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| To: | <[email protected]> |
|---|---|
| Subject: | Re: LF: Re: Faulty FKP-1 |
| From: | "James Moritz" <[email protected]> |
| Date: | Sat, 13 Aug 2011 01:28:55 +0100 |
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| In-reply-to: | <[email protected]> |
| References: | <[email protected]> <52970A62D1224CAF9A966D1743DCBF00@PcMinto> <[email protected]> <4FB899125601413A9077646E2728C784@PcMinto> <[email protected]> <[email protected]> <002101cc5947$0a6ce300$4001a8c0@lark> <[email protected]> |
| Reply-to: | [email protected] |
| Sender: | [email protected] |
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 Moritz73 de M0BMU |
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