Re: LF: Re: Faulty FKP-1

["James Moritz" <james.moritz@btopenworld.com>]

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. 
So they will survive a certain amount of over-voltage events. But this can 
only happen a finite number of times...
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.
Cheers, Jim Moritz
73 de M0BMU