[Amps] 20kw Dummy load?

[craxd <craxd1@ezwv.com>]

Hal,
I delved into this once using the large ceramic tubular resistors. They 
make there in almost any size. I have a air cooled load here that came 
out of a Harris piece of equipment. It has six 300 ohm resistors like 
that in parallel, each about 12 inches long. It handled 5 Kw easy on a 
test one day. I mounted the assembly in an aluminum enclosure I had made 
which is vented along the bottom. The top cover has four 4-3/4" muffin 
fans blowing across the resistor bank and out the bottom vents. At this 
it still had some inductance so I added an air variable cap in parallel 
with about a 40 pF max. I tuned it to about 30 pF as a guess by the 
mesh, or to where the SWR dropped to 1:1 or 1:2. When it heats up, it's 
no worse than about 1:5 and this after a minute or so of hammering it. 
The old Collins loads had a 30 pF doorknob in parallel with the four 
resistors in the load they made. There's no part numbers on these 
resistors either so I don't know who made them. I had intended on 
building this up from scratch until I found the resistor bank on ebay of 
all places. The guy had that it became a friend of mine. He had bought 
it for the same purpose.

It's funny you mentioned Palstar. When I was thinking about all this, I 
called Palstar and spoke to the owner (2+ years ago?). This was before 
they had this model you have there. Then, I asked him if he had to 
cancel out any inductance, and he said no. I mentioned then about using 
some capacitance like in the Collins, and he said it wouldn't work. Then 
I happened to visit their website a while back and seen this new load. I 
swear, it looked an awful lot like he added a doorknob capacitor to that 
resistor bank to cancel out any inductance. This after telling me that 
it wouldn't work and would have to be tuned all the time =)

The resistor manufacturers I contacted was Ohmite, who was very helpful, 
Kanthal Globar, helpful also, and Caddock. Price wise, Globar I think 
was cheapest and had what I wanted. Ohmite, if I recall had a long lead 
time and they were a non-stock item. Caddock didn't have anything large 
enough except some 100 watt, heatsink type resistors. Globar had some at 
12" long, 1" diameter, and about matched these. They were rated at 275 
watts each at 10 Kv peak max., if I recall. The 12" x 1-1/2" dia. were 
only rated at 150 watts and 75 Kv peak. Of course nothing will even be 
close to 75 Kv, or even 10 Kv. They make one that's 24" long, 2" dia. 
and is rated at 1 Kw, 22 Kv max. I was thinking about using two 100 ohm 
resistors of this in parallel for 5 Kw with a 50% derating. The 1 Kw 
rating is a continuous rating so a derating of 50% would be ok for short 
test times. Plus that 1 Kw rating is for open air. Fan cooling one would 
raise it's rating a good bit. In my load, going by Globars values, my 
six resistors would handle about 1650 watts cont. in open air. I can 
attest they'll take 5 Kw for about a minute+, fan cooled. Below is a 
link to Globars website.

http://www.globar.com/ec/tubular.php.html

http://www.globar.com/ec/axial.php.html

http://www.globar.com/ec/slab.php.html

http://www.globar.com/ec/water.php.html

http://www.globar.com/ec/app.php.html

What one might do is catch some of these on ebay like I did. A big load 
will cost some $ no matter how you go about it.

Best & 73's

Will Matney




Dear Will,

Some months ago Rich Measures was talking about ordering some special
resistors
from Ohmite or the-like to fill a need for a project he was doing.

In that time I was bench testing the Emtron DX-3 and was using the MFJ
oil-filled Cantenna clone, and that got really hot after 10 seconds and
the red plug
popped off and my hotel room smelled like the french-fry concession at
Yankee Stadium in the Bronx, so I had to upgrade.

When the Palstar 5KW fan-cooled dummy load arrived I took it apart to 
see if UPS had incurred there usual shipping damage on anything fragile
going through their system and noticed the carbon-pile type cylindrical
resistors being used: 100.34 ohms, nominal,l about 12 inches long, 2
inches
in diameter, sintered gold plating on the ends, clips holding them in and
some HV caps at the hot ends to counter the reactance that the case
and resistors probably conjured up. This device seems to work 
very well, because the fans kick in and stay lit until the pile cools
down.

I took apart the Cantenna and measured the smaller but similar carbon
pile
resistor in that unit. It was 65.7 ohms and MFJ sent me out a free
replacement
resistor, which was 50.75 ohms, well within their 10 percent tolerance.

The whole point of this is if it's possible to obtain carbon pile type
resistors
of sufficient dissipation to construct a 20KW resistive load.  Mr.
Measures
uses the garden hose to cool down the resistor body in a carefully
engineered
conical assembly, but what about those experimenters who just want to
have
an air-cooled deal that will fit in the shack?  Power-dividing and using
the resistor
elements from Cantennae suspended in the bathtub is a practical solution,
but that
gets real old when the YL spots it.

Sorry for the long wind, messieurs. The recent spate of posts about dummy
loads
catches my attention.

Hal Mandel
W4HBM 


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