Placing Tesla Coil Capacitor WITHIN the Secondary

I intend to build a classic spark gap Tesla coil. I understand the value of the timing cap must be selected with reference to the resonant frequency of the coil.

I was wondering what would happen if I was to custom build (wrap) a tubular cap, from al foil and mylar, and slide it _within_ the TC's secondary so both are on the same axis.

Would this configuration tend to make selection of the capacitance value less critical?

More precisely, would being within the magnetostrictive field of the secondary affect the cap's characteristics in any way that might tend to optimise its resonance with the coil?

Robert Miller

Reply to
Robert Miller
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The top part of the secondary coil would probably arc to the cap inside.

magnetostrictive field of the secondary?

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
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Reply to
John Larkin

No. How would it? You just got done claiming you "understand the value of the cap".

At best, the cap will get hotter, from eddy currents. (The Tesla coil is a transformer, after all.) At worst, it'll melt, or arc as JL noted.

These are words, but they aren't being used correctly.

Magnetostriction is a material property, not a spacial property. Intrinsic, not extrinsic.

At frequencies as low as used in TCs, there are no self-resonance properties. Even if there were, they certainly wouldn't be spooky, and they would almost certainly make performance /worse/.

Tim

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Seven Transistor Labs, LLC 
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Reply to
Tim Williams

This is a really bad idea for a couple of reasons. A typical small to medium-power Tesla Coil (TC) often develops hundreds of volts per turn on the secondary. Adding a coaxially-oriented capacitor inside the secondary will create severe corona and arcing between the bottom and top of the secondary winding and the capacitor's outer foil layer. Also, Mylar is also quite lossy at TC frequencies when compared to other films such as polypropylene or polyethylene.

A better choice would be a series string of pulse-rated HV ceramic capacitors, where the total DC withstand voltage for the string is significantly greater than the expected peak output voltage. Some examples are TDK Series FD capacitors or Murata N4700 series caps. This configuration will share the voltage stress across the caps evenly similar to the voltage distribution across the secondary. It also reduces the resonant frequency of the secondary. This approach has been used in some coil designs and it does work. Avoid using ceramic HV DC filter capacitors, since the capacitance of these can dramatically decrease at higher operating voltages, also affecting the resonance point with changes in output voltage.

No. A well-constructed secondary has a Q between 200-350 at its self-resonant frequency (SRF). However, because there's a lossy spark gap in series with the primary LC circuit, the primary has a significantly lower Q (i.e., a much broader resonance curve). As long as the primary is tuned "close" to the secondary SRF, it will work just fine.

Fine tuning of the system is usually done by changing the inductance of the primary. Usually, this is accomplished by using a flat spiral spaced primary wound with bare wire of copper tubing and adjusting the position of a moveable tap. The Primary-Secondary coupling coefficient is purposely designed to be low - typically between 0.1 and 0.2. A high coupling factor is actually undesirable in a spark gap coil. Some excellent information can be found here:

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No. Magnetostriction does not occur with any of the materials discussed above.

Reply to
Bert Hickman

I suspect the presence of conductive foils anywhere near the secondary would kill the Q.

Reply to
Clifford Heath

Can this not be prevented? I intended to enclose the cap in hi-dielectic film, such as several layers of e-grade Mylar, use silicone insulated leads, and make the secondary former fairly wide.

I liked the idea of the coaxial cap since I thought it would be more likely to interact (somehow?) with the internal field of the secondary.

How it would interact, given that configuration, was the basis of my question.

Would the cap's surface area at right angle to the secondary's magnetic field not affect its displacement current? You mentioned above that internally situated caps can lower the resonant frequency.

Or would this be more likely to occur with a series of horizontally stacked plates?

I was referring (imprecisely) to any effect upon the cap's internal charge caused by the magnetic field lines within the secondary.

Then again, it could be a BaTiO3 cap. I have a few transducers from a sonobouy in my junk box.

Are those TDK and Murata caps perhaps of the same material?

Robert Miller

Reply to
Robert Miller

Some violet rays used to wind a paper cap round the outside of the coil, near the grounded end. I think the idea was the heat helped keep the paper dry.

NT

Reply to
tabbypurr

It depends on the dimensions of the coil form, OD and length of the capacitor roll, and output voltage of the secondary. ANY high frequency corona and ozone inside the coil form will eventually carbonize, and destroy, any organic insulating system, Mylar included. Try it and see for yourself.

It will interact, but not necessarily in a desirable fashion.

Not in any measurable way. The frequency is lowered simply because you're adding capacitance across the secondary winding, lowering the resonant frequency of the secondary.

No difference in circuit performance. However, stacking the capacitor plates in this direction makes the voltage distribution across the series of capacitors closer to that of the secondary winding, reducing voltage differentials between the winding and capacitors, and helping to reduce corona problems.

No. Your transducers are made to emphasize electrostriction. They're made from a Class 2 ceramic blend and are really not optimal for your application. But they're free, and the worst that can happen is that they won't work or will break down. The HV capacitors I suggested are made from a Class 1 ceramic blend, usually strontium titanate. See the following for more information.

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Reply to
Bert Hickman

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