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Jamie Cansdale

Thu Oct 06 2011, 05:19PM

Registered Member #3904 Joined: Sat May 21 2011, 10:32AM
Location:
Posts: 28

I was wondering what any experienced coilers can tell from the attached waveforms. My coil has a spark gap which is currently driven by DC (I'm trying to keep the waveform simple for tuning)

The blue trace is from a x1000 probe placed across the tank capacitor. The yellow line is from a probe with a wire attached to it.

I'm particularly wondering about the second screenshot, which is zoomed in to show half a primary cycle. Am I correct in thinking the secondary is badly out of tune? Would adding a much larger top load help?

I've just added a new top load (an IKEA tea light holder :). The secondary's waveform is looking much more healthy!

It appears to be out of tune, but not by as much as I thought. I'm going to add some more bottle capacitors and see if that improves things.

Mads Barnkob

Sat Oct 08 2011, 06:33AM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

Have you tried plotting all your coil data into JavaTC? This will give you an idea of where to start with the tuning.

It do also seem a bit over coupled and other things you will discover using this tool:

Jamie Cansdale

Sat Oct 08 2011, 04:19PM

Registered Member #3904 Joined: Sat May 21 2011, 10:32AM
Location:
Posts: 28

Mads Barnkob wrote ...

It do also seem a bit over coupled and other things you will discover using this tool:

Thanks for the link. I'll give JavaTC a try.

What indications are there that it's over coupled?

Herr Zapp

Sat Oct 08 2011, 04:59PM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

Jamie -

The first thing you need to do is accurately determine the resonant frequency of your primary and secondary circuits. You can use JAVATC to calculate the approximate resonant frequencies, but since you've already got a decent o'scope, if will be more fun and instructive if you can actually measure the pri and sec resonant frequencies, and see the effects of your changes.

Do you have access to an RF signal generator? With a signal generator, its very easy to drive both pri and sec circuits with a low-power RF signal and measure their exact resonant frequencies with the o'scope.

If you can't get your hands on a signal generator, than you can quickly build the very simple & low-cost adjustable-frequency signal generator (Terry Fritz' "Tesla Coil Tuner") as shown here:

Blindly changing top load configuration in a "trial and error" effort to achieve resonance can be frustrating if you don't have a way to see how your changes are affecting the resonant frequency.

Herr Zapp

Jamie Cansdale

Sat Oct 08 2011, 11:56PM

Registered Member #3904 Joined: Sat May 21 2011, 10:32AM
Location:
Posts: 28

I've increased the size of my capacitor bank and reduced the size of the top load. The trace finally appears to show ringing. :)

I do have access to a signal generator. I'll try measuring their resonant frequencies and see how close they are to being in tune.

I've measured the resonant frequencies of the primary and secondary using a signal generator. They appear to be 740 kHz and 840 kHz respectively. This corresponds pretty well to the peak FFT frequencies when the coil is driven via the spark gap.

Does this look reasonable or should I try to tune it some more?

Herr Zapp

Sun Oct 09 2011, 07:01AM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

Jamie -

Looked a little closer at the photo of your coil, which triggered a few more questions:

1. What is the Ikea "tealight holder" made out of? Solid metal? Glass, metallized on the inside? Plastic? How is the top of your secondary winding connected to the conductive surface of this topload?

2. It looks like (aside from the primary coil itself) all the wiring in your primary circuit may be quite small gage wire, with everything connected together with mini-alligator clips. Remember that even in a small coil like this, the primary circuit is handling peak currents of scores of amps. ALL your interconnections in the primary circuit should be soldered, or at least be solidly screwed together.

3. Can you provide specs and photos of the rest of your coil? What type of spark gap are you using?

4. You mentioned that this is a "DC" coil. Simply powered by a HV DC power supply, or a resonant DC PS with a charging inductor? Describe the PS, including voltage and current ratings.

5. Based on the apparent dimensions of your primary and secondary coils, your system may indeed be "overcoupled". Typical coupling factor for spark-gap coils ranges from .06 to .18. See the section on "frequency splitting" at Richie Burnett's website at:

Herr Zapp

Jamie Cansdale

Sun Oct 09 2011, 11:59AM

Registered Member #3904 Joined: Sat May 21 2011, 10:32AM
Location:
Posts: 28

Herr Zapp wrote ...

