"Jim Lux" wrote in message
...
Copper sulfate in water makes a nice resistor, too. It's used a lot for
"energy dump" resistors in pulse power applications.

From a theoretical standpoint, copper sulfate solutions should increase
their resistance with frequency more rapidly than the sodium and ammonium
chloride solutions used in the ARRL article. This is mainly due to ion
mobiity in solutions.

The power limiting thing on resistors like this is the packaging. If
you put it inside PVC pipe, then the thermal resistance of the PVC
limits how fast you can get the heat out (and the maximum temperature,
too)

Heat transfer was the primary reason for using transformer oil or mineral
oil in the old Heath Cantenna. Without the oil, the Globar carborundum
resistor would rapidly overheat. With the oil, the resistor could could
dissipate 200 watts continuously. If you started with room temperature
mineral oil, it could dissipate a kilowatt for about a minute before the
oil got too hot and reached the flash point. Medicinal mineral oils have
an open-cup flashpoint between 171 and 221 ºC depending on the specific
manufacturer, while that of technical grade oils is somewhat lower. Of
course, Heath suggested removing power to the Cantenna when it reached 60
ºC [the temperature at which the can is too hot to touch continuously].
At the time the HN-31 was produced, transformer oils generally contained
polychlorinated biphenyl (PCB) additives which reduced their
flammability. This is why Heath rated the Cantenna at 1 KW for ten
minutes or less when it was filled to the proper level with transformer
oil.

I have a Bird dummy load rated at 1 kW continuous. It has a larger
resistive element, still immersed in oil. However, the case is finned to
help dissipate the heat.

Jim is quite correct about the PVC pipe limiting the heat transfer. I
would suggest using power chip resistors. Digi-Key carries Bournes chip
resistors rated at 50 ohms and 100 watts for less than $8 each.

73, Dr. Barry L. Ornitz WA4VZQ

Nordic Breeds WA4VZQ wrote:
"Jim Lux" wrote in message
...
Copper sulfate in water makes a nice resistor, too. It's used a lot for
"energy dump" resistors in pulse power applications.

From a theoretical standpoint, copper sulfate solutions should increase
their resistance with frequency more rapidly than the sodium and ammonium
chloride solutions used in the ARRL article. This is mainly due to ion
mobiity in solutions.


Copper sulfate is popular because it is compatible with copper
electrodes, too. Sodium chloride tends to corrode the electrodes.

There might also be some difference in solubility, which would affect
the range of resistances available.

http://home.earthlink.net/~jimlux/hv/rwater.htm has a discussion of
building water resistors for HV applications.

On Mon, 06 Dec 2010 16:52:21 -0500, Nordic Breeds WA4VZQ rearranged some
electrons to say:



Jim is quite correct about the PVC pipe limiting the heat transfer. I
would suggest using power chip resistors. Digi-Key carries Bournes chip
resistors rated at 50 ohms and 100 watts for less than $8 each.

73, Dr. Barry L. Ornitz WA4VZQ


Amen on the thick-film chip resistors. I built a 200W dummy load using
two 100 ohm 100W devices in parallel.