DomainKey-Signature: a=rsa-sha1; q=dns; c=nofws;
  s=s1024; d=btopenworld.com;
  h=DKIM-Signature:X-Yahoo-Newman-Property:X-YMail-OSG:X-Yahoo-SMTP:Received:Message-ID:From:To:References:In-Reply-To:Subject:Date:MIME-Version:Content-Type:Content-Transfer-Encoding:X-Priority:X-MSMail-Priority:X-Mailer:X-MimeOLE;
  b=kdzLe5smBYXMVbopATUXZtbnQYSIWmZ8uXAOuQxQ2hGmMIAAJhCcp5dOcKA5ZZ968pM4WOOLuCiwnHPAAYB0kMoGEkDlGvk4wdoSsl+1soOJgzj4Cs9GADTxkeRcNcn1RatDd5DzOkB45+Z5zVT394gD5SAv+J3uSIxcLSfXOPA=  ;
Message-ID: <46BE41500D6B4F71B5BBFFDDAF4E0FA6@JimPC>
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
References: <4E4527EF.2000304@iup.uni-heidelberg.de> <52970A62D1224CAF9A966D1743DCBF00@PcMinto> <4E456AB7.6090403@iup.uni-heidelberg.de> <4FB899125601413A9077646E2728C784@PcMinto> <4E45A678.5010908@iup.uni-heidelberg.de> <4E45B0F9.1000007@iup.uni-heidelberg.de> <002101cc5947$0a6ce300$4001a8c0@lark> <4E45B929.70802@iup.uni-heidelberg.de> <F7DB147AC432438F8811F0E44DE2A13C@JimPC> <4E467FE1.9080808@iup.uni-heidelberg.de>
In-Reply-To: <4E467FE1.9080808@iup.uni-heidelberg.de>
Date: Sun, 14 Aug 2011 00:07:12 +0100
MIME-Version: 1.0
DomainKey-Status: good (testing) 
Subject: Re: LF: Re: Faulty FKP-1
Content-Type: text/plain;
	format=flowed;
	charset="UTF-8";
	reply-type=response
Content-Transfer-Encoding: 7bit
Sender: owner-rsgb_lf_group@blacksheep.org
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org
X-AOL-SCOLL-URL_COUNT: 0  
X-AOL-SCOLL-AUTHENTICATION: mail_rly_antispam_dkim-m268.2 ; domain : btopenworld.com DKIM : pass  
x-aol-sid: 3039ac1d60184e4703d91546
X-AOL-IP: 195.171.43.25
X-AOL-SPF: domain : blacksheep.org SPF : none


Dear Stefan, LF Group,

> So what do you suggest? Using the caps in series to bring down the voltage 
> per cap or switching smaller values in parallel?

Well, The first priority is to ensure that the AC breakdown voltage is high 
enough - the capacitors will not survive long otherwise. Practically, this 
will require series capacitors to reach your required operating voltage ...

> 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 ;-)

If, for example, one looks at the Wima data for 1000pF, 2kV DC FKP1 
capacitors, the RMS voltage limit is 700V up to about 55kHz, above which it 
decreases proportional to 1/f. This implies an RMS current limit of only 
0.24A. For a 10nF value, the maximum current works out to about 1.1A - the 
current limit does not increase in proportion to C. So it would be quite 
hard to produce a series combination with a reasonable number of capacitors 
with the required current rating.

But the Wima data is based on only 10degreesC teperature rise - presumably 
to allow operation at high ambient temperatures without exceeding the 
100degreesC maximum operating temperature. In a reasonably well-ventilated 
amateur TX circuit, the ambient temperature probably will be below 40 
degrees, so perhaps 60 degrees of internal I^2R heating would be the maximum 
allowable. This would allow the current rating to be increased by a factor 
of about 2.4. So now the 1000pF capacitor could be operated at  0.58A 
maximum - the RMS voltage at 137k would then be 680V, still within the 
"rupture" limit.

For 500W out, the current into 50ohms load would be 3.2A, so 7 x 1000pf 
capacitors in parallel would have an adequate current rating ... A 2 x 7 
series/parallel combination should be adequate for the applied current and 
voltage, and give the 3.5nF you had before.

I am not sure if capacitor manufacturers would approve of this calculation, 
but practical experience shows that the capacitor current can be 
considerably higher than the values implied by the data sheet graphs.

Incidentally, I believe the reason the HF RMS voltage limit of these 
capacitors is always well below VDC / sqrt(2)  is due to corona discharges 
occuring in microscopic voids in the film, which cause the dielectric to 
deteriorate over a period of time. This does not happen when a DC voltage is 
applied. So the AC voltage has to be derated to a level below the onset of 
corona.

Cheers, Jim Moritz
73 de M0BMU