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Doorknob Capacitors - One large One or Multiple Smaller Ones?

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KeeDx3

Thu Apr 13 2017, 08:01AM

Registered Member #61435 Joined: Thu Jan 19 2017, 06:46AM
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Posts: 25

I'm building my first small TC and since I'm wanting to do as little fabrication as possible (this is just a 'test coil'), I am leaning toward using ceramic caps.

My transformer is only 10KV at around 20ma, so I don't need much, but I'm wondering if I should install one (or a pair of) 30KV 1000pf caps, or parallel a larger number of 30KV 470pf's? Other than cost, are there benefits/drawbacks either way?

Sulaiman

Thu Apr 13 2017, 03:29PM

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

Assuming an NST or OBIT type of transformer, (current limited by magnetic shunts)
10 kV 20 mA is best suited to primary capacitance around 5 to 10 nF

if costs are similar I would go for multiple smaller units as this allows flexibility in configuration.

I like ceramic doorknob capacitors for their ruggedness,
Based on only a little TC experience,
I believe that dielectric suitability for TC use is generally inversely related to dissipation factor,
so in order; (* = not tried in TC use by me)

Vacuum *
ptfe *
polypropylene, polyethylene
ceramic
mylar/PET
glass

KeeDx3

Fri Apr 14 2017, 04:04AM

Registered Member #61435 Joined: Thu Jan 19 2017, 06:46AM
Location:
Posts: 25

Sulaiman wrote ...

Assuming an NST or OBIT type of transformer, (current limited by magnetic shunts)
10 kV 20 mA is best suited to primary capacitance around 5 to 10 nF

That might actually be a bit of a sticky wicket. The transformer I'm hoping to use is some custom-wound unit from the ignitor of a cinema projector. From what I can see, it is not shunted. It's basically a big iron "O with the 120v primary on one side and the secondary on the other. It is marked as .200KVA, 10,000V O.C.
I wrote about it and put a picture in the High Voltage forum here

because I noticed that the primary side gets pretty warm after drawing an arc after 30 seconds or so.

I do have a few 15/30's and a 7200/30 at my disposal, but I wanted to save them for when I actually start building a nicer, larger coil. I was also hoping to avoid building a Terry Filter for this one.

Sulaiman

Fri Apr 14 2017, 05:56AM

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

I had a look at the photo of the transformer, caps, inductor etc.
I do not understand the circuit ...
. the spark gap is directly across the transformer secondary
. the left-side capacitor is across the spark gap so also across the secondary
I am surprised that it works !

Your transformer overheating problem;
does it overheat if you dis-connect one of the transformer secondary wires ?
If not then I suspect the spark gap is power-arcing, and a re-designed circuit may work ok.

Even if the transformer is usable for a sgtc (does not overheat) then it is still not obvious to me how to do it,
a sure bet would be dc resonant charging

since the original application for this transformer has a spark gap directly across the secondary,
I am wondering if there is some effective ballasting on the primary, or internal ?
which would allow conventional sgtc use.

Why are you looking for capacitors for this transformer ?
the caps in the photo's look to be servicable,
and they are rated for this transformer :)

KeeDx3

Fri Apr 14 2017, 08:00AM

Registered Member #61435 Joined: Thu Jan 19 2017, 06:46AM
Location:
Posts: 25

I wish I had included more of the module in the picture - the capacitors feed directly in to a large inductor with three bars in the center (probably ferrite). In essence, it was a SGTC, just very weak, and I believe produced more of a pulse (tick, tick, tick) to strike the xenon lamp.

It is an odd circuit though. I just figured it might be an easy transition to a 'real' tesla coil :). I will try the experiment you described, but I'll bet there would be little to no heat. The transformer didn't seem to get hot unless it was actively arcing between the secondaries, but I will confirm.

It is also possible that there was some external ballasting. The entire machine was connected to a large ballast (for the lamp), but it's possible that the ballast had a winding for the ignitor as well. Then again, it was also built with a short duty cycle in mind.

As for the caps, I was advised that 30KV would be a safer bet. I actually have four of these caps, but two are chipped and I'm not sure how resilient ceramic caps are to that kind of mechanical damage. One of the chips was actually a result of some arcing that took place while the unit must have still been in use - seems that one of the capacitor leads was pinched and arced out against the capacitor itself. I am tempted just to hook it all up and see what happens. I tried it on a variac a few weeks ago and the spark gap made quite a noise, but I didn't have the guts to try and tune it or really mess with it beyond that. I've since built a different spark gap using brass knobs. I feared that this nylon one might melt down under heavier use.

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