Question re: light bulb characteristics

Discussion in 'General Electronics Chat' started by kingdano, Jul 6, 2010.

  1. kingdano

    Thread Starter Member

    Apr 14, 2010
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    I recall you mentioning in a post in the past looking at load characteristics of light bulbs.

    I am in the middle of a difficult problem at work and i am trying to look at the load characteristics of gold, platinum and ceramics with respect to temperature.

    I was wondering if you (or anyone else) could link me to some websites, im having a hell of a time trying to find info via google.
     
    Last edited by a moderator: Jul 6, 2010
  2. SgtWookie

    Expert

    Jul 17, 2007
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    When choosing a subject for a general inquiry thread, it is generally a poor idea to specifically address an individual, as that may discourage others from replying.

    IRT incandescent lamps, as the tungsten filament is heated due to the current flowing through it, the resistance of the filament increases. You can verify that assertion by measuring the DC resistance of a bulb at room temperature, and compare that to the bulbs' wattage rating. At room temp, the resistance will be far lower than would be required for the wattage rating at the operating voltage.

    What you seek is the "temperature coefficient of resistance".
    There is a table right here at AAC:
    http://www.allaboutcircuits.com/vol_1/chpt_12/6.html

    Note that the coefficients of tungsten, platinum and gold are all listed.
    Note also the absence of citation/reference of the source of the information. In order to keep your keister out of hot water, you need to refer to a professionally recognized source of information, such as IEEE or a national standards reference.

    Ceramics is a continually evolving field; it's quite fascinating. You will have to refer to datasheets from the manufacturers themselves, as a typical ceramic is an insulator.

    If you are considering using ceramic materials as a substrate, you'll need to match the thermal coefficient of expansion of the materials; otherwise you will likely experience early failures due to metal fatigue.
     
  3. kingdano

    Thread Starter Member

    Apr 14, 2010
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    My bad on the topic title, a mod can change it.

    I just recalled a specific comment from you and didnt want you to skip over the thread title

    thanks again for the direction, much appreciated as always.

    also thanks for the tip of not using this as a final source of information - we would always confirm theories with lab experiments to back up the thought before showing findings to the VPs and directors of engineering and research.

    i suggest anyone else reading this does the same - its safest for your paycheck :)

    and yes, the ceramic (which is apparently an RL203 material which is "very common" we were told by our French research equivalent...common enough to not return a google hit.....) is a substrate in which gold tracks are laid and platinum vias connect the traces to the rear surface of ceramic.

    the front is coated with several layers of dielectric material (unknown at this time) to prevent breakdown between traces which are driven with opposing phase (180 degree shift) 550VAC RMS signals. the traces are bounded on each side by earth ground traces.
     
    Last edited: Jul 6, 2010
  4. SgtWookie

    Expert

    Jul 17, 2007
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    OK, glad that helped you. Note that the numbers given are for pure metals. Gold and platinum are very malleable, which should help with differences in thermal coefficients of expansion, but also could result in material "creeping" over/in the ceramic substrate. I'm not a physicist, so I'm simply speculating.

    Note that many IC failures occur due to thermal cycling, which stresses the bonding wires that connect the semiconductor die to the package connecting pins. Hi-rel components use gold/platinum for bonding wires. Cheap ICs for the commercial markets frequently use aluminum or even steel; thus have a far higher failure rate when subjected to thermal cycling. But I digress...
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    Gold on alumina substrate is stable enough to pass military specs. I've worked with two companies, both made/make space rated products, that depend on the stability of this process. Actually mil spec is conservative for this process, since it is also used in deep space probes. Aluminum wire is also used in this process, as well as ultra pure gold (99.99%).

    I could be wrong, but I don't believe it isn't as simple as sputtering gold onto ceramic though, it does use several intermediate layers for things like resistors and straightforward adhesion (I believe titanium nitrate was our resistive material). There is nothing secret about this process, it is available for anyone who wants to do the research. It is commonly called microelectronics.

    Raw chip die, both transistors and ICs, are glued onto gold/ceramic PCBs with both conductive and non-conductive epoxies. Gold wire is used to connect these chips to the PCB (a process I very familiar with). It is about as state of the art as you can get, but we were doing it during the 60's. In some ways it was an alternative to ICs.
     
  6. kingdano

    Thread Starter Member

    Apr 14, 2010
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    thanks for the info guys

    the corollary to ICs is rather helpful to help wrap my brain around this.

    this forum is a great resource because of the people who post here.
     
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