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copper current capacity ?

Discussion in 'Electronic Basics' started by Dominic-Luc Webb, Jan 26, 2004.

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  1. I have made a filter for a classic light dimmer, not having a local
    source for what I need; two lower pics at:

    http://www.megspace.com/science/stp/alum/dimmer/dimmer.html

    It uses 1.4 mm diameter transformer wire @ 58 turns wrapped
    around a piece of 15 mm diameter wooden dowel with a 6 mm
    diameter steel rod driven through the length, as seen in
    the pic. This was covered with heat shrink. I do not have
    convenient way to measure the Henries, but I cannot detect
    any radio frequency interference at any dimming level and
    operation is quiet. I am wondering how to rate the current
    carrying capacity. There are two values listed on relevant
    websites (i.e., www.powerstream.com/Wire_Size.htm) indicating
    very different capacities for power transmission or chassis
    wiring. Any recommendations on how I should rate this?

    I could note that typically these are recommended to
    be 20-100 uH @ 4.5 amp. I would like to be able to
    operate this a bit higher, like 10 amps (I plan to
    switch the triac to a similarly configed 25 amp version)

    I am curious about criticisms of this construction overall,
    of course. While we are on the subject, any neat tricks
    to measure L?


    Dominic
     
  2. The smallest wire gauge usable for a given current depends on two
    factors. One is voltage drop. You do not want to waste more than a
    minor fraction or your voltage getting the current wherever it is
    going. The second factor is temperature rise and risk of damage or
    fire. For wound arrangements, the second factor far outweighs the
    first almost every time, since the heat is concentrated and the escape
    of heat is inhibited. So ampacity tables that are made for wiring (1
    to 3 strands run between points) are not at all conservative enough
    for wound components. And specific limits are a lot harder to
    generalize, since there is such variety in volume to surface ratio and
    material temperature capability. Your design has a strike against it
    by including a flammable material (wood) under the heat source
    (wire). If I were you I would either purchase a current rated
    inductor for this job such as the 5626 to 5628:
    http://rocky.digikey.com/WebLib/JW Miller/Web Data/5600 Series.pdf

    This type of low cost inductor is manufactured by many vendors. They
    are wound on an epoxy coated ferrite drum core. These cores are also
    available if you want to wind a custom version with much more
    effectiveness per amount of copper and heat generated compared to your
    design.

    If you must make your own inductors, you need to do some testing by
    putting a thermocouple under some insulation at the middle of your
    coil and measure the temperature at full current and see if the
    temperature rise is low enough. For materials like wood, I would
    think that should stay below 100c. You could also look at the data
    sheet above to get an idea what wire size is used for variously rated
    coils. For instance, in the above sheet, .082 inch wire (2.1mm) wire
    is used to wind the 14 amp 100 uHy version, while .065 inch (1.7mm)
    wire is used for the 250 uHy 9 amp one.
     
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