Hi Alan,
Yes i know, but these caps perform very well in a my PAs so far. Have
never had a problem with them. The are rated at 2 kV DC and 700 V AC. I
alwyays ignored the 700V AC level. But probably the voltages in the
series L-C were much higher than that, probably even higher than 2 kV
rms. I will do measurements at lower power soon. So then, the solution
would be to make series parallel combinations, e.g. 3x 10 nF in series...
Am 13.08.2011 01:24, schrieb Alan Melia:
Hi Stefan, The voltage rating alone is not enough. The capacitors in the
Class E stages must be "pluse rated" They have to pass massive amounts of
current. I tried originally to use old valve caps but they are not rated for
that level of current and the connection of the individual foil plates to
the lead-out wires kept fusing and the capacitance went down and down. I
would have though the WIMA caps were intended for SMPSUs so should be
suitably rated.
Yes, they are excellent for impulse issues. I had some switching
impulses in the H bridge when operating without a load. A 33 nF cap
directly between the upper drain and the lower source was a nearby
perfect solution. Before i observed up to 3x UDC peaks.
Because it is in the antenna circuit is it possible it might have been
damaged by a nearby lighning surge??
No, almost impossible. Even in the case of a direct lightning strike i
expect the would survive, in that "position" where they are in the
circuit. I have a lightning spark gap across the isolator which has
about 4 cm distance. So just a short impulse would be applied to the
coil until it gets shortcutted by the gap. The voltage in that
arrangement, dorwn transformed to 50 Ohm wouldn't be high enough i assume.
But an interesting point!
73, Stefan
Alan G3NYK
----- Original Message -----
From: "Stefan Schäfer"<[email protected]>
To:<[email protected]>
Sent: Saturday, August 13, 2011 12:02 AM
Subject: LF: Faulty FKP-1
Minto, LF,
Maybe interesting for other homebrewers in the QRO region as well...
Actually the only thing that caused all the faults was a damaged WIMA
FKP-1-2 kV cap. I never have had problems with these caps before. Never
expected that. They are even suitable as a shunt cap for a 10 MHz 300W
class E PA or as the shunt cap in a 1 kW 137 kHz class E... But now one
looks quiet big here. I even heard it dying but thought the sound is
coming from the FETs, making some noise when switching, you know. Thus
the C was much lower and i didn't come on resonance. Probably the
undefined C (sometimes even an inner shortcut?) caused strong mismatch
and lastly destroyed the FETs.
That was the weakest link of the chain. QRO optimisations... The C was
0.33 nF + 1 nF + 2.2 nF in parallel, this in series to the small
variometer part in the shack. It allows resonating within the entire band.
Now replaced, all works well again...
73, Stefan/DK7FC
Am 13.08.2011 00:17, schrieb Stefan Schäfer:
Hi Minto.
The signal must have been about 6 dB lower than yesterday. Still have
to go back to the old system, unfortunately. Difficult to do good
measurements here in the "shack" where all the stuff is floating
either on mains voltage or on 137 kHz or a mixture of that. Have a
battery supplied oscilloscope but anyway.
I didn't got more than that what you saw, no higher power was possible
even when running the SMPS at 320 VDC. Unfortunately now i saw what
the reason was/is: Some of the turns of the loading coil moved a bit
and so i was outside the resonance. Since the Q of the TX antenna is
very high, the power goes down quickly when beeing outside the
resonance. I haven't had the tuning-meter (U,I, phase) in line, and
just tuned the shack-internal variometer for a maximum signal.
After the unwanted fault and replacing the FET i had to find other
problems like a non conducting shunt resistor (current sensing). This
wasn't visible. For some reason the current limiting didn't work then.
A second loud BAAAAAAANG caused me to *let it be* for this week! Now
two mains fuses here at QRL are OFF and i don't have the key to put
them in again.
Anyway, i'm in holiday for the next 2 weeks and they will be back in
when i am back and no one will remember that, hopefully, hi.
Actually all that stuff should be exchanges in a QSO :-) BTW how about
YOUR PA? :-)
73, Stefan/DK7FC
Now trying to bring the coil back to its previous L...
Am 12.08.2011 23:07, schrieb Minto Witteveen:
Hi Stefan.
I see your CQ on my grabber at this moment.
How does your ERP now compare with yesterday (during your QSO with Mal)?
It looks like S/N is now worse here then yesterday. Reason might be
the loop versus the miniwhip (with the miniwhip the winner). But on
the other hand there seems to be a lot of QRN (because of lightning
at this moment.
73’s Minto pa3bca
-------------------------------------------------------------------------
-
Ceterum censeo Carthaginem delendam esse
*From:* Stefan Schäfer<mailto:[email protected]>
*Sent:* Friday, August 12, 2011 20:02
*To:* [email protected]<mailto:[email protected]>
*Subject:* Re: LF: PA3BCA
Hi Minto,
Am 12.08.2011 19:42, schrieb Minto Witteveen:
How much power were you generating?
73’s, Minto pa3bca
It changes all the time. Not even sure since i just have an E field
probe to decouple some power from my TX antenna and feed it to a 50
uA amperemeter, shunted by a potentiometer. This poti was adjusted to
show 100% full scale when using 120 W RF before some weeks. Now
reduced the parallel R on the poti so it shows just 25 uA then. Got
50 uA anyway by QRO and more (not sure). So it should be arround
500W. This is -6 dBfs :-)
But the antenna, i.e. the coil and its cover is very lossy it seems.
When there was rain, i have up to twice the losses :-( Need to search
for an alternative cover (mustn't burn, so a water barrel is no
alternative...).
73, Stefan/DK7FC
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