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Resistor Power Rating Spec

Discussion in 'Electronic Basics' started by Jim, Jul 18, 2003.

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  1. Jim

    Jim Guest

    I need to spec a 1 Ohm resistor to handle 4.5 Amps.
    (4.5^2)*1 = 20.25, and I'd spec a resistor with the appropriate power rating
    depending on the environmental conditions.
    In this case, the current will only last a fraction of a second (0.1 s, max)
    and will not be repeated for a while (probably 5-10 minutes at the
    absolutely fastest). All of this will be occurring in an air conditioned
    Can I get away with specifying a lower power rating? I'd be concerned
    going with 1/8W, but I'd think that 2-3W should be able to handle it (but
    they may get a bit warm).

  2. A 2 watt should do fine, but get a wire wound, not a metal film.
    These have more resistor metal mass to soak up the heat pulse.
  3. Hi,

    And mount it with plenty of air all around so that if the
    system latches-up it won't set fire to anything. I used to employ
    a similar trick (high peak power but a low wattage resistor) to
    delay the closing of some relay contacts on a largish power
    supply and found that they made wonderful fuses.

    Cheers - Joe
  4. DarkMatter

    DarkMatter Guest

    At that duty cycling of 0.1 sec with a 5 minute rest in open air,
    even a 2 watt job would probably work.The work period and rest period
    are the key. That is a 0.33 percent duty cycle! :-] The very short
    duration on cycle would almost certainly work into a 2 watt Metal
    Oxide or a wirewound version resistor. The lead on those devices can
    handle it, and I'd be willing to bet the device can without excessive
    or any abnormal heating to speak of. Still, if there is any chance of
    the on period getting stuck on, or such, I'd go at least 5 watts, if
    not the full monty... err watty. :-]
  5. meirman

    meirman Guest

    In sci.electronics.basics on Fri, 18 Jul 2003 15:58:39 -0400 "Jim"
    I can't help you with the rest, but in the scheme of things, I don't
    think it matters much that it is AC. The difference between 90 and
    70F or even 110 and 70 seems like a lot to us because it's the
    boundary between comfortable and not. But I think 20 or 40 degrees is
    not so great when compared with the melting or burning temperature of
    most electrical components. Well, maybe AC would matter here???, I
    don't know, but I just wanted to raise the point in general.
    Try it a few dozen times and then see how hot it gets. First hold
    your fingers near it. If it's not radiating too much heat, then lick
    your fingers and be quick, like people do with a clothes iron, so you
    won't burn yourself.


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