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Resistence of linear halogen bulb when cold

Discussion in 'Lighting' started by JS, Dec 7, 2005.

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  1. JS

    JS Guest

    I am working out the wattage of some of my spare 118mm linear halogen
    bulbs. I have access to only a regular meter.

    So I have measured the resistence of the bulb when cold.

    (Q1) Is the resistence likely to change significantly from my cold
    reading compared to when the bulb is at operating temperature? Is
    there a rule-of-thumb multiplier for such bulbs.

    (Q2) Is the filament material used likely to vary from manufacturer
    to manufacturer in a way that noticeably affects the relationship
    between the cold resistence and hot resistence of a bulb? (If you see
    what I mean.)

    FWIW I am in the UK with 230 volt mains and my cold resistence
    readings are 12.1 ohms, 13.5 ohms, and 8.1 ohms. Presumably the
    first two are 300W bulbs and the third is 500W.
     
  2. Yes, there is a huge change in resistance from cold to hot.
    The ratio is 16.44:1 between 20 C and 2727 C, but since it
    is a strong function of the filament operating temperature,
    any rule-of-thumb that does not specify the operating
    temperature will have an error. If my math is correct, a
    300-watt 230-volt lamp would have a hot resistance of 176
    ohms. If it operated at 2727 C, then the cold resistance
    should be about 10.73 ohms.
    Most filaments are made from essentially pure tungsten, with
    some added materials to make the wire easier to draw and to
    strengthen the final filament. The change in resistance is
    therefore dominated by the properties of tungsten. The
    largest error will be knowing the operating temperature. In
    fact, the change in resistance of tungsten is so large and
    so predictable that the ratio of hot to cold resistance is
    used to determine the operating temperature in many lamps.
    --
    Vic Roberts
    http://www.RobertsResearchInc.com
    To reply via e-mail:
    replace xxx with vdr in the Reply to: address
    or use e-mail address listed at the Web site.

    This information is provided for educational purposes only.
    It may not be used in any publication or posted on any Web
    site without written permission.
     
  3. David Lee

    David Lee Guest

    JS wrote...
    No you aren't! That's a common misconcepetion but UK mains voltage is 240V
    and will not be changed in the foreseeable future. 230V is the NOMINAL
    European standard but the standard includes tolerances that encompass the
    national standards of all the member countries. The point of the standard
    is that a European appliance should be safely usable anywhere in the EU and
    not that it will necessarily work properly. You can buy 230V lamps but if
    used on UK 240V they will burn a bit brighter and have a lifetime degraded
    to only 55% of their design life. Hence you should make sure that you are
    being supplied with the correct product for Great Britain. Buying from a
    reputable supplier is not necessarily a guarantee of this - not so long ago
    I was supplied a batch of 230V halogen theatre lamps by one of the major UK
    theatrical suppliers.

    David
     
  4. Ben

    Ben Guest

    Is that reversible ? Would it make sense to order, let's say, beamer lamps
    in the UK, for 240 V, and have a lifetime of 1.5 times the "European" lamps
    when used on 230 V AC ?

    Thanks for your reply,
    Ben

    http://home.planet.nl/~benzandstra and www.pe2bz.nl
     
  5. Bremecker

    Bremecker Guest

    Yes it is. But you will loose a lot of the luminous flux you need. There
    is a nice diagram that shows the influence of the voltage to luminous
    flux, power current and lifetime. You have to got too:

    http://www.osram.com/service_corner/glossary/index.html

    In the glossary list you have to scroll down to "incandescent lamp" In
    the pop up window that opens have a look at the second diagram
    "Operating characteristics of incandescent lamps".

    The problem with dimming of incandescent lamps is, that you reduce power
    just a little but you loose a lot of light output, that means the light
    efficiency that is already very low decreases much more than the power
    reduction.

    Regards

    Willi
     
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