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What is the streamer length related to?

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LAN_403

profdc9

Mon Dec 04 2017, 02:22AM

Registered Member #58522 Joined: Tue Mar 15 2016, 08:33PM
Location:
Posts: 50

Hello,

I have my Tesla coil work (see the attached video), but I want to lengthen the arcs. Currently I have a 26 nF capacitor on it.

Would a larger capacitor help? Is the streamer length related more to the total power, the pulse energy, or both somehow? What are the factors that determine streamer length?

I was thinking of buying a bunch of Cornell Dubilier capacitors from Mouser

Unless there is a better source? But before I buy hundreds of $$$ of capacitors, I would like to have some idea that its going to help.

Thanks,

Dan

]teslavidsmall.mpg[/file]

paulj

Mon Dec 04 2017, 07:18AM

Registered Member #59353 Joined: Sun Apr 17 2016, 02:08PM
Location: France
Posts: 84

Hello,
your question lacks detail.

put all the information in one post.

To increase the length of the sparks, the secondary circuit (core and secondary coil) must be at the same frequency as the primary circuit (primary coil, pulse capacitor).

"like a swing", you have to send the energy at the right moment

Before buying your capacitors, you need to know the inductance of your coils and the capacitance of your capacitors.

links :

if you do not have an oscilloscope, you can use this small circuit easy to manufacture with a 555:

If you are lazy, use JAVA TC to know your frequency of resonance, but personally, I find the application little prcise, it is sutrout for the base

Ps: I find your primary coil too wide.

start by changing the number of turns by changing the turn connection

Dr. Slack

Mon Dec 04 2017, 11:35AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

If your coil is in tune, then streamer length could be in the ballpark of 1.7*sqrt(power), this is John Freau's emperical equation with length in inches and input power in watts.

If your coil is out of tune, it could be much much less.

profdc9

Mon Dec 04 2017, 05:12PM

Registered Member #58522 Joined: Tue Mar 15 2016, 08:33PM
Location:
Posts: 50

Ok sorry I left off details, I was in a bit of a hurry.

I used Java TC to design the coil. The circuit you provided is a good idea, but maybe I should get a frequency counter, otherwise it might be hard to compare the frequencies? I have moved the tap point on my primary coil around to try to get the primary and secondary to have the same frequency by maximizing the streamer length. Is the circuit a better way to tune the coil?

The power supply is 2 MOTs with a voltage doubler, so it provides about 11 kV peak pulses at 60 Hz. The capacitors in the voltage doubler at 0.95 uH microwave oven capacitors. I built a Terry filter using 10 1800 V MOVs, 2200 pF X 2 at 36 kV capacitance RF filtering, with 600 ohm series resistors. This seems to be protecting the voltage doubler diodes from damage. My voltage doubler rectifier is four microwave oven diodes in series.

The secondary coil is 18 inches tall on a 4" PVC pipe wound with 26 AWG wire. It has about 1100 turns. The turns are encapsulated in epoxy to prevent arcing. The top load is two stainless steel salad bowls put together into an elliposidal shape, 18" across and 12" tall. JAVATC predicts the resonance frequency of this arrangement is about 200 kHz. I tried to achieve a 4:1 ratio of length/diameter like I read was optimal.

The primary coil is 1/4" copper tubing starting at 6 inches from the center axis of the secondary coil, and ending at 10 inches from the center axis, with the turn spacing being 1/2 inch. The bottom of the primary is 2" above the bottom of the secondary, and the top of the primary is 4" above the bottom of the secondary, the coil is tilted slightly upwards. JavaTC says that the coupling coefficient is 0.14 to 0.15, and the optimal is 0.129. I do not know if this difference is enough to be concerned about. Instructions I read say that one should err on the side of overcoupling rather than undercoupling. I was having problems with arcing from the primary to the secondary, so I removed turns from the inside of the primary coil.

My capacitor is home-made, which is why I am thinking of replacing it with Cornell Dubilier 942C capacitors. It consists of sheets of aluminum roof flashing with the corners carefully rounded, wrapped in polyethylene painters tarp. The plates are interdigitated. The capacitor is huge, each plate is 22" by 14" inches, and the plates are separated by 1/8" of polyethylene. I originally designed it the capacitor for 60 kV of peak voltage, because I was trying to use a home made transformer that has a high voltage (30 kV+), because I designed my own high voltage switching power supply. However, the RF pulses coming from the Tesla coil using this high voltage were unfilterable and would blow up the transformer. I may wind a new lower voltage transformer. The total capacitance is 26 nF. I do not think there is significant leakage in the capacitor plates, as they retain the charge a significant amount of time after power is turned off, and I have a 300 Mohm resistor series set to bleed the charge off the primary capacitor which takes a little while (I usually just discharge the capacitor using a chicken stick to the spark gap).

I have two spark gaps. Once is a Richard Quick gap consisting of seven copper tubes side-by-side with a strong fan on it. That works ok, but I think it is lossy. The second is a series pair of gaps using tungsten contacts, also with a fan blowing on it. The second gap is very hard to cool as the tungsten rods I have are only 1/4" wide. The ends of the tungsten rods at the gap are slightly chamfered. I am trying to build a rotary spark gap using an angle grinder, but I am having a problem making a balanced load wheel for it; it shakes a lot. The angle grinder keeps shaking itself out of position.

Thanks for your patience, hopefully this provides help to get a little more insight.

Dan

profdc9

Thu Dec 07 2017, 11:27PM

Registered Member #58522 Joined: Tue Mar 15 2016, 08:33PM
Location:
Posts: 50

So I have made some progress. Still haven't ditched the home made capacitor, but I got the rotary spark gap to work.

Turns out the angle grinder flange had a lot of runout, it was a $15 Harbor Freight special. I ground off the wobbly part from the flange, and got the wheel to turn relatively true.

The video is below. Any advice I could get on whether a Cornell Dubilier 942C capacitor would help, or about the coupling between the primary and secondary and how to adjust that, would be appreciated.

In particular, for a 2 MOT system with 1.5 to 3 kVA, is there an optimal capacitance? Can you use a small capacitance and just have the pulse rate be very high? How do you decide how much capacitance for a particular voltage primary and power level? From what I've read, I should be aiming for 60 nF @ 20 kV or so, but this is just what I've seen on the net.

Dan

Enjoy the video!

Dan

]tesla-coil-dec-7-2017.mpg[/file]

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