A Project that Flopped
don't always work the way you expect and I've had my share of projects
that flopped. The most recent was a Heathkit Cantenna rebuilt.
all know they are a bit light duty for use with today's 1500 watt
amplifiers. I had one around that wasn't very healthy after some
extended amplifier testing and it bugged me. The mental gears
started turning and meshed on the idea that I should use the new thick
film resistors. After all, if that 3/4" x 4" carbon pole resistor
(rated 50 watts?) could handle a Kw for a couple of minutes in oil then
four 50 ohm 100 watt thick film resistors in series parallel on a 4 x
6" copper 1/4" slab in oil should be a lot better. The numbers
showed that the pole resistor had about 7 square inches of conducting surface to the oil and the copper plate had about 44 square inches in
the oil (48 less 4 for the resistor housings). To boot, I
reasoned it was going to take some serious emery to even heat up that
mass of copper.
WRONG! Those thick film
resistors are rated at exactly the maximum amount of heat is they can
transfer out through a relatively small ceramic pad on the back.
Turns out, 100 watts means 100 watts with the best heat diffusion
you can muster.
But, if you need a 100 or 250 watt dummy load that is very accurate, the thick film resistors
are a nice way to go.