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SGTC Assistance - I'm stumped

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intellectualbean

Sun Jan 13 2013, 12:46AM

Registered Member #9024 Joined: Wed Dec 26 2012, 02:48AM
Location:
Posts: 4

I am in need of some assistance. I recently finished my construction of an SGTC loosely based on these instructions -

Here are some details regarding my coil:
- 6.5kV 30mA NST
- 0.015uF 14kV capacitor bank - 7 strings of 7 capacitors in series (each with parallel resistor as instructed)
- 7 turn primary coil of 10 awg stranded stereo wire
- approximately 885 turn secondary coil of 28 awg magnet wire (3 inch diameter)
- spherical topload with 3.25 inch radius
- spark gap using two bolts

Using DeepFriedNeon's calculator, I tuned both my primary and secondary circuits to approximately 360 kHz, yet the first time I turned on the coil, absolutely nothing happened.

My first guess was that there was a break in my cap bank, but after checking nearly ever single connection for continuity, as well as confirming the cap value of the bank with my meter, that was obviously not the case.

To make sure my NST and spark gap were good, I separated them from the rest of the circuit, and the gap was crossed without a problem.

I reconnected the spark gap and NST, leaving only about a mm of space between the two bolts. I flipped the switch, and once again...nothing. I checked all my connections for continuity/shorts, and double-checked the schematic to make sure I was making the correct connections. All seemed to be correct.

So, now I'm stumped. Even if my circuits were not perfectly tuned, shouldn't I at least get some corona discharge? And shouldn't the spark gap fire? I'm running out of ideas on what could be wrong. I've had a decent amount of experience with high voltage projects, but this is my first attempt at construcitng a tesla coil. Any tips you could offer me would be GREATLY appreciated.

Here's a picture of my coil -

Houdini0118

Sun Jan 13 2013, 12:50AM

Registered Member #8558 Joined: Thu Dec 06 2012, 11:38PM
Location: usa
Posts: 57

if you pull the neon sign lead off while its *running* do you get an arc?

intellectualbean

Sun Jan 13 2013, 01:00AM

Registered Member #9024 Joined: Wed Dec 26 2012, 02:48AM
Location:
Posts: 4

Houdini0118 wrote ...

if you pull the neon sign lead off while its *running* do you get an arc?

Yes. It arcs from the NST lead to the spark gap lead.

Houdini0118

Sun Jan 13 2013, 01:42AM

Registered Member #8558 Joined: Thu Dec 06 2012, 11:38PM
Location: usa
Posts: 57

then something is shorted whats happens when you put a ohm meter across the spark gap?

intellectualbean

Sun Jan 13 2013, 03:23AM

Registered Member #9024 Joined: Wed Dec 26 2012, 02:48AM
Location:
Posts: 4

Houdini0118 wrote ...

then something is shorted whats happens when you put a ohm meter across the spark gap?

I've checked for continuity across the spark gap, and there is none.

ZakWolf

Sun Jan 13 2013, 06:52AM

Registered Member #3114 Joined: Sat Aug 14 2010, 08:33AM
Location:
Posts: 608

i might be wrong, but isnt the NST AC and needs to be rectified ?

Ash Small

Sun Jan 13 2013, 12:34PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

ZakWolf wrote ...

i might be wrong, but isnt the NST AC and needs to be rectified ?

I may also be wrong, but don't you also want a high value resistor on the output of the NST, after the rectification?

(I'm not sure, but if you can't find any other problems, I'd say the current is flowing from the NST to the cap bank, then back through the NST to charge the cap bank in the 'other polarity' with each cycle of the NST, and the voltage in the cap bank never gets high enough to 'jump the gap' and start the tank circuit oscillating. Of course, I may be wrong.)

intellectualbean

Sun Jan 13 2013, 04:35PM

Registered Member #9024 Joined: Wed Dec 26 2012, 02:48AM
Location:
Posts: 4

Ash Small wrote ...

ZakWolf wrote ...

i might be wrong, but isnt the NST AC and needs to be rectified ?

Hmm... I suppose that makes some sense, but the instructions I followed said nothing about any such rectification or resistance

Ash Small

Sun Jan 13 2013, 05:31PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

intellectualbean wrote ...

Ash Small wrote ...

ZakWolf wrote ...

i might be wrong, but isnt the NST AC and needs to be rectified ?

Hmm... I suppose that makes some sense, but the instructions I followed said nothing about any such rectification or resistance

Well, as I understand it, IF the caps get sufficiently charged in one half cycle to 'jump the gap', it will oscillate for a bit, then the same will happen on the other half cycle. My only experience with this 'SGTC' circuit used a DC flyback (LOPT), with built in rectification, and a high voltage resistor between the flyback output and cap bank (I think it was 330kOhm, but I'm not certain). Mine was driven at ~20kHz (I didn't have a 'scope at the time). Once the voltage was sufficient to 'jump the gap', it would oscillate for a bit, then the process would repeat.

Someone else may be able to suggest something else, though. (Don't take my word for it)

ZakWolf

Sun Jan 13 2013, 08:50PM

Registered Member #3114 Joined: Sat Aug 14 2010, 08:33AM
Location:
Posts: 608

When ever i make homemade caps and charge them with ac i just end up pulling an arc off of them and not charging them at all, try rectifying it or half wave rarefaction

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