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

On Tue, 27 Jun 2017 08:56:54 +1000, Robert Miller
<rmiller@teledyne.com> wrote:

>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.

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

>
>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?

magnetostrictive field of the secondary?


>
>Robert Miller

-- 

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement 

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

"Robert Miller" <rmiller@teledyne.com> wrote in message 
news:ois3fl$qtg$1@gioia.aioe.org...
> 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?

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.


> 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?

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

-- 
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design
Website: http://seventransistorlabs.com 

Robert Miller wrote:
> 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.

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.

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

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:
https://web.archive.org/web/20161119042110/https://www.richieburnett.co.uk/tesla.shtml

>
> 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?

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

>
> Robert Miller

On 27/06/17 08:56, Robert Miller wrote:
> 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?

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

On 27/06/17 12:13, Bert Hickman wrote:
> Robert Miller wrote:
>> 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.
>
> 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.
>

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.

> 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.
>

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 this configuration tend to make selection of the capacitance
>> value less critical?
>
> 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:
> https://web.archive.org/web/20161119042110/https://www.richieburnett.co.uk/tesla.shtml
>

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?

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

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

On Monday, 26 June 2017 23:56:59 UTC+1, Robert Miller  wrote:
> 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

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

Robert Miller wrote:
> On 27/06/17 12:13, Bert Hickman wrote:
>> Robert Miller wrote:
<snip>
> 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.

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.

<snip>
>
> 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.

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

<snip>
>
> 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.

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.

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

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.

>
> 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?

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.
https://en.wikipedia.org/wiki/Electrostriction
https://en.wikipedia.org/wiki/Ceramic_capacitor

>
> Robert Miller
>