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Re: Info and requests for book
From: terryf-at-verinet-dot-com[SMTP:terryf-at-verinet-dot-com]
Sent: Wednesday, September 17, 1997 3:19 PM
To: Tesla List
Subject: Re: Info and requests for book
Hi all.
Mark and I said in part:
>From: Mark S. Rzeszotarski, Ph.D.[SMTP:msr7-at-po.cwru.edu]
>Sent: Wednesday, September 17, 1997 2:27 AM
>Hello all:
>Terry Fritz said in part:
><snip>
.....................
>>I am looking for any info on the following:
>>1. Any information on the losses on the secondary such as secondary
>>terminal capacitance change vs voltage, Energy loss vs voltage and time,
>>ect. would be useful.
> Do you have a really good method for measuring the voltage of a
>tesla coil during spark breakout? If so, I am interested. I do not have a
>model for this either. Recent off-list discussions suggest that sparks
>introduce additional top capacitance on the order of 1-3 pF per foot of spark.
>
If no one else has any ideas, I was going to build up an elecrostatic
capacitive voltage divider probe that could be inserted up the center of the
coil. This would be very similar to the voltage transducers used on
high-voltage power lines (I used to repair these long ago). This would have
to be custom built to get good high frequency response and low insertion
capacitance. It may need an electrostatic sheild as well due to all the
"noise" around the coil (that would need much thought). It would also need
much protection on the output to protect the scopes and such it would be
plugged into (gas suppressors and transorbs). And it would need to be able
to live inside the secondary coil without causeing any bad effects. It
could be made easily with thin pvc pipe, a few good caps (the top high
voltage one would obviously be real special - but straight forward), and
lots of oil. Obviously, the actual dynamics that occur during the disharge
affect the values of the coupling, pulse times, energy transfere and
loss........ It is a very important topic. If we have a good idea of what
happens to the capacitance and energy loss then we can model and optimize
the other factors much better.
>>2. Has anyone tried using varistors (transient surge absorbers) instead
>>of spark gaps for transformer protection? I realize they my need to be
>>isolated with chokes due to their capacitance but in quantity they are
>>fairly inexpensive (1800V 4500A $1.88ea from DigiKey) and they can be easily
>>chained together to reach a desired voltage. I would think they would give
>>much better protection than a spark gap.
> They are worth investigating, but I have my doubts. How do they
>perform at RF frequencies? Can they switch on fast enough? Will a
>resistive voltage divider work once these things start to turn on? Peak
>kickback can be more than three times the peak A.C. output voltage of the
>transformer.
>
All great questions... They state a rise time of 8uS which is not quite
good. However, a few years ago I talked to an engineer from on of these
manufacturers and he said that their measurments left much to be desired.
Perhaps good RF layout would increase their speed to accepatable levels.
Once they are "on" they will clamp anything until they explode. - Another
minor problem :-) They need to be able to absorb the full energy of the
primary circuit without damage. That's like 150 Joules for us high power
folks :-)) but lower power systems should have more luck if this is a limit.
Perhaps this is is not a concern since they SHOULD turn "off" once the
voltage returns to an acceptable level(they wouldn't have to absorb all the
energy). There are gas tubes and transorbs (which would be the ultimate
fix) but their cost would be high. I really am looking for a dead sure way
to protect the power supplies and caps from damage. It is far easier and
cheaper to protect them that repair this equipment. I will try to look into
these more. Just wanted to see what people already knew.
>>3. Any info on predicting secondary parasitic capacitance or resonant
>>frequency. This is a big killer for most Tesla coils and the info I have
>>seen tends to fall far short of what is needed. There are certainly better
>>ways to wind the secondary to reduce this capacitance but I have never seen
>>any good details on doing this. The programs and equations I have seen have
>>never predicted any of my coils very well at all. Virtually all Tesla coils
>>tend to be big delay lines instead of antennas.
> Distributed capacitance is easily modelled and is essentially
>isotropic in nature, related to the area of the coil on the former. It is
>not turns dependent. It is not a killer unless you build really big coils,
>and top capacitance does much to tame it.
>
Big top capacitance goes a long way toward swamping the coil capacitance and
getting all the energy to the top. However, it also takes much energy to
charge these big toriods. This takes time (ligthning doesn't wait around)
and it also limits maximum voltages. I have to disagree about how easy it
is to model or discount this capacitance. Idealy we wouldn't have any
internal or top capacitance except what is generated by the giant "spark" an
ideal system would produce. I would rather use thin toroids to control
E-fields rather than produce large capacitances. However, this is something
I need to study more.
Also, has anyone tried clear "KRYLON" spray paint as a coil coating? I
worked on designing high voltage power supplies (200kV) a few years back and
we sprayed that stuff around like holy water. I was always very impressed
with its high voltage performance (hard to puncture, seemed to not mind
corona, etc.). It was very nice to work with (fast drying, no drips, etc.)
Those were DC systems however and I hate to try it out if someone knows a
problem with it and Tesla coils.
................
Terry