Re: Fair caps test
>>From 74040.3317-at-CompuServe.COMWed Jul 3 22:15:52 1996
>Date: 03 Jul 96 10:48:39 EDT
>From: Charles Brush <74040.3317-at-CompuServe.COM>
>Subject: Fair caps test
>Those of you interested in the Fair Radio caps might be interested in a test I
>just tried. I'm in NYC and have only been able to run one of these caps in a
>very small system in my apartment. When I have time, I'd like to take them out
>to my folk's place on Long Island and try them in my larger neon system, but
>who knows when that will be (not enough free time!). Anyway, I thought I'd try
>a little "stress test" just to see how these caps held up. This consisted of
>running with just a transformer and gap across one of them. Results:
>-6kV 30mA for 5 minutes continuous, studs got slightly warm to the touch, no
>discernable change in case temp.
>-15kV 60mA for 5 minutes continuous, studs got a little warmer, case definitely
>getting warm, but not hot.
>I used my "enclosed" series static gap, since the air blown one was just too
>loud and I didn't want any complaints. By the end of the second test the gap
>was really cooking. Five minutes can be quite a long time!
>I'd be interested in hearing any thoughts on this test. Any idea how much
>other types of caps might heat up under similar conditions? If I could find my
>indoor/outdoor thermometer I'd get some numerical data.
I think that the results of your tests as to the performance to be
expected by the Fair Radio capacitors in Tesla service are
encouraging, but not conclusive. I think that the conclusive test
will be in an actual tuned Tesla coil that has been used tuned with
capacitors of known quality previously so that the spark ouput length
can be used as a performance gauge. For example, in my limited
experience with a small system (12kV-at-60mA) using a homemade glass plate
and oil capacitor (glass is a known poor lossy dielectric with undesireable
memory characteristic), and then going to a homemade polyethylene and oil
dielectric capacitor, the superior polyethylene cap very nearly doubled the spark
output length (both caps were the same value at 0.005 mfd and all
other system parameters were identical).
It is one thing to see if a capacitor will survive Tesla service, and
quite another if it will both survive AND perform well. Your test so
far probably indicates the survival expectation to be reasonably good
as long as the duty cycle is kept within reasonable limits to keep
undesireable dielectric heating at safe levels. If this capacitor
were the desired polypropelene in silicon oil, there would be no
detectable dielectric heating from your test, although the terminals
might still get warm.
As I recall, these capacitors were quiite expensive, was it $100.00
for a 0.01 mfd? I think at this price, if they don't give you twice
the spark length in a given system as compared to an identical value
glass plate and oil cap, then they are not worth this high price
As for monitoring the case temperature, that is a good idea. I fear
however that if your indoor/outdoor thermometer is a modern digital
unit with shielded cable to thermistor sensors, the high EMI field in
the vicinity of the capacitor in the test you described may well
permanently scramble the delicate electronics of the thermometer.
I would also keep in mind that the thermal transfer of heat from deep
inside the capacitor to the outside case is a slow process and that
it takes a long time for the instantaneous heat that you make by
abusing the dielectric in a short test to reach the case. In a first
run from cold, when the case starts to get warm the insides are
probably really hot. If the case temperature continues to rise
after shutting off the power, then this is indeed occuring.
Finally, I am suprised that your neons did not fail during the test
as described. You were feeding some whicked high frequency RF back
into those fragile transformers, apparently without the protection of
series RF isolation chokes.
You asked for my thoughts and here they are.
Happy Coiling!, rwstephens