Bear,
The resistor used by Heath is a 100 watt carborundum. You can figure the rating
in mineral oil at around
10:1 for the temporary rating of 1 minute. The air rating stays the same for
any use unless fan cooling
them. It actually raises the raiting in air by using a fan but not
signifigantly. I have just been around
the horn on this with Ohmite and Carborundum for the loads we build here. Also
in building a load you
will find you need to cancel out any inductance by adding a capacitor in
paralell with the resistor bank.
The metal shield around the resistor in the Heath Cantenna does this same
function. Collins loads which
were air cooled used a 30 pF doorknob capacitor this way. Hope this helps you.
Best & 73's
Will Matney
U.S. Amplifier
=======================================================
Anyone out there have solid technical information on Dummy Loads &
Carborundum Resistors??
My old Heathkit Cantenna has a ~1" dia x ~5" (iirc) grey carborundum
resistor,
sitting in a can of oil. (rated 1kw short time)
I've been offered a batch of ~1" x 18" carborundum resistors @ 50 ohms
and/or
a batch of ~2" x 18" @ 25 ohms - these are said to be rated at 750 watts.
Here are my Q's:
If the latter one is rated at 750 watts, what is the dry rating of the Heath
likely to be??
(the heath is asymptotic to 200 watts over time...)
What is the bandwidth likely to be ??
(the heath is <1.5:1 up to 300 mhz.)
If I put it in a bath of transformer oil, what is the increase in power
handling?
(does the added surface area play in to this??)
I'm thinking that unless there is something wierd about these resistors that
the 50 ohm unit ought to be at least 500 watts in air, or more... the 25
ohm
unit would seem to be rated very conservatively, yes?
I've owned a bird style >2kw unit at different times and they seem to be
about
this depth and have fins for cooling (of course) - do they just leave
some air in
the chamber for heat expansion of the oil, do they have a blow off vent??
What else should I know???
_-_-bear
--
_-_-bearlabs
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