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general questions about tesla coils

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LAN_403

Hazmatt_(The Underdog)

Mon Nov 20 2006, 02:08AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

What I have noticed in simulation is that your tank ring-down voltage peaks at roughly twice the input charging voltage. That means if have a 10KV NST and you are charging an LTR capacitor, the decaying ring-down will oscillate between +10kV and -10KV while decaying exponentionally.

You should also consider the current loops. During simulation you can expect several hundred amps through the tank circuit during ringdown. This is a low impedance path, but the transformer is a high impedance path (reative to primary). The current will go through both primary inductor and it will feed back through the transformer as well, ie. two current paths. You could think of it as the primary tank getting 99% and the transformer having 1% surging back through it, conceptually speaking.

Now if you wanted to put a safety gap in parallel with the transformer and the capacitor in parallel with the transformer, then that gap would fire shorting the transformer and the capacitor and close the circuit to the Lprimary. The ringdown would be reflected to the transformer as the current pulses back and forth through the capacitor. This could spell a lot of trouble for your transformer.

Generally you want to keep the high ringdown effects isolated from the transformer so that it operates as intended without a lot of stresses. The insulation class is pretty poor on these things. If you talk to Franceformer, you will find out that transformers are insulated for .707 Vpk because they're intended to be connected to a diode all the time (neon sign tube)

A good filter design can reduce these peaks back down to nominal plate ratings. I've already finalized my filter design and it looks great.

I would highly recommend to anyone to simulate their system and find the bugs before building. Simulation is a great tool to help you 'see' whats happening.

Steve Conner

Mon Nov 20 2006, 05:34PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The spark gap (both safety and regular) should always be across the NST terminals. Terry Fritz proved experimentally that this stresses the transformer least, because it doesn't see the oscillating RF voltage (as mentioned by hazmatt) that it would see across the capacitor.

IamSmooth

Sun Dec 03 2006, 09:59PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

I have a question about proper grounding:

I know that the secondary RF ground should be separate. However, does the ground for the NST (Mains) have to be separate from the ground for the Terry Fritz filter or can they be combined? I am referring to the grounded center of the spark gap that is used to catch high-voltage spikes where each end of the spark gap is connected to the HV terminals of the NST.

Hazmatt_(The Underdog)

Sun Dec 03 2006, 11:26PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

The NST housing and midpoint on the non GFCI units share a ground that should be grounded to your AC line ground to prevent any voltage rise on the housing. I would use this midpoint ground as my 0 reference for safety gaps.

The newer GFCI units have a midpoint isolated from the housing that is about ~60 V above line ground. This voltage is on this small isolated bushing for the GFCI reference/fault condition detector. If you have a GFCI unit I would recommend using the isolated midpoint for your protective gap.

IamSmooth

Mon Dec 04 2006, 01:54AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

thanks

IamSmooth

Fri Dec 08 2006, 06:22PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

I want to start adjusting my static safety gap that I have with my sych gap. I also want to set the NST fail-safe gap on my filter. Is there a problem with running the NST attached to the primary LC circuit *without* the secondary at half power? Or, will this power not get dissapated fast enough and cause overload problems if there is no secondary to transfer the power for streamer production?

Second question: if one has a properly tuned coil, does it matter what is the value of the resonant frequency? That is, is there a difference in performance if the frequency is 250 khz or 80 khz? The only thing I considered was that at higher frequencies there could be higher RF losses.

Hazmatt_(The Underdog)

Fri Dec 08 2006, 07:55PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

I'm not understanding your first question the way you wrote it. But I can say this, if you are attempting to adjust a static or rotary gap and are varying the power input while connected to your LC primary tank, you pose a real threat to your transformer by means of dielectric breakdown. What do I mean by this? Well while your LC tank is connected to your transformer and say you have no switching of the gap, there is a voltage rise in the transformer above what it is rated for, so eventually you're going to cause a carbon track across one of the transformer secondaries and fault the transformer. I know this because I was doing some dumb things trying to see if my static gap was set far enough apart, and it was too far apart. The transformer autopsy revealed the carbon track and I finally knew why I had lost ~half my streamer length.

IN short, if you turn on the power to your primary circuit and you don't get any breakover then the gap is too wide and you might be burning the insulation on the winding.

Second question: The way I see it is a function of input power. When I try to design a coil I am trying to make it a good size for the power applied, which then determines the resonant frequency. Low power--> tiny coil--> high Fo. If you apply too much power to the coil then it will arc to itself, its strike rail, primary, possibly catch on fire, all that stuff.

Then with tuneing I just go for maximum streamer length. I did measure the inductance of all the connecting leads and tuned it for LpCp = LsCs, and it was right there in the ballpark the first time. So if you know the secondary characteristics, measure the primary leads, calculate, and you'll be really close to where you need to be.

IamSmooth

Fri Dec 08 2006, 08:33PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

With regards to the first question: If I want to make sure my safety gap works and I am using a rotary gap, what is the safest way to test the static gap without frying my NST?

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