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a question about power

Discussion in 'Electronic Basics' started by Gunnar G, Jun 7, 2005.

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  1. Gunnar G

    Gunnar G Guest

    Hi.
    I'm a total clueless newbie and I've just bought my first soldering tool and
    I'm about to construct a resistor from a small thread with a resistance of
    30 Ohm per meter.
    The question is: how much power can this thread take? It's going to be a 10
    Ohm resistor and the DC current will be like 0.8 A and it gives a power of
    6.4 W. I don't want things to start burning, is there a risk that 6.4 W
    too much power for this small thread?
     
  2. Probably. It depends on how hot that element can get and still remain
    a stable resistance, and how easily heat is conducted from it to the
    environment around it. It takes an object about as large as your
    little finger to get rid of 6 watts and remain cool enough to not
    blister you. So it may take two strands in parallel (and twice as
    long, each) wound around something about that size, and covered in a
    layer of epoxy, to keep the wire temperature reasonable at that
    wattage. Bigger is better.
     
  3. Gunnar G

    Gunnar G Guest

    Probably. It depends on how hot that element can get and still remain
    It says "konstantan" on the package. I think I've heard that that is a
    material that changes its resistance very little with temperature.

    The calculation with my little finger and 6 watts, what do you base that on?

    The plan is to wire the strands around a piece of wood. The epoxy, is that
    to fix the wires so they don't get in contact with each other, or does it
    have any other possivitve effects when it comes to heat disapation?
    I have some of that termal paste you use under your heatsink in the
    computer, might that be of any use?


    Sorry for the bad english.
     
  4. It takes some area to dissipate some power to heat in the air.

    If you heat up a big objekt, like a car, it can warm up the air around
    it, and 5Watt would easily be moved over from the car to the air around
    it. The car would just have to be slightly warmer than the air.

    Think about a small object, like the thin wire inside a bicycle
    lamp. If you heat that wire with 5Watt of power it will have problems
    getting rid of that heat. It can do it, but it needs to be very much
    hotter than the air to do it.

    That is why size means so much for the ability to move heat, to
    dissipate heat, without getting very high temperatures.
    Wood and epoxy are usable up to a certain temperature, if your wires
    don't get too hot it can work.
    What are you really trying to do?
     
  5. It is true that over some operating temperature range, that alloy
    changes resistance little.

    According to:
    http://www.goodfellow.com/csp/active/STATIC/E/Constantan.HTML
    the maximum operating temperature is 500 C.
    The normal size of resistors conservatively rated for similar power
    dissipation. They are normally coated with ceramic or silicone
    material which has a higher temperature rating than epoxy.
    The coating not only provides electrical insulation, it helps the heat
    to spread from the thin wire to the nearby surface, to allow it to get
    out into the air with a lower temperature rise. By the way, epoxy is
    limited to something like 100 C and wood will shrink at less than
    that. I suggest you use something mineral, ceramic or glass. If you
    can scrounge up some much higher resistance 10 watt resistor, that
    would make an ideal body for your custom resistor.
    Not if you intend to use epoxy, also. Those materials are usually
    based on silicone grease, which prevents epoxy from bonding. But the
    thermally conductive solids they contain make a good additive to
    epoxy. Some examples would be zinc oxide, alumina and powdered
    aluminum. But this enhancement is of little importance because the
    main thermal resistance is not that between the wire and the surface
    it is wound on, but between that surface and the surrounding air.
    Very good thermal contact with the resistive wire is only important
    when brief, large pulses of power must be handled.
    I hadn't noticed. If anything I say is not clear, please ask for me
    to say it another way to help your understanding.
     
  6. Gunnar G

    Gunnar G Guest

    Good question.
    The project changed dramatically last night when I (please don't laugh, I
    thought I knew this stuff, but obviously I didn't) realized that an adapter
    marked 7,5V 1A does not send out a constant voltage of 7,5 AND a constant
    current of 7,5V. It's rather a 7,5V voltage and a maximum of 1A.

    My earlier questions are therefor irrelevant.

    To answer your question, I was trying to make a resistor that would be
    warmed up with 6W. With last nights revelation, its no longer something I'm
    going to do.
    Silly me. But at least I learnt something!
     
  7. JeffM

    JeffM Guest

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