[AMPS] dummy loads

[George T. Daughters]

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

> 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.

(snip)

> 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. 

**********************************************************************
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!"

****************************************************************

frankly, i think it should be fine, but i'm gonna call and talk to 
them anyhow.

people have suggested Bird (tm) dummy loads, and i'm sure they're 
great.  but i just bought a Bird wattmeter and i'm fresh out of bucks 
right now.

thanks for your input, and i'll keep you (and the ampsters) posted.

73, k6gt


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