>Not so sure about 15, though. However, this argument assumes that the
>ambient temperature of the environment in which that resistor sits is low
>enough that the resistor still has dissipation capability available
>sufficient for the reduced dissipation in it resulting from the lower RF
>voltage across it, on lower frequency bands. For example, if the dissipation
>on 10 metres is say, 10 watts, then it will be 2.5 watts on 20. If the
>resistor has to be derated because the ambient is 80 deg C, then even the
>lower dissipation on the lower bands may well be enough to 'cook' it.
Thank you, Peter. Well put.
I did all my damage to Rich's resistors on 10M and I was using it cause
it is the worst case scenario.
Here is an idea that I have to perhaps get a better handle on how hot
these things actually get in an amplifier:
1.) Construct a parasitic suppressor of Rich's design and a parasitic
suppressor of "traditional" design. What is important is that we know
precisely the value of the inductors.
2.) On the resistor body, attach either a thermocouple or some
temperature sensitive paint that changes color with increasing
temperature (they do make this don't they?).
3.) Hook up one of the suppressor assemblies to an amplifier in normal
fashion
4.) Run the amp at key down at rated power into a dummy load for five
minutes starting on the lowest band of operation.
5.) Measure and record the temperature of the suppressor resistors - A
thermocouple is accurate, but I am not sure if one would want extra wires
hanging around high voltage which is why I suggested the paint. Of
course the best way would be to measure the temperature of the resistors
with a thermal imaging camera. Then you could see where all the hot
spots are, etc.
6.) Calculate the dissipation in the resistors based on Rich's formula.
7.) Compare the dissipation calculated with the maximum dissipation
rating based on the manufacture's spec sheets.
8.) Do a FIT analysis and calculate the MTTF of the resistor given its
dissipation and temperature.
9.) Repeat all steps for each band through 10 Meters.
10.) Repeat steps 1-9 for the second suppressor assembly.
Unfortunately, I don't have access to this sort of test equipment. Also,
I feel an independent person should do this as I have a bias. If there
is anyone who would like to volunteer to do this it would be a great
thing.
My point is to determine the following:
1.) Are the suppressor resistors being operated ABOVE their specified
dissipation ratings at the temperature of operation?
2.) What is the effect of dissipation and temperature on the overall
lifetime of the device?
I think many people may be surprised by some of these results.
If I end up being wrong about my theories on resistor failures as a
result, I'll admit it and shut up. But we won't get anywhere unless
scientific measurements are taken by an independent person who has
nothing to gain or lose from this.
73,
Jon
KE9NA
-------------------------------------
Jon Ogden
KE9NA
http://www.qsl.net/ke9na <--- CHECK IT OUT! It's been updated!!!!!
"A life lived in fear is a life half lived."
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