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Registered Member #4704
Joined: Wed May 02 2012, 11:51PM
Location:
Posts: 2
Hi Guys,
It's my first Tesla coil and I'm having some difficulties and hope you can help. The problem is that I'm simply not getting sparks off the secondary. Here's my tesla setup in a nutshell;
2 x 5000 V 30 mA neon sign transformers wired in parallel to give 300 Watts. The digital meter confirms almost 60 mA when wired in parallel. As my meter only goes up to 600 V I haven't been able to confirm the voltage. I'm assuming it is ok and as stamped on the transformers. Both are the same make and model, and are correctly phased.
The capacitor is a homemade bottle cap. Testing with a digital meter shows 8.96 nF. If I connect one terminal of the cap to one terminal of the Transformer, then use a wire (connected at one end to the other terminal on the transformer) and bring it close to the second terminal on the capacitor, I get a spark. Crude, I know, but it seems to work.
The primary coil is 12.7 mm enamelled copper tubing.
The spark gap is made from 2 x 10 mm stainless bolts and is set approximately 5 mm apart. If I connect only this to the transformers, I get a continuous, sustained spark.
I've tested continuity throughout the circuit and everything seems ok.
The secondary is wound on 90 mm poly tubing and is approximately 945 turns of 0.5 mm enameled copper wire. Unfortunately I could not get a single continuous spool of wire and as such have joined 5 lengths of approx. 60 meters each. I've successfully tested continuity on the winding and found no shorts (even at the joints).
The torroid is spherical, made from 2 x stainless steel kitchen mixing bowls. The bottom end of the secondary winding and the two transformers are earthed using the same earth wire.
I used deepfriedneon.com to determine all the calculations and the res freq of the primary is around 299.874 kHz with an inductance of 31.4 uH. The secondary is close at 300 kHz and 31.2 uH.
I've checked and rechecked the calcs, the wiring, and the transformers and capacitor. For the life of me I can't find why I'm not getting a spark off the secondary. I was hoping you guys could help?
When I last tested, the spark gap fires for several seconds, and then stops. I unplug mains power, then retest, and the same thing happens. I would expect the spark gap to continue firing if everything was ok. Can anyone explain why this might be happening? Air temperature here at the moment is relatively low at around 10-12 degrees Celsius.
I also noticed, that on occasion, when I first power on the coil, I get a very small and short lived spark at the point of the first join in the secondary winding closest to the primary coil. I'm wondering if maybe I'm getting some shorting issues?
I was doing some reading this morning about testing primary circuits etc. As such I placed a flouro tube on the primary coil, and when I turned the system on, the flouro tube lit up. This is as expected and further confirms my primary circuit is working. Although it went out several seconds later when the spark gap stopped firing.
I've also tried shortening the length of the primary coil half a turn at a time. This didn't help.
Could it simply be not enough wattage? The NST's are delivering around 300 W. Any help you could offer would be greatly appreciated.
Registered Member #3900
Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600
5kv breaks down *roughly* 5mm. I would bring the gap a bit closer together to see if the sparks is just unable to maintain itself. Static gaps tend to do this if they are not close enough. The only other reason I can think of is internal nst safety features kicking in. If it was a breaker you would have to reset it. Make sure your nst's don't have any of these feature built in. My first tesla coil use one 5kv 21ma nst and provided 1-2 inches of spark even when I was clueless as to how to tune an lc circuit back then. I calculate now that the secondary was resonating around 2mhz and the primary around 150k. I must have got lucky and hit some sort of harmonic but my point is that you should be getting something. Once you get that gap firing again of coarse!
Deepfriedneon says you need 17.6nF for resonance. You need 1.618x resonant capacitance for a proper stationary spak gap type tesla coil. That comes to 28.47nF or 0.0287uF. Your capacitor needs to withstand at least 20kV for safety as well, but that usually isn't an issue for bottle caps. Bottle caps are lossy though which means they are not efficient at delivering the power thy store and they cannot store the power for long.
Secondly if your NSTs are GFCI protected they will not work for tesla coils. Likewise you can't use a GFCI outlet to power one.
Also, I want to confirm you have everything wired up correctly, so please draw up even a simple schematic.
Registered Member #4704
Joined: Wed May 02 2012, 11:51PM
Location:
Posts: 2
Thanks everyone. This has all been great advice. I'll definitely be adding more capacitance. I might look at buying some and wiring them together. It's amazing...this all started out as a project for my son at school...now it's an addiction...
What you should do, what I did, was attach a grounding wire to a long stick. (of course, ground the wire) Bring the stick close to the toroid. There might be a spark. My coil (currently) doesn't fire unless I do that. With a similar topology to yours, I think that now I have better caps, it should fire better, (I don't know yet, don't have time to work on it until summer vacation starts), and I think yours should too. Build another one of those bottle caps, and put it in parallel with the other. That should get you close to the preferred capacitance.
Registered Member #4187
Joined: Fri Nov 04 2011, 08:08PM
Location: Spain
Posts: 43
attach a grounding wire to a long stick. (of course, ground the wire) Bring the stick close to the toroid. There might be a spark.
I am not expert, but if you do that make very sure that coil is unpowered when you bring the stick near the toroid and hold it in place with some tool different from your body parts .
Otherwise you can get a very nasty discharge with some bad luck (i.e. if you lose the grounding wire when getting sparks by any reason)
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