Thanks Marco,
I added the 940C to my list.
I don’t know if the ESRs in the 940C data sheet are specified at 120 Hz or 1 kHz, but in any case the 940C ESR and other specifications look good.
Looking at the 940C package brings to mind that I have been series/parallel stacking the rectangular PHE450 capacitors incorrectly: no air gap for cooling. I wonder if a stack of 940Cs would be better than rectangular capacitors (PHE450 etc.) in a forced-air configuration. Or in de-ionized ice water as Stefan mentioned.
73,
Jim AA5BW
From: [email protected] [mailto:[email protected]] On Behalf Of [email protected]
Sent: Thursday, January 31, 2019 2:14 PM
To: [email protected]
Subject: R: RE: LF: TXing on indoor/outdoor loops and Silver mica caps
Hi Jim,
just to share my feelings / thoughs ...
Since I started to play with LF I collected a number of R.I.P. caps ;-) and failure reason, on my opinion, was that the current was exceeding the capability of the cap.
I was used to refer to the insulation voltage and thus seen the trepassing of a lot of caps, the found this serie
http://www.cde.com/resources/catalogs/940C.pdf
and never had further trespassing at least in low impedance, relative high power: for instance for the caps insulating the dc component and carrying the RF power from a 500W pushpull @28Vdc.
For other similar application like for instance the caps in the output LC of Decca style PA or Class E PA I used succesfully either the above CDE caps or WIMA FKP1 combining series /parallel in order to have the wished capacitance / voltage insulation and current capability.
I guess should be nice organize a day to remember all our ex-devices ;-)
73, Marco IK1HSS
----Messaggio originale----
Da: [email protected]
Data: 31-gen-2019 18.55
A: <[email protected]>
Ogg: RE: LF: TXing on indoor/outdoor loops and Silver mica caps
I have been using what appear to be similar polypropylene capacitors [data sheet specifications (see below) including spectral loss tangent seem very similar] for multi-turn indoor and outdoor (35 meter x 35 meter) single-turn VLF transmitting loops for about five years (when time is available) and have found the following:
1) Temperature rise has been roughly consistent with the spectral loss tangent table
2) No change noticed in capacitance after approximately 200 of hours of transmitter operation at capacitor case temperature of 115 F (46 C)
Used 80-PHE450HD6470JR6L2 (0.47 uF) and also used 0.1 uF through 1.0 uF from the same PHE450 product line; always series/parallel stacking capacitors of the same value to enable higher current and/or voltage at a given capacitance.
Data sheet:
https://www.mouser.com/datasheet/2/212/KEM_F3033_PHE450-1104182.pdf
Reached somewhat of a practical limit of series/parallel stacking of these capacitors, for indoor multi-turn loops from 1 meter diameter to ten square meters, and outdoor loop 35 m x 35 m; and have hoped to find capacitors that would allow more current and voltage. I thought I had investigated (through calculations, not on the workbench) silver micas and found these polypropylene capacitors to be better based on calculations, but based on the messages between Stefan, Dimitris and Marko I must have missed something (perhaps temperature rating?); thanks for the inputs on this topic; looking forward to silver micas.
73,
Jim AA5BW
Polypropylene capacitors can actually self-heal to such an extent that there's little left of them. The melting point of polypropylene is not very high and they can end up as just a liitle blob of plastic.
Don't worry though, I still use them suitably rated for the frequency in use and most of them are 600Vac or more.
----- Receiving the following content -----
Time: 2019-01-31, 00:25:55
Subject: Re: LF: TXing WSPR/MF on an indoor loop, part 2
Hi Stefan,
In my experience these polypropylene capacitors when they fail, they
just lose some capacitance. I think they call it 'self healing', maybe
they imply that when they fail there is no short circuit?
I have some really nice silver mica capacitors (ex MW broadcast stuff)
that would be perfect for this task...
Anyway, I am slowly gearing up for some VLF action over here. The
other day I did some tests at 16.72 kHz into an ugly loop (very
roughly 120 metres by 3 metres, single turn) and I was able to receive
my signals using an unoptimise ferrite antenna at a bit over 2 km from
the TX. I don't think I am in the near field at that distance, am I?
:-)
73, Dimitris VK2COW
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