Hello Jim, tnx for comments.
Am 13.08.2011 02:28, schrieb James Moritz:
Dear Stefan, LF Group,
These metallised film capacitors are "self healing" - if a voltage
breakdown through the film occurs, the thin metallisation layers close
to the puncture are vapourised, clearing the fault, with a small
reduction in capacitance.
Yes, as written in the datasheet http://www.wima.de/EN/WIMA_FKP_1.pdf
So they will survive a certain amount of over-voltage events. But this
can only happen a finite number of times...
Yes. In my case it seems the number of events was above the limit. And i
expect the withstand voltage increases then since the arrangement
becomes rather inhomogenious. And there must be a blow up due to the
heat exposure, which is well visible in my case ;-)
If you look at the manufacturers' rating data, they show graphs of
max. AC voltage vs. frequency. At low frequencies, the voltage is
constant, the limit being the value at which the dielectric breaks
down. Above some frequency which depends on the capacitor value, the
voltage decreases proportional to 1/f, equivalent to saying that the
capacitor current is the limitation at high frequency. If you work out
what this current is, it is often suprisingly low, and many amateur TX
circuits exceed the limits. But the criterion for the rated current is
usually an internal temperature rise of 10 degreesC above ambient,
which is quite conservative for a reasonably well-ventilated capacitor
in an amateur shack, so significantly higher currents are OK in practice.
So what do you suggest? Using the caps in series to bring down the
voltage per cap or switching smaller values in parallel? I think it was
an overvoltage problem so i will try the series arrangement.
Just realised that it is easy to calculate things: The LC tank circuit
is directly connected to the PA output. So running 120 W RF at 50 Ohm
means 1.55 A rms is passing that C. At 3.5 nF this means an applied
voltage of 512 V rms. This was no problem at all during the last months
(TXing in QRSS-60 and so). The caps are specified to 700 V rms at 1 kHz.
So when running 500 W RF, as i did during the tests of the last days,
about 1 kV rms was applied. Surely this caused the breakdown. So if i
design the caps for 2 kV rms, all will be fine :-) 6 A rms will be no
problem for the caps i think, rather for the variometer ;-)
Cheers, Jim Moritz
73 de M0BMU
73, Stefan/DK7FC
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