[Amps] peak voltage rating for load cap

[Peter Voelpel]

The Russians doorknob capacitors do well.

Check Kevins page on qrz.com:

https://www.qrz.com/db/VK4AKV/

DL6EAT is using them with 8x MRF157 on Kevin´s board.

73
Peter, DJ7WW


-----Original Message-----
From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of Manfred
Mornhinweg
Sent: Samstag, 19. August 2017 18:54
To: amps at contesting.com
Subject: Re: [Amps] peak voltage rating for load cap

Conrad, and all,

keep in mind that the voltage rating of the capacitor is just one part 
of the whole thing. The other is its current rating. Once you have found 
out what's the highest RF voltage the capacitor will ever see, simply 
use that value in Ohm's Law, along with the reactance your capacitor has 
at the operating frequency, to get the current flowing to it. Then look 
for a capacitor that can handle both the voltage and the current, with 
some safety headroom.

You will quickly find that for most capacitors no current rating is 
given. Usually these are ABSOLUTELY unsuitable for RF applications at 
anything close to their rated voltage! In these applications you need 
"transmit-type" capacitors, which are designed to handle high current.

I have the same problem right now, only that I have it at least 18 
times. I'm looking for capacitors suitable for a set of legal-limit low 
pass filters. That requires capacitors of roughly 100 to 2000pF, each of 
them able to withstand AT LEAST 500V and 10A. Preferably some more, to 
have a better safety factor for high SWR situations. Can anybody point 
me to good sources of such capacitors? Everything I have found is either 
unsuitable, or unspecified and thus risky to use, or extremely expensive.

There are lots of hams out there trying their luck and using capacitors 
that lack a current rating. And also there are lots of reports out there 
of these capacitors blowing up in use... Not all do blow up, but I think 
it's a good idea to use capacitors rated to handle the job, rather than 
run risks. Specially with solid state, a blown capacitor can blow up a 
$200 LDMOSFET.

Manfred

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