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[TCML] Strike rings - to use or not to use; was: Newbie Grounding Question
Hi Gary,
I do understand what you're saying about the conservation of energy not
allowing the primary capacitor to be "overvolted" by a streamer striking
the primary coil during operation. And that would lend credence to Bart
Anderson's practrice of not using a strike ring at all. However, I have ex-
perienced running my big Green Monster coil on a windy day where the
wind actually "blew" the output streamers back into the side of the lower
third of the secondary coil. Although my primary capacitor and transfor-
mer (pole pig) did survive the hit, my 18 kv rated distribution arrester
failed as a short circuit. This didn't really make sense to me either as the
base of the secondary coil as well as the primary coil is tied securely to
the RF ground. And yes I WAS using a grounded strike ring in this case,
so the strike ring didn't do me any good. The streamer completely by-
passed the strike ring and crashed into the lower third of the secondary
coil, thanks to the influence of the wind. Of course what you're saying
makes
perfect sense, too. There simply should not be enough available energy in
the
output streamers to be able to charge the primary capacitor above the
voltage
that it initially discharged into the primary coil. It's akin to trying to
overvolt
a 10 kV, 100 uFd energy discharge cap by charging it with a few cycles
from a 50 kV, 5 mA flyback driver. Although there's 50 kV potential
available, at only 5 mA of current, that means that there is only 250
joules per second available to charge the capacitor: 50,000(.005) = 250
That means that you would have to charge a 10 kV, 100 uFd (5 kJ)
for a minimum of 20 seconds contiunously with the flyback driver
before you'd be in any danger of overvolting the said capacitor.
Still, the momentary streamer hit to my secondary did cause an overvoltage
that did leave its mark - on my distribution arrester. Maybe others can add
their commnets to this subject as well.
David Rieben
----- Original Message -----
From: "Lau, Gary" <Gary.Lau@xxxxxx>
To: "'Tesla Coil Mailing List'" <tesla@xxxxxxxxxx>
Sent: Friday, December 28, 2007 9:36 PM
Subject: RE: [TCML] Newbie Grounding Question
If one is considering damage to the cap, this can only occur by the voltage
between the two plates exceeding the voltage rating of the cap. Let's say
that just prior to the gap firing, the cap voltage was 20kV. The gap fires,
the primary rings down, the secondary rings up, an x-hundred-thousand volt
spark jumps from the top load to an exposed terminal on the cap. But there
is no way, even if we assume no losses in anything, that that spark can
recharge the cap to anything higher than 20kV.
Thinking about it another way the total Csec is in the tens of pF. The
voltage on such a small cap would need to be VERY high to recharge the much
larger primary cap to a voltage that exceeds its voltage rating,
particularly if you include losses.
I suppose that damage to the plastic case of the cap could occur if a
streamer was in direct contact with the case for an extended interval, but
that sounds unlikely. I can't think of any other way that a high voltage
can damage a cap.
Regards, Gary Lau
MA, USA
-----Original Message-----
From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
Behalf Of Scott Kraus
Sent: Friday, December 28, 2007 8:06 PM
To: Tesla Coil Mailing List
Subject: RE: [TCML] Newbie Grounding Question
Hi Gary,
I have a question in reference to your statement about strikes to the
primary not
being able to overvolt the cap because of conservation of energy. I
understand that
the energy can't be higher but couldn't the voltage from the topload be
dangerous
for the cap? I suppose that there are easier paths to ground in the
primary circuit
than through the cap dialectric. My pig coil runs with one side of the
xfmr., gap,
strike rail and inside primary turn all tied to the rf ground.
I have seen other references to this in the archives but this seems
like a good
time to ask.
Scott
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