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Re: [TCML] (no subject)

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.

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