> hi tom,
>
> thanks for the e-mail. i'm a physicist, so i'm not confused by
> things like :
>
> power = energy/ time; or , if you like, energy = power x time
I'm "trained" to think that way. I should have read the e-mail
header.
> > The resistor in that unit has a joule rating (power over time)
> > not wattage. It'll handle fifteen kilowatts for one second, or 150
> > watts for 100 seconds according to the joule rating.
>
> > But gradually at low powers, the heat transfer to air prevents
> > overheating no matter how long you leave it on.
>
> yes, that's when the power being dissipated is equal to the power
> being carried away, so the thing is in thermal equilibrium (at some
> hi temperature maybe, but it's in equilibrium. it's not getting any hotter.)
>
> the *continuous* rating for the 264 is 100 watts; for the 250 it's 200
> watts. (IN THE FACTORY OIL!)
> i think rich was saying that the *free air* rating for the resistor
> was 75 about watts... in the oil it would be considerably higher, of course.
That might be what he was thinking. But the real problem is ALWAYS
heat over time, not the CCS free air rating.
> > I can try to find out what the rating calculates to be, if you like.
>
> well, i intend to call mfj myself, tomorrow, and ask them. what i'm
> concerned about is some maximum voltage rating or the like. some
> power resistors have a maximum voltage rating, even when their
> dissipation rating isn't being exceeded. that's what was worrying
> me.
The connector or lowest external arc point would be the limit, not
the resistor. The voltage rating of that resistor is many kV!
> **********************************************************************
> scenario:
>
> me: " i french fried the dummy load! i put 1.5 kw into it for 10 seconds.
> that's only 15,000 joules, and the derating curve on the can shows 1kw for 10
> *minutes*... that's 600,000 joules! what i did shouldn't even have
> warmed it noticeably!"
>
> mfj: " ahh, but you exceeded the maximum recommended power! if you do
> that for even a microsecond, the resistor goes off like a flashbulb,
> and so does your warranty!"
>
> ****************************************************************
Yes, but the correct answer would be "I don't know, let me check and
get back to you".
> frankly, i think it should be fine, but i'm gonna call and talk to
> them anyhow.
I think you are right, although I don't have a load like that, I
have seen one at a Hamfest. I'm sure it will take 1500 watts for
short periods of time. Composition resistors are not damaged by brief
severe overloads, but by heat over time. (Unless they arc over of
course, then the heat is concentrated in one spot.)
Resistors make VERY poor fuses.
73 Tom
73, Tom W8JI
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