Received: from post.thorcom.com (post.thorcom.com [195.171.43.25]) by mtain-mp06.r1000.mx.aol.com (Internet Inbound) with ESMTP id AD02538000090; Sun, 14 Aug 2011 12:15:50 -0400 (EDT) Received: from majordom by post.thorcom.com with local (Exim 4.14) id 1QsdKy-000826-Cr for rs_out_1@blacksheep.org; Sun, 14 Aug 2011 17:14:56 +0100 Received: from [195.171.43.32] (helo=relay1.thorcom.net) by post.thorcom.com with esmtp (Exim 4.14) id 1QsdKx-00081x-MS for rsgb_lf_group@blacksheep.org; Sun, 14 Aug 2011 17:14:55 +0100 Received: from imr-ma04.mx.aol.com ([64.12.206.42]) by relay1.thorcom.net with esmtp (Exim 4.63) (envelope-from ) id 1QsdKu-00075x-U2 for rsgb_lf_group@blacksheep.org; Sun, 14 Aug 2011 17:14:55 +0100 Received: from mtaout-ma06.r1000.mx.aol.com (mtaout-ma06.r1000.mx.aol.com [172.29.41.6]) by imr-ma04.mx.aol.com (8.14.1/8.14.1) with ESMTP id p7EGEjWr024238 for ; Sun, 14 Aug 2011 12:14:45 -0400 Received: from White (nrbg-4d057361.pool.mediaWays.net [77.5.115.97]) by mtaout-ma06.r1000.mx.aol.com (MUA/Third Party Client Interface) with ESMTPA id 75088E000142 for ; Sun, 14 Aug 2011 12:14:42 -0400 (EDT) Message-ID: <4193E2C26B874319A8FAC77EE03475CC@White> From: "Markus Vester" To: 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> <4E467FE1.9080808@iup.uni-heidelberg.de> <46BE41500D6B4F71B5BBFFDDAF4E0FA6@JimPC> Date: Sun, 14 Aug 2011 18:14:56 +0200 MIME-Version: 1.0 X-Priority: 3 X-MSMail-Priority: Normal Importance: Normal X-Mailer: Microsoft Windows Live Mail 12.0.1606 X-MimeOLE: Produced By Microsoft MimeOLE V12.0.1606 X-AOL-SCOLL-SCORE: 0:2:436651424:93952408 X-AOL-SCOLL-URL_COUNT: 0 X-Spam-Score: 0.0 (/) X-Spam-Report: autolearn=disabled,HTML_MESSAGE=0.001 Subject: Re: LF: Re: Faulty FKP-1 Content-Type: multipart/alternative; boundary="----=_NextPart_000_000F_01CC5AAE.1249AF50" X-Spam-Checker-Version: SpamAssassin 2.63 (2004-01-11) on post.thorcom.com X-Spam-Level: X-Spam-Status: No, hits=0.6 required=5.0 tests=HTML_20_30,HTML_MESSAGE, MISSING_OUTLOOK_NAME autolearn=no version=2.63 X-SA-Exim-Scanned: Yes Sender: owner-rsgb_lf_group@blacksheep.org Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group X-SA-Exim-Rcpt-To: rs_out_1@blacksheep.org X-SA-Exim-Scanned: No; SAEximRunCond expanded to false x-aol-global-disposition: G X-AOL-SCOLL-SCORE: 0:2:466546848:93952408 X-AOL-SCOLL-URL_COUNT: 0 x-aol-sid: 3039ac1dc14a4e47f4b640fc X-AOL-IP: 195.171.43.25 X-AOL-SPF: domain : blacksheep.org SPF : none Dies ist eine mehrteilige Nachricht im MIME-Format. ------=_NextPart_000_000F_01CC5AAE.1249AF50 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable Jim, thank you for this clear and well thought out explanation! 73 de = Markus (DF6NM) From: James Moritz=20 Sent: Sunday, August 14, 2011 1:07 AM To: rsgb_lf_group@blacksheep.org=20 Subject: Re: LF: Re: Faulty FKP-1 Dear Stefan, LF Group, > So what do you suggest? Using the caps in series to bring down the = voltage=20 > per cap or switching smaller values in parallel? Well, The first priority is to ensure that the AC breakdown voltage is = high=20 enough - the capacitors will not survive long otherwise. Practically, = this=20 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,=20 > about 1 kV rms was applied. Surely this caused the breakdown. So if i=20 > design the caps for 2 kV rms, all will be fine :-) 6 A rms will be no=20 > problem for the caps i think, rather for the variometer ;-) If, for example, one looks at the Wima data for 1000pF, 2kV DC FKP1=20 capacitors, the RMS voltage limit is 700V up to about 55kHz, above which = it=20 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=20 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=20 with the required current rating. But the Wima data is based on only 10degreesC teperature rise - = presumably=20 to allow operation at high ambient temperatures without exceeding the=20 100degreesC maximum operating temperature. In a reasonably = well-ventilated=20 amateur TX circuit, the ambient temperature probably will be below 40=20 degrees, so perhaps 60 degrees of internal I^2R heating would be the = maximum=20 allowable. This would allow the current rating to be increased by a = factor=20 of about 2.4. So now the 1000pF capacitor could be operated at 0.58A=20 maximum - the RMS voltage at 137k would then be 680V, still within the=20 "rupture" limit. For 500W out, the current into 50ohms load would be 3.2A, so 7 x 1000pf=20 capacitors in parallel would have an adequate current rating ... A 2 x 7 = series/parallel combination should be adequate for the applied current = and=20 voltage, and give the 3.5nF you had before. I am not sure if capacitor manufacturers would approve of this = calculation,=20 but practical experience shows that the capacitor current can be=20 considerably higher than the values implied by the data sheet graphs. Incidentally, I believe the reason the HF RMS voltage limit of these=20 capacitors is always well below VDC / sqrt(2) is due to corona = discharges=20 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=20 applied. So the AC voltage has to be derated to a level below the onset = of=20 corona. Cheers, Jim Moritz 73 de M0BMU=20 ------=_NextPart_000_000F_01CC5AAE.1249AF50 Content-Type: text/html; charset="utf-8" Content-Transfer-Encoding: quoted-printable
Jim,  thank you for this clear and well thought out explanation! 73 = de=20 Markus (DF6NM)
 

