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ZVS SGTC problems

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Alex M

Sun Jun 10 2012, 06:25PM

Registered Member #3943 Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552

Hi, I attempted to make a small SGTC today using a flyback transformer and "ZVS driver" (or what ever the correct term for that thing is). Problem is I get no streamers out of it, just a small amount of corona visible in the dark at the breakout point and half centimetre arcs to ground.

I have it connected like this (its only small one).

And the secondary looks like this, think its something like 32SWG magnet wire.

Primary coil is just a piece of single cored 2.5mm2 mains wire that I had about 4 turns of (I tried lower but no difference, this was wound around the base of the secondary coil).

I think, but am not sure, my capacitor might be too small as it is only around 1nF. It is made out of a few salt-water jar capacitors in parallel.

I have tried 24v and 36v input and whilst with 24v input the spark gap fires with loud discharges (using bolt heads for electrodes), but on 36v I get one snap and then just a continuous hot fiery arc. Could this be becuase of the tank capacitor being too small?

As I said the only output I get from the secondary of the tesla coil is a small amount of corona and perhaps half a centimetre arcs to ground.

Any help regarding this would be appreciated.

Thanks.

Sigurthr

Sun Jun 10 2012, 07:10PM

Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597

The spark gap needs quenching (airflow to break the flow of electricity) because if it stays conducting the capacitor never charges. Inadequate quenching will greatly reduce the output from any spark based tesla coil.

Also, try swapping the placement of your capacitor and spark gap. The parallel capacitor setup you have in the schematic is not advised for fragile transformers (flybacks, NSTs, etc) and allows HF ringing to be sent back to the driver circuitry. Instead having a parallel spark gap and series capacitor adds the safety of keeping the HF out of your drive circuitry and transformer while adding no drawbacks or additional losses.

Adding more capacitance cannot hurt, and will only improve output, but fix the other issues first.

brandon3055

Mon Jun 11 2012, 12:27AM

Registered Member #4548 Joined: Mon Apr 23 2012, 03:52AM
Location: tasmania
Posts: 271

The tc in this thread

runs off a small OBIT but it runs twice as well off of my zvs but dont be fooled by its size I can get the same sparks out of it when I replace its secondary with one that's only 20x4cm that's y I'm rebuilding it

Alex M

Mon Jun 11 2012, 11:40PM

Registered Member #3943 Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552

Ok, I will try making a better spark gap and put a fan on it. I will also add some more capacitors.

I am looking into buying some copper tubing to make an easily adjustable primary coil, but don't know what sort of diameter and length I need to buy. Can anyone suggest a rough size of tubing that would work for this? I have found this

but don't know if it will be too thick (15mm).

Thanks.

Sigurthr

Tue Jun 12 2012, 01:23AM

Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597

It is a pretty tiny coil, don't bother with tubing for the primary just use heavy gauge wire... 10ga should do. Length of wire depends on how many turns you use.. the fewer the turns the higher the voltage on the secondary and the more current your TC will draw. You will have to experiment to see what gives you the best output at a level of current your ZVS can safely handle.

The more air you can force through the spark gap the better, set the gap spacing so it arcs continuously when the primary and capacitor are not connected (remember to use a parallel spark gap, and not the series one you have now!) but doesn't arc if any wider. Having the gap too small wastes power and makes it harder to quench correctly.

brandon3055

Tue Jun 12 2012, 02:15AM

Registered Member #4548 Joined: Mon Apr 23 2012, 03:52AM
Location: tasmania
Posts: 271

Sigurthr dose having the spark gap as big as possible also apply to RSG's?

Sigurthr

Tue Jun 12 2012, 01:04PM

Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597

Unfortunately I am not experienced/familiar with rotary gaps, so I can't answer that and be absolutely sure.

I can say that you do NOT want a misfire with a rotary gap. From that knowledge I would assume that the actual electrode spacing when aligned must be short enough to guarantee a firing, every time.

Alex M

Mon Jun 18 2012, 11:19AM

Registered Member #3943 Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552

Right I just added more capacitance and a better primary coil and spark gap and it sparked into life right away giving around 2 inch streamers to air on first light with no topload (remember its only a small one and is still not 100% tuned).

The bad news is it arced to the primary coil and started a small fire (the primary is not even that close to the secondary either, the low end of the secondary coil is grounded locally to the ground pin of the LOPT since I am inside).

I only have around 2.6nF of capacitance too right now so I think an extra nF or two might give some improvements.

HV power source is a small "ZVS" driven LOPT @ 24v since I don't think one or two of my jar capacitors could take it on 36V.

I will keep tweaking it and probably share a video too later, just feeling happy since this is the first tesla coil I have managed to get working correctly

Thanks.

Alex.

Update: Just found a 2.5 inch by 18 inch PVC pipe in the garage so I am going to try making a larger secondary coil when I get some more magnet wire.

Can anyone recommend what gauge magnet wire I should use for this? I was thinking something around 30SWG (28AWG) but I want to try and do things correctly this time first time round.

Sigurthr

Tue Jun 19 2012, 02:00AM

Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597

I threw together a ZVS LOPT SGTC the other day for kicks and used my 8"x3.25" 30ga secondary with no issues. Had 2" streamers when output was capacitively loaded. You should be fine with 30ga.

You will have arcover issues between pri/sec when using a LOPT as the power supply for a SGTC because the voltage running through the primary is at least 2x what is normally used for a SGTC. The solution is to lower coupling - place primary on a coilform much larger than the secondary's diameter.

You also cannot use the LOPT ground pin for the secondary ground as this will over-volt the LOPT and likely destroy it quickly. You WILL need a separate ground. I have an isolated RF ground installed in my office but you can use a piece of insulated 14ga wire running to your cold water pipe as well.

The thing about the secondary of a TC is that if you were to not ground one side, you would see HV ac at both sides, this arangement is called a bipolar TC. By grounding one side you force it to stay near earth potential which allows you to place the primary near that point (which makes coupling easier). You still need to consider the fact that the primary voltage is very high in reference to ground (especially with a LOPT), but in a bipolar TC the voltage between primary and the proximal location in the secondary is even larger. The ground pin of a LOPT can actually be floating in reference to ground, not to mention it is a horrible quality ground at RF frequencies like out of the secondary. All these factors contribute together and lead to an arcover.

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