Date: Fri, 4 May 2018 11:42:20 -0400
From: Peter Bertini <radioconnection@gmail.com>
To: amps@contesting.com
Subject: [Amps] Russian capacitor kbap ratings to current?
<I have several 470pF @ 15kV RF doorknob caps that have a 40kbap rating.
<Seller says the kbap markings are the same as our kvar.
<How do I calculate the maximum RF current these can handle?
<Is this the correct formula? (assuming they are well within their frequency
rating.)
*I = kvar x 1000 / Vrms x 1.73 *
73
Pete
## You cant. I just went through this exact same thing last week with
russian doorknob caps.
## The best I could come up with is to go to HECs site, high energy corp in
the usa,
and compare their graphs for similar rated doorknob caps, similar vdc, similar
pf ratings,
and similar kvar ratings.
## EG, the HEC HT-57 series.... or centralab version. BTW, centralab is
now ITT / jennings.
HT-57 series is 15 kv rated, and 35 kvar. BUT the current rating depends
on freq used, and also
cap value. KVAR is a bit of a misnomer. IF say you want it for plate block
cap use, you have to 1st
calculate the current through the block cap. And plate block cap current
will be equal to the current
that flows from anode to grid on a GG Triode. Take key down B+ voltage
and multiply by .6
Take that result and divide by the XC of the anode to grid C. You have to
know what the anode to grid C
is... WITH the tube plugged into socket. Which is always higher than when
its on the wooden bench.
XC of course will vary with freq in use. Lower freqs will have higher XC...
and higher freqs will have lower XC.
## Next calculate the XC of the plate block cap on a specific freq, like
both 160m and also 10m. Once u have calculated the current through the
block cap,
then multiply the block cap current by the calculated XC of the block cap.
That result will give u the RF RMS V drop across the block cap.
Then its easy to calculate the KVAR through the cap. block cap current
squared times XC. Or RF rms V squared..... divided by XC.
## The voltage across the cap is typ not very much. BUT, it is in addition to
the B+ across the block cap.
## IF on the other hand its going to be used for padding a load cap etc, then
just calculate the XC of the cap on a specific freq. Then calculate
the rms RF voltage across the cap. Current through the cap will be V
divided by XC.
## U need to go to HECs site... and find a comparable looking cap to the
russian one. If it’s a puck style, HEC also has those listed.
Don’t assume anything. U will see at a glance, that depending on cap value,
the rated current will vary greatly. EG, the highest current rated cap
in the HEC HT-57 series is the 200 pf cap...rated for 15 A. Go above or
below this value... and current ratings drop off.
## Typ generally speaking, on the lower freqs, the current rating drops
off..and max V across the cap increases. On higher freqs, the current rating
increases..and V rating drops off. HEC will depict on their graphs for
each cap, a graph of freq vs current, voltage and also KVAR.
http://highenergycorp.thomasnet-navigator.com/category/ceramic-capacitors
## HEC will stamp the series, like HT-57 on their caps, and also XXX pf
and also 15 kv, but nothing else. No current rating and no kvar rating.
Later... JIm VE7RF.
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