Hi, AA. Aren't plaster of paris and clay susceptible to cracking under thermal expansion/contraction cycling? If you're doing a "home brew", wouldn't it be better to use plain ole furnace cement (available at any hardware store), which is better under thermal cycling? I've had good results with that.
Just asking. You're correct, of course, that the coating around wirewound resistors improves their thermal characteristics.
Thanks Chris
Chris,
I have just used the items I have commonly avalible around the house. Their may be better items to use, but I havent had any problems with these. I generally don't thermally load the ceramic or Plaster of Paris to heavy. ie I make them larger then necesary. Furnace cement would also probably work fine, but its improtant that you have no air pockets in your mold.
Another method I've seen "back in the day" was to wire up an array of resistors to fit into a paint can, and fill the can with oil. The oil is a good electrical insulator and decent thermal conductor. As I recall, there was a product called a "Cantenna" for hams to use as a test load for their transmitters.
Note that besides whatever increase in ultimate cooling due to the large area of the can surface, the real virtue of this approach is its huge heat capacity for short-term tests, which don't reach equilibrium. Besides transmitters, it might be good for bench-testing big power amps, or for power supply overload tests, etc. Anything that needs a large dissipation, but only for a limited time.
Best regards,
Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis
--
Not only are you quite rude, I\'m quite sure you\'re wrong and I\'ve
devised an experiment to prove it.
I have a 1 ohm +/-5%, 5 watt wirewound resistor (Dale CW-5) from
Vishay, some Durapot 804 ceramic potting compound from Cotronics, an
HP6285A DC Power Supply and an Omega 450 AKT thermocouple
thermometer which I\'ll use to monitor the temperature of the
resistor with while it\'s dissipating 5 watts in still air at 25C and
after it\'s been encapsulated.
Since you also claim, in my opinion falsely, that a 4 watt resistor
can safely dissipate 10 watts after it\'s been encapsulated, I\'ll run
my 5 watt resistor at 12.5W after it\'s been encapsulated and post
what I find with respect to the derating curve you can find at:
http://www.vishay.com/docs/30215/cw.pdf.
If you like, you can also look up the spec\'s for the ceramic at:
http://www.cotronics.com/vo/cotr/pdf/801%20-%20820%20Potting%20Compounds.pdf
The resistor measures about 1" long by about 0.3" diameter, for a
surface area of:
A = (pi D H) + 2 (pi r²)
= (3.14 * 0.375" *1") + (2 * 3.14 * 0.188²
= 1.4 square inches
so if you\'d like to participate in the experiment by giving me the
dimensions of the box into which you\'d like to see the resistor
encapsulated, I\'ll see what I can do.
By the way, since you\'re new here, our custom is to bottom post and
to inline post when it\'s necessary, (or desirable) so I\'m sure
nearly everyone would be pleased if you did the same.
Thanks,
should
In his other post he mentions a coil in the circuit but I can't be bothered to look at it again. Perhaps the coil will have enough resistance anyway.
-- Experiment posted to abse.
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