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small SGTC paralell plate capacitors

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KUN

Tue Oct 03 2017, 12:36PM

Registered Member #61824 Joined: Mon Oct 02 2017, 05:38PM
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Posts: 40

Hello, i built small spark gap Tesla coil with a paralel plate capacitors made with aluminum foil. Coil is driven with flyback transformer. Whenever i turn on the thing more noise comes from capacitors than from spark gap. It seems that capacitor plates are arcing to each other but there are no burnmarks. Although output is ok. I get 4 cm arcs with only driving flyback at 12 2 A. It is driven by a simple 2 component flyback driver(mosfet + resisitor). So i think that i could get even more power if capacitos wouldnt arc. In the dark i can see small corona arcs jumping from one plate to another at almost all of he edges of the plate. So i was wondering can i somehow stop it, or is it maby normal?? If i put more dielectric between it only lowers the performance but the noise stays the same. The reason i am using plate capacitors is that i can pull one plate slowly up until i get max output. then i stop and cut the plate. This way i can make sure that it is in resonance. Any suggestions?

hen918

Tue Oct 03 2017, 02:39PM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

The plates should definitely not arc. either your capacitor is too small for the voltage you are putting on them or you have imperfections on the foil surface where charge can build up.
Make sure the plates are perfectly smooth, then if they are, you'll have to drop the voltage or make the cap bigger in both dimensions (more distance and more area).

KUN

Tue Oct 03 2017, 04:46PM

Registered Member #61824 Joined: Mon Oct 02 2017, 05:38PM
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Posts: 40

Oh imperfections could be the reason. I"ll make new ones. And I also noticed that it is only arcing from sharp spots(edges and ends of wires that connect to plates)

Sulaiman

Wed Oct 04 2017, 11:04AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

This is a common problem with diy eht capacitors,

unfortunately, no matter how smooth your plates are, the radius of curvature at the edges is very small
hence corona voltage is relatively low
Submersing your capacitor in oil (lubricating/mineral/baby/cooking) will help
but is messy and could be a fire hazard.

Unless you come up with something new, you will have to operate your diy capacitors below about 5 kV each, which makes tuning more difficult.
Google images of kV rated variable capacitors for size/spacing estimates

There are reasons why you thought that no one else had this fantastic idea

KUN

Sat Oct 07 2017, 03:05PM

Registered Member #61824 Joined: Mon Oct 02 2017, 05:38PM
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Posts: 40

aa yes, I just find best capacitor plate spot where coil is in tune completely, so i guess i am going to put it in oil. But i will soon build a zvs driver so i guess ill have to retune the coil.

KUN

Sat Oct 07 2017, 10:41PM

Registered Member #61824 Joined: Mon Oct 02 2017, 05:38PM
Location:
Posts: 40

Oh and yes I forgotten to ask. Am I getting max output power for the power i am putting in. DC power supply(12 v, 2 A) drives a flyback(2 component driver), it outputs 1 cm spark at best my tesla is 31 cm high, and 2.7 cm in width, i calculated that it has around 1100 turns. So, with that input i am getting 3.5 cm sparks at best, sometimes only 2.5 cm. So i am wondering is it really the max that it can give with that power supply. I tried changing different things but i cant get it any better. I have standard spark gap, and a flat pancake primary coil with 11 turns.

KUN

Wed Oct 11 2017, 09:29PM

Registered Member #61824 Joined: Mon Oct 02 2017, 05:38PM
Location:
Posts: 40

Hay i have an update! I built a zvs driver to drive a flyback better. It worked on first try. i get some tick arcs of around 3.5 cm, at 24 v, 2 A. I then connected it to the tesla coil, and it worked somehow output wasnt something that spectacular(5 cm) i suppose it can do better. What do you think? And then after some time operating spark gap melted, and capacitor stopped discharging there was a arc between gap that looks like zvs arc and it was. So i thought that capacitor arced over, but it didnt, I built a new spark gap, checked the cap and it was good. Then the coil worked again i started tuning it by removing layers of capacitor or adding them. Then when there were only 2 layers left capacitors again stopped discharging. i am not sure what is causing it. When i adjust spark gap size slowly it starts discharging and at some point i get red hot zvs arc. So it is discharging cap but other moment it doesnt..... The problem seems to improve when i added cap layers back. So please if someone could help me understand what is going on here. I am sure nothing is arcing over. I got an idea that it is happening because of too low cap capacitance, and zvs is so strong that it charges caps quicker than they can discharge so electricity that is left goes directly to the spark gap and arcs over. Is that what is happening or what am i missing?

KUN

Thu Oct 12 2017, 07:11AM

Registered Member #61824 Joined: Mon Oct 02 2017, 05:38PM
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Posts: 40

Very wierd, now i noticed that if spark gap is bigger it wont do that easily but still does. No matter what cap i connect it acts the same. Only thing is that now it cant create a 1 mm arc on spark gap, so i thought that my zvs burned or sth, i disconected tc and ti still works the same. I reeally dont konw that it might be

Flachzange

Sat Oct 14 2017, 07:50AM

Registered Member #61569 Joined: Sat Apr 15 2017, 05:12PM
Location: Germany
Posts: 28

Hi,

1. The problem with the plate capacitors is that you are using aluminium foil. It is very thin and as Sulaiman already mentioned therefore corona will develop. The "arcing" noise comes from the foil itself. If you charge the capacitor the aluminium foil will the attracted to each other. If it is discharged the attracting force by the electric filed will be gone and it spring back to its original position. So the movement of the foil itself makes the noise you hear. It is also a problem in compressed gas capacitors for high voltage measurements. We have a 800 kV compressed gas capacitor and if you measure voltages with the frequency close to the mechanical resonance frequency of the capacitors electrodes you can also hear a sound from them.

2. The problem with your sparkgap: ZVS and a flyback will generate DC (almost pure DC without any ripple due to the high frequency). So you have no zero crossing of the current like with AC. So the arc has no time to extinguish. The problem will get even worse with more power and the longer you will run the coil, because the spark gap will heat up.
In order to get a better performing spark gap, you should try more spark gaps in series. If you are using a 5mm spark gap now, then try to build 5 spark gaps with a 1mm distance. The best solution would be a rotating spark gap.

KUN

Sat Oct 14 2017, 10:22PM

Registered Member #61824 Joined: Mon Oct 02 2017, 05:38PM
Location:
Posts: 40

Oh for 1 i noticed that with aluminum foil, but how do i solve it, would putting whole capacitor in oil help? Do i need to use ticker sheets of metal or what?

For 2. i noticed that it starts happening after some time coil is running, and gets worse if capacitor is lower capacitance. I also noticed that i get best arcs when spark gap is as little as possible lets say 0.8-0.5 mm. Sparks get up to 7 cm. Think it should do much better. So my question is is it normal that spark gap needs to be this small?
Arcs will probably improve when i make a decent cap, and redesign a spark gap. For the top load i am using small sphere around 2.5 cm diameter. It gives best arcs without discharge needle.

So to keep things short: 1. How do i make plate cap that wont form corona? and 2. Does larger spark gap mean more power in primary coil, and is larger spark gap better than having smaller at higher freq. if yes how do i make my coil work good with larger spark gap. Ill post a video of my tc in action some time soon.

Tnx. for help!

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