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Jim, as usual, speaketh the TRVTH.
An 8kV transformer will charge a capacitor to close to 12 kV, which is
fine for a Tesla coil. Your cap should be rated for at least 30kV,
since it will be charged both positively and negatively, and should have
some slack on top of that. That will work for a rectified coil too,
since the voltage will stay on one side of zero. My coil runs on 9 kV,
with 50kV rated caps. If you go much above 15 kV of operating voltage,
you will encounter unauthorized arcs, sparks and coronas which will
require a new level of high voltage art and finesse.
Excellent advice to build a coil with one NST for starters.
--- Carl
On 8/15/2017 10:01 AM, jimlux wrote:
On 8/15/17 9:11 AM, Jeff Allen wrote:
Hello All, I have 8000vdc transformer at 1 amp. Do i need to use a
voltage
doubler to get up around 16000v? Can some one explain the differance
useing
ac or dc voltage for the Tesla? Im looking to build a large Tesla and
big
arckes. Thanks
is it 8kV DC, or AC? Most transformers put out AC but sometimes they
are packaged with an integral rectifier.
For a first coil, I'd start with a (iron core) neon sign transformer,
build that up and get through all the peculiarities of building a coil.
But DC is a bit trickier to use - WIth AC, the circuit is pretty
simple - the supply voltage charges a primary capacitor until the
spark gap breaks down connecting it to the primary inductor. That
resonant LC is coupled magnetically to the LC secondary circuit, the
energy transfers from primary to secondary, the voltage comes up on
the secondary and you get sparks.
With AC, the supply goes through zero periodically, so the spark gap
"turns off" naturally. With DC, you have to help it along -typically
by using a series inductor and a big HV diode - so now, you not only
have your HV supply, the primary capacitor and primary inductor, but
you have a "charging inductor" and a HV diode (that has to take a
pretty abusive environment... no 1N4001 in this application<grin>)
with 8kVA, you're also most likely looking at needing a rotary gap -
while people have built static gap coils at that power level, it's
tricky - probably a blast gap of some sort (high velocity air to "blow
out" the spark between half cycles, or at least cool the elecrodes).
Hence the suggestion to try a lower power NST based coil with a static
gap so you get the "feel" of tuning and stuff on something that won't
outright kill out or start a fire or spread shrapnel from a rotary gap
or worse if something goes wrong.
Or, find someone near you who's built coils before so they can give
you advice.
You'll also need a suitable primary capacitor - with 8kV you're going
to need a healthy HV capacitor that can take the RF current - current
techniques are "repurposed" and derated pulse discharge capacitors
from companies like Maxwell Labs/General Atomic OR series parallel
strings of high current polypropylene dielectric snubber capacitors.
Neither of these is cheap at this power level (hundreds of $)
Do you want a doubler? That kind of depends - 8 kV is certainly
enough to make sure the spark gap breaks down, but you're also
doubling the current. Most AC coils run at 14-15kVrms which is about
20kV peak. But building a high power doubler is no trivial matter
either.