understanding is right, i can not determine if the bulb is 100 watt or not? because the wattage of the bulb can be changed drastically depending on filament windings, length, and thickness.

    The post i was following to build this motorbike light project says to use a 20 amp fuse so in case of any power surge in the motorbike system, the fuse will blow before the bulb does.

    I am not very experienced with electronics, would a 20 amp fuse blow before a 100w bulb does? if it should, that proves that the bulbs really weren't 100 watt.

    Thanks!
    Thomas.
     
    Last edited: Jul 22, 2015
  6. Jul 22, 2015 #5
    it depends on the current you use and what the bulb is rated for. a 20 amp fuse is going to blow when a current greater than 20 amps passes through it. the bulb manufacturer should list the proper conditions for the bulb.
     
  7. Jul 22, 2015 #6
    the current is 12v i don't know anything else, i guess the watt changes depending on engine RPM, the bulb was so post to be 12v 100W, when i convert 12v 100W to amp, it says 8.333333 amp, so that means that fuse would not help at all even if the bulb was really 12v 100 watt? so does that mean i need like a 8 amp fuse? otherwise the bulb will burn out?


    anyway... never mind guys. thanks so much for your help. I think i just got scammed by Chinese on eBay...

    Thanks!
    Thomas.
     
  8. Jul 22, 2015 #7

    NascentOxygen

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    The easiest way to work out a 12v bulb's wattage is to measure the amps it draws from the battery. You can buy multimeters pretty cheaply, and most have a 10A switch position. A 30W bulb will use about 2.5A, but a 100W bulb uses about 9A for full brilliance. To measure the current you connect the meter in series with the bulb NOT IN PARALLEL or you risk destroying the meter.

    A different trick to watch out for is in buying household LED ceiling bulbs. Some sellers will claim high efficiency/high brightness, but you may find their bulbs draw more power to achieve this light output than is written in the brochure. So a "high efficiency 9W bulb" actually may be using 14W and not saving you as much money as you'd thought.
     
  9. Jul 22, 2015 #8

    NascentOxygen

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    If you already have a 100W 12V bulb of a similar type, you could use a multimeter to measure its filament's cold resistance, it should be the same as that of your "suspect" bulb's.
     
  10. Jul 23, 2015 #9
    Thanks, but i burnt them both out, i don't really care what their problem was anymore, i just want to work out if a 12v 100W (8.33333 amp) H3 bulb will work on my 12v system on my motorbike, straight from the lighting coil, no battery.

    I will get my multi meter out and Analise the bike's current output, but how can i measure the amps? because if i understand correctly, the multi meter max amps is 10, and i think the bikes output will be quite a bit higher. once i work out the current the bike is putting out then i will need to ask if the 12v 100W H3 bulb will work on that current or if it will blow, is this more suited to a electronics forum now or is it okay here?

    I'm not sure if i have my idea of Amps totally wrong or not... i need to research further.

    Thanks everyone for the help! i am learning!
    Thomas.
     
  11. Jul 23, 2015 #10
    i found this:
    "A neat analogy to help understand these terms is a system of plumbing pipes. The voltage (volts) is equivalent to the water pressure, the current (amps) is equivalent to the flow rate, and the resistance (Ohms) is like the pipe size."

    does this mean if my generator could put out 10000 Amps i could still run a 1 amp light or something on it and it would not burn out the light, and the 9999 amps would just not be used? or am i wrong?

    Hard to get my head around it.....
     
    Last edited: Jul 23, 2015
  12. Jul 23, 2015 #11

    NascentOxygen

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    That is correct. The amps capability of your generator is not important, so long as it's adequate. If it's a 12V bulb, so long as you connect it to 12V DC and not greater, it will be fine.
     
  13. Jul 23, 2015 #12

    NascentOxygen

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    DO NOT get out your multimeter and try to measure the amps of your generator like you suggest. That is not how it's done, nor is it meaningful.

    To measure the current in a bulb, you connect one lead of the meter to the generator and the other meter lead to one terminal of your bulb. The other terminal of your bulb connects to the remaining terminal of the generator. This way, current passing through the bulb must also go through the meter. But before you do any of this, switch your digital multimeter to its 10A range and plug the red lead into the special meter socket for the 10A range.
     
  14. Jul 23, 2015 #13

    NascentOxygen

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    The only problem I can see is that the generator of your bike probably puts out more than 12V, maybe 15 or 16. This will shorten the life of a 12V bulb, it will soon burn out if it gets much more than 12v. The battery when in place holds the voltage down to 14 or less.

    Use your multimeter switched to DC volts to measure the bike generator voltage with no battery and no bulb.
     
  15. Jul 23, 2015 #14
    Thanks! I tried that today, you were right, it seems to range between 18 to 12 volts, so that was the problem.

    I don't have a bulb at the moment which i can do this with. I think the problem like you pointed out is the voltage. It seems like i will either have to add a voltage regulator, or a battery. Do i need to install a voltage regulator or anything? or can i just connect the frame to negative on the battery (negative is grounded) and the positive wire to positive and it will just charge and cycle correctly? If i do that i could even connect up the electric starter!

    Thanks for your help!
    Thomas.
     
  16. Jul 23, 2015 #15
    The generator puts out AC am i wrong? also, when routed into a battery, does it them become DC? I'm pretty sure I'm getting that all wrong. o:)
     
  17. Jul 23, 2015 #16

    NascentOxygen

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    If your meter gives a sensible voltage reading on DC then your generator is a DC type. If you needed to set the meter to AC VOLTS to get a sensible reading then it would be an AC generator (alternator). The battery itself does not turn AC into DC, it needs rectifiers to do that.

    Your thread has morphed from a light bulb rating into modifying the electrical system. I can't help further than with the bulb. You *could* build a regulator to keep the bulb happy at its rated voltage despite generator fluctuations, but whether that would be more economical than replacing the battery, and what other electricals considerations you need to take into account, I wouldn't know.

    Good luck with your project!
     
  18. Jul 23, 2015 #17
    Thanks, i will try that again today with both ac and dc settings.

    Thank You, and everyone very much for your help! i do appreciate it! i should be able to sort my self out now.... and if i cant i will resort to a electrical forum.


    Thanks.
    Thomas.
     
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