From: James Moritz
Sent: Sunday, August 14, 2011 1:07 AM
Subject: Re: LF: Re: Faulty FKP-1

Dear Stefan,=20 LF Group,

> So what do you suggest? Using the caps in series = to bring=20 down the voltage
> per cap or switching smaller values in=20 parallel?

Well, The first priority is to ensure that the AC = breakdown=20 voltage is high
enough - the capacitors will not survive long = otherwise.=20 Practically, this
will require series capacitors to reach your = required=20 operating voltage ...

> So when running 500 W RF, as i did = during the=20 tests of the last days,
> about 1 kV rms was applied. Surely this = caused=20 the breakdown. So if i
> design the caps for 2 kV rms, all will = be fine=20 :-) 6 A rms will be no
> problem for the caps i think, rather for = the=20 variometer ;-)

If, for example, one looks at the Wima data for = 1000pF,=20 2kV DC FKP1
capacitors, the RMS voltage limit is 700V up to about = 55kHz,=20 above which it
decreases proportional to 1/f. This implies an RMS = current=20 limit of only
0.24A. For a 10nF value, the maximum current works out = to=20 about 1.1A - the
current limit does not increase in proportion to C. = So it=20 would be quite
hard to produce a series combination with a = reasonable number=20 of capacitors
with the required current rating.

But the Wima = data is=20 based on only 10degreesC teperature rise - presumably
to allow = operation at=20 high ambient temperatures without exceeding the
100degreesC maximum=20 operating temperature. In a reasonably well-ventilated
amateur TX = circuit,=20 the ambient temperature probably will be below 40
degrees, so = perhaps 60=20 degrees of internal I^2R heating would be the maximum
allowable. = This would=20 allow the current rating to be increased by a factor
of about 2.4. = So now=20 the 1000pF capacitor could be operated at  0.58A
maximum - the = RMS=20 voltage at 137k would then be 680V, still within the
"rupture"=20 limit.

For 500W out, the current into 50ohms load would be 3.2A, = so 7 x=20 1000pf
capacitors in parallel would have an adequate current rating = ... A 2=20 x 7
series/parallel combination should be adequate for the applied = current=20 and
voltage, and give the 3.5nF you had before.

I am not sure = if=20 capacitor manufacturers would approve of this calculation,
but = practical=20 experience shows that the capacitor current can be
considerably = higher than=20 the values implied by the data sheet graphs.

Incidentally, I = believe the=20 reason the HF RMS voltage limit of these
capacitors is always well = below VDC=20 / sqrt(2)  is due to corona discharges
occuring in microscopic = voids in=20 the film, which cause the dielectric to
deteriorate over a period of = time.=20 This does not happen when a DC voltage is
applied. So the AC voltage = has to=20 be derated to a level below the onset of
corona.

Cheers, Jim=20 Moritz
73 de M0BMU
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