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Hi Dimitris, LF,
here is a bunch of 33 nF / 2 kV
FKPs working in my indoor LF transmit loop experiment:
With 200 W TX power, the total current is 32 amps
rms. It is split up into 5 branches (6.4 amps each) in 5p-2s
configuration. A large series C has been inserted into one branch for fine
adjustment of resonance.
Attached is a graph from the WIMA datasheet showing
the voltage derating with frequency. Even at low frequency the AC voltage rating
(750 Vrms) is significantly less than DC (2000 V) to avoid aging due
to internal partial discharges. At higher frequency, voltage and
current are further derated due to self heating from ESR and
dielectric losses. The curves are for 10°C overtemperature in still
air, allowing 200 V x 6 amps rms at 136 kHz. Indeed
the capacitors became just noticibly warm during long transmissions. I
think that with fan cooling or oil immersion, you might be able to use
them up to 2 to 3 times higher currents.
Best 73,
MArkus (DF6NM)
Sent: Thursday, August 09, 2012 2:34
AM
Subject: Re: LF: Re: Caps for Class-E
amp
Hi Alan,
thanks for the tip, I will keep an
eye open for RIFA PHE428 or PHE450
which seems to be the newer version of
that capacitor type.
In my PA, I used two .1 uF caps in series to make
.05 uF, then three
of these in parallel to make .15 uF hoping that the
currents will be
distributed, however I think, once one of them gave up the
ghost, the
others immediatelly followed suit, so I ended up with one in
every
two-in-series capacitor being busted.
By the way, that
self-healing effect is quite useful if you ever want
to create a bunch of
lower value caps with non standard values :-) I
now have a77 nF, 45 nF, 16 nF
and even a 0.3 nF out of my .1 caps! I
am kidding of course. :-)
73,
Dimitris VK1SV
2012/8/9 Alan Melia <[email protected]>:
>
Hi Demitris, I am not sure of the types codes but I used 1000v or
greater
> rating, the problem often is that the caps are not rated for
high pulse
> currents. The best kind of caps that are used are rated for
pulse operation
> in switch-mode power supply. Normal caps will not take
the current
> pulses.....the individual plates get disconnected from the
lead wires.
> One of the caps I used has come to hand its a RIFA PHE428
rated 2000v. I
> used this on 36v Class E running about 300W. I believe
WIMA make similar
> caps but I am not sure of their type
numbers.
>
> Alan
> G3NYK
>
> ----- Original
Message ----- From: "Dimitrios Tsifakis"
> <[email protected]>
> To:
<[email protected]>
> Sent: Wednesday, August 08, 2012 11:03 PM
> Subject:
LF: Caps for Class-E amp
>
>
>> Hello LF
group,
>>
>> I am building a 200 W Class-E transmitter for LF
but keep killing the
>> capacitors in the output circuit (both C1 and
C2 according to the
>> schematic in Sokal's QST article). I have
used a mixture WIMA
>> polypropylene MKP10, MKP4 and FKP1 caps
and I haven't damaged any FKP
>> ones so far. Should I not bother with
MKP and use only FKP caps or is
>> there any other type of capacitor
what will do the job? I need a
>> couple of hundred of nanofarads. The
voltage rating of the caps I
>> destroyed was 400 VDC or 250 VAC. I can
see that these caps have a
>> decreasing AC rating as the frequency
goes up, but that's what I had
>> in the junk box at the
time...
>>
>> The mode of failure of these MKP10 0.1 400 VDC
caps is interesting
>> too, they seem to go down in capacitance as the
damage progresses.
>>
>> Any advice is
appreciated.
>>
>> 73, Dimitris
VK1SV/SV1DET
>>
>
>
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