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Capacitor Help!!

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hfcoil

Sun Jan 10 2010, 10:03PM

Registered Member #2617 Joined: Sun Jan 10 2010, 09:52PM
Location:
Posts: 6

I have just started building my first tesla coil. I am fairly experienced with electronics and high voltage but I can't seem to get past building a homemade capacitor that will work. My first attempt I built eight Leyden jars in parallel that were constructed from glass canning jars and metal tape. Next I tried lining a 5-gallon bucket inside out with aluminum foil. Lastly I tried assembled a rolled capacitor with aluminum foil as the plates and wax paper as the dielectric (about 10 square feet). All of these have failed in a similar fashion; I hook them into the circuit and the spark gap wont fire. There is no arcing between plates, however the bucket and jar constructions made a faint noise similar to a capacitor charging. I have seen websites and videos of working tesla coils with only three or four Leyden jars. I don't know whether the homemade ones just donâ€™t have enough capacitance or if it is something else. If anybody has any input on the problem please help.

Thanks

I am using a 14kv 35ma 60hz oil burner ignition to power the coil.
attached is a schematic of the circuit configuration.

uzzors2k

Sun Jan 10 2010, 10:33PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Perhaps you need to reduce the spark gap distance? What is it set to now? Not much else comes to mind, as I'm sure you've constructed good capacitors. If they're not arcing-over, they will work.

doctor electrons

Sun Jan 10 2010, 10:46PM

Registered Member #2390 Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381

If you cannot get the spark gap to fire, there just is not enough voltage (assuming the caps are good). The spark gap will fire when the voltage is
high enough to break the gap between the points. As for the capacitance, the only way to know for sure is to use a meter
capable of measuring capacitance. If you did not adjust the gap (with less distance between the points) to try to get it to fire
then that could be part of the problem. If you did, then it has to be the caps. OR, If the load of your primary coil (ohms) is dissipating the voltage
in the caps, they are not storing enough voltage. Is the primary getting hot? Like uzzors said, they should work!
Hope this helps a bit!

hfcoil

Tue Apr 13 2010, 05:53PM

Registered Member #2617 Joined: Sun Jan 10 2010, 09:52PM
Location:
Posts: 6

hfcoil wrote ...

I have just started building my first tesla coil. I am fairly experienced with electronics and high voltage but I can't seem to get past building a homemade capacitor that will work. My first attempt I built eight Leyden jars in parallel that were constructed from glass canning jars and metal tape. Next I tried lining a 5-gallon bucket inside out with aluminum foil. Lastly I tried assembled a rolled capacitor with aluminum foil as the plates and wax paper as the dielectric (about 10 square feet). All of these have failed in a similar fashion; I hook them into the circuit and the spark gap wont fire. There is no arcing between plates, however the bucket and jar constructions made a faint noise similar to a capacitor charging. I have seen websites and videos of working tesla coils with only three or four Leyden jars. I don't know whether the homemade ones just donâ€™t have enough capacitance or if it is something else. If anybody has any input on the problem please help.

Thanks

I am using a 14kv 35ma 60hz oil burner ignition to power the coil.
attached is a schematic of the circuit configuration.

Update:
i just recently built an audio modulated coil to help a friend with a class project, it uses a flyback tranformer driven by a 555 timer and mosfet running at 18v. with this new tranformer i have used my leyden jars and a bucket lined with tin foil as capacitor, and they both work (decided to use the bucket capacitor for the end product). i have tried to connect the 14Kv oil burner ignition tranformers and get the same result as mentioned in my first post. does anyone have any insight as to why these capcitors would work with the flyback transformer and not the oil burner ignition transformer.

Thank you!!

klugesmith

Tue Apr 13 2010, 06:24PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

hfcoil wrote ...
... i have tried to connect the 14Kv oil burner ignition tranformers and get the same result as mentioned in my first post. does anyone have any insight as to why these capcitors would work with the flyback transformer and not the oil burner ignition transformer.

How do you know the OBIT is any good?
Does the spark gap fire when connected directly to the transformer, with no cap or coil?

If the spark electrodes are rounded, they need to be much closer than a sharp wire gap which you might have used to test the transformer. Maybe less than 1/4 inch. Breakdown electrical field strength of dry air is about 30 kV per cm.

hfcoil

Tue Apr 13 2010, 06:40PM

Registered Member #2617 Joined: Sun Jan 10 2010, 09:52PM
Location:
Posts: 6

when the OBIT is connected directly to the spark gap, no capacitor, it will fire up to 2.5cm apart. the spark gap is just 2 bolts stuck in some wood. while trying to get the OBIT to work with my capacitor i have adjusted the gap all the way down to 1 mm with no luck.

Sulaiman

Tue Apr 13 2010, 06:56PM

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

First I want to give a warning,
NST and OBIT will fail if the output voltage is too high,
(resonance of transformer secondary inductance with primary capacitor)
SO make a 'safety' spark gap.
Just a simple spark gap directly across the NST or OBIT output,
adjusted to just about fire with nothing else connected.
I have destroyed both NST and OBIT due to no safety gap.
Make a good main spark gap - not pointy - smooth arcing surfaces,
preferably with a fan blasting air through it.
The arc in a good spark gap is loud with thin blue/white arcs, a poor spark gap has a fat (usually) 'orange' arc.
The overall 'quality' of a SGTC is determined mainly by the spark gap and primary capacitor.
Of course more power is always more fun.

I can think of two possible reasons for your obit/capacitor not working,
1 - capacitor leakage, carefully observe your setup in the dark,
looking (and listening) for corona, a bluish/purpleish glow with a 'hissing' sound.
2 - maybe your capacitance is too great, try with less.
(the voltage doesn't rise enough during 1/2 cycle of the ac.)

radiotech

Tue Apr 13 2010, 08:31PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

You might try a spark gap on glass or porcelon. The moisture conrtent of wood is tested by two sharp nail probes, and certaily not at peak AC value oF a OBIT. The other thing is OBITs often are changed out, even when they are working, but not well enough to satisfy HVAC people on annual service calls.

The ark gap is a interesting thig. The terminals to the gap are
summing junction for RF being reflected from the TC/CAP and the RF from the spark. Maybe the length of the leads back to the transformer from the gap could be lengthened so when the RF
goes back that way, it isn't shunted by the inner capacitance of the OBIT, which is really made for delivering 60 Hz current into the atomized mist of oil and lighting it into a conductive plasma.

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