Re: Primary Voltage

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Terry Fritz" <teslalist-at-qwest-dot-net>

Hi Paul,

At 07:15 PM 3/16/2003 +0000, you wrote:
>Thanks,
>
>
>Hello,
>
>I have now managed to get all the bits of my Tesla Coil in place and have
>been doing a few test and come up with some interesting measurements and
>just wanted to confirm them.
>
>I wired the spark gaps up and they were all working fine, I then brought the
>gaps in to fire at 2KV so I could test the primary circuit. When the primary
>was connected I ended up getting a resonant gain in the order of 20 (40KV
>across the cap and inductor), so if I apply the full 10KV my inductor and
>capacitor are going to see 200KV.

My first thought is that something is affecting your measurement system to 
report a higher voltage than you are actually getting.  Could the super 
high currents be leaking into your measurements or the high voltage be 
leaking into the probes?

If you start with a cap say 30nF at 5000 volts, you have 1/2CV^2 of energy 
available or 0.375 Joules.  In order for the cap to charge to 20 times that 
voltage you need 150 Joules!!!  Clearly, there is no place for that added 
energy to come from.  You should see a 2kV ring-down to zero.  See the 
paper at:

http://hot-streamer-dot-com/TeslaCoils/MyPapers/modact/modact.html


>But looking at some designs on the web I notice very few people have
>actually designed there tank capacitor to withstand these high voltages.
>Usually using about 2.5 times over the supply rated voltage.

They really only have to stand the peak voltage.  For a say 15kVrms input 
that is 21213 volts peak.  One can turn up the variac to 140 VAC and things 
like that, but the voltage never rings higher than the available source 
voltage.  It would violate the conservation of energy of the system.

Some people like to over size cap voltage just for safety 
margin.  Personally "I" and many "EMMC" designs underrate the voltage with 
no problem.  RMS current is the real killer of caps.  If a cap can stand 
the peak transformer voltage, that is good enough.


>Am I missing something ? Yes I have access to a High Voltage lab with test
>equipment that can withstand these voltages, but my caps are not rated to
>200KV ! Should they be?

I would recheck things.  I think the high currents are messing with you 
test setup.  There is also a super messy high frequency harmonic filled 
burst when a gap first fires that you may be pickup up too (also at the 
zero current crossings when the gap "winks out" briefly).  This is caused 
by the "instant" shorting of the capacitance of the spark gap:

http://hot-streamer-dot-com/TeslaCoils/MyPapers/spark1/sparkgap.html

This burst will have far higher frequencies than the normal tank circuit 
ringdown but it sure can shoot into test equipment giving odd readings


>Any info would be helpful !

Hope this helps.  If I knew more about the equipment you used to measure 
this I might have more ideas.  You may try just moving the location of the 
test cables around.  If that changes your readings a bunch, you got noise...

Cheers,

         Terry


>Paul