1. What is the Ikea "tealight holder" made out of? Solid metal? Glass, metallized on the inside? Plastic? How is the top of your secondary winding connected to the conductive surface of this topload?

I've got one large tealight holder and a smaller candlestick holder that I'm using now. They're made out of solid metal and are nice and smooth. The hole where a candle would go is a handy place to put breakout point objects. There's a wire attached to the secondary that touches the bottom of the topload.

Herr Zapp wrote ...

2. It looks like (aside from the primary coil itself) all the wiring in your primary circuit may be quite small gage wire, with everything connected together with mini-alligator clips. Remember that even in a small coil like this, the primary circuit is handling peak currents of scores of amps. ALL your interconnections in the primary circuit should be soldered, or at least be solidly screwed together.

You're right it was all strung together with some pretty poor hookup wires. I've now replaced all of the primary circuit hookup wires with copper pipe. It isn't soldered or screwed, but the contact is over a reasonably large area.
ed together.

Herr Zapp wrote ...

3. Can you provide specs and photos of the rest of your coil? What type of spark gap are you using?

I've attached some more photos. The spark gap was made using a fuse holder and sat under a glass. It was far too many hookup wires away from the rest of the circuit (optimized to reduce the noise, not efficiency)! I've replaced this with a gap between two copper pipes in the primary circuit.

Herr Zapp wrote ...

4. You mentioned that this is a "DC" coil. Simply powered by a HV DC power supply, or a resonant DC PS with a charging inductor? Describe the PS, including voltage and current ratings.

The supply is a flyback transformer driven by the ZVS circuit on Grenadier's site. My cheap analog ammeter is showing 5mA with the spark gap firing at around 8.5kV.

Herr Zapp wrote ...

5. Based on the apparent dimensions of your primary and secondary coils, your system may indeed be "overcoupled". Typical coupling factor for spark-gap coils ranges from .06 to .18. See the section on "frequency splitting" at Richie Burnett's website at:

Thanks for the link. I'll read up and play with this some more tonight. :)

Herr Zapp

Sun Oct 09 2011, 05:16PM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

Jamie -

1. In the photo showing your salt-water capacitor bank, the individual wires from the inside of each bottle appear to just be poked into the neck of the center bottle. The center wire from each bottle must be directly tied to the main conductor (preferably soldered), or you don't really have an "all bottles electrically in parallel" circuit.

2. Are you certain that you've got a good electrical connection between the top winding on your secondary and the bottom of your topload? If that's machined aluminum, there may be an insulating anti-tarnish coating (clear anodizing, clear lacquer, etc.). With the topload sitting on the secondary, use your multimeter to verify continuty from the surface of your topload to the bottom of your secondary.

3. What are you using as an RF ground (or counterpoise) for your secondary?

4. What is the capacitance value of your salt-water capacitor bank?

5. If your DC power supply is truly only capable of providing 5ma charging current, this coil might not operate, except possibly in a very slow "pop............pop............pop" mode. 5ma is not much current to drive even a small TC, and with lossy salt-water capacitors, you just don't have much to work with. I think you'd be MUCH better off starting with a conventional AC spark-gap coil, powered by very small NST (5 KV/25-30ma) or OBIT.

Herr Zapp

magnet18

Sun Oct 09 2011, 08:19PM

Registered Member #3766 Joined: Sun Mar 20 2011, 05:39AM
Location:

Posts: 624

Do you have oil over the water in your capacitor?
If not you'll probably get carona losses and take a severe cut to efficiency (& spark length)

Herr Zapp

Sun Oct 09 2011, 09:39PM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

Jamie -

Also, I forgot to mention that the large difference in resonant frequency between your primary and secondary circuits (assuming that your measured values are accurate) needs to be corrected. Initially, you can attempt to match the frequencies, or perhaps set the resonant frequency of the primary 2-5% lower than the secondary.

I'd make adjustments to get both pri and sec resonating at the same frequency first, then tweak for best output (assuming that 5ma can actually drive this coil).

Herr Zapp

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