Re: Coil done but not working well with new caps
Hazmatt_(The Underdog), Tue Jul 28 2015, 03:09AM

Keep moving your tap around, you'll find it eventually.

Is your strike rail a full turn? if its connected together, it is shorting. Strike rails are not connected together.

Re: Coil done but not working well with new caps
Mads Barnkob, Tue Jul 28 2015, 07:06AM

I can see that there is a gap in your strike rail, so that is fine.

I can also see that you currently have your primary tap point at the outer most turn, your 15th turn! That is properly waay too much. Your coil is so much out of tune as it can be.

Have you used JavaTC to calculate your optimal tuning point? It is the most precise online tool to find a good starting point for optimal tuning and then slowly moving the tap point for a lower primary frequency.

The optimal tuning point can be hard to find, it lies within a few centimeters, so it easy to miss when moving the tap.

You risk damaging your secondary coil by drawing arcs from it, you should at all times avoid racing sparks or drawing sparks from it. If you first punch through the varnish isolation you have to unwind the damaged area, repair and clean from all carbon tracks and varnish the damaged area again.

Re: Coil done but not working well with new caps
crashstudio, Tue Jul 28 2015, 09:37PM

I've been using TeslaMap and just found the problem. I forgot to change the primary capacitance value, when I entered the new value it says I should tap the 13th on the primary.

Do you think I would get better results with a spun aluminum toroid? I found a 3" x 12" on ebay for just over $100
Thank you

Re: Coil done but not working well with new caps
Mads Barnkob, Wed Jul 29 2015, 06:32AM


Better results yes, but that is not your biggest problem at the moment, a spun toroid vs. ducting might not give you more than 5-10% longer sparks from less corona losses and higher breakout voltage if not breakout point is used.

Could you please tell us all the details of your coil, I would like to plot the data in JAVATC and see what it says.

Re: Coil done but not working well with new caps
crashstudio, Wed Jul 29 2015, 11:54PM

NST
* Input 120v
* Freq 60Hz
* Output 15KV
* Current 60mA

Primary cap tapped at 17.08nF

Richard Quick static gap. 6" PVC with 11 pieces of 3/4" copper pipe all spaced at 1/32"

Primary Coil
* Diameter .25"
* Spacing .25" roughly
* Center hole diameter 6"

Secondary Coil
* AWG 28 magnet wire
* Height 20"
* Coil form 4"

Toroid 4" x 17"

A/C Line filter
* Tokin Noise Fliter 115/220 VAC 15amps
* 2 Microwave oven caps in parallel to = 165nF (166nF suggested by TeslaMap)

Let me know if you want the specs on the Terry Filter too.

When I turned it on the last time my first reaction was that something was wrong. After your last post I decided to check the secondary and think I found a small hole. I provided a Drive link to the pics below. I was already planning on winding another with 26ga just not this soon. I'd rather do that than to unwind and fix like you said.




Just found the real problem. Inside the secondary the voltage has been riding the PVC and melted a good portion of it. I added the pics to the link in my last reply. The hole in the pics could have been made when the coil dropped on the bench.

Could this have happened from the threaded rod from the toroid? It looks like it was arcing from that to the lower connection. Apparently all internal connections have to be completely insulated? I guess it would make sense considering their less than 2 feet apart.

Re: Coil done but not working well with new caps
Mads Barnkob, Thu Jul 30 2015, 07:16AM

Wow, that is some internal failure of the secondary you have there.

Generally I would advise against having any metal on the inside of the secondary and that also goes for the end caps. Secure everything with nylon bolts and mount metal screws in a additional piece of insulating material that is mounted with nylon screws, this way you can completely avoid having metal on the inside.

I plotted your data in JAVATC and you could use a smaller MMC capacitance to still have larger than resonant with the NST and your tapping point is properly below the 12th turn, maybe 11 to 11.5 to tune the primary lower than the secondary to compensate for streamer loading.

Re: Coil done but not working well with new caps
crashstudio, Thu Jul 30 2015, 09:53PM

[quote]
Wow, that is some internal failure of the secondary you have there.

I know, it's crazy. I'm sure it was happening every time I ran the coil. Looks like it found a path and stuck with it.
I'm thinking about patching it up and properly insulating it. Maybe even inserting a 3" diameter pvc in the middle. Do you think it would work? I've yet to see it with the new caps and seeing it work better will give me even more motivation to wind another secondary. Oh and thank you for the data.

Re: Coil done but not working well with new caps
Mads Barnkob, Fri Jul 31 2015, 06:21AM

[quote]
[quote]
Wow, that is some internal failure of the secondary you have there.

I know, it's crazy. I'm sure it was happening every time I ran the coil. Looks like it found a path and stuck with it.
I'm thinking about patching it up and properly insulating it. Maybe even inserting a 3" diameter pvc in the middle. Do you think it would work? I've yet to see it with the new caps and seeing it work better will give me even more motivation to wind another secondary. Oh and thank you for the data.
[/quote1438323335]

Repairing the damages from arc over that has left such a severe burn and carbon tracks can be a real pain to ever get to work properly again. You will have to cut, mill, sand out every little residue of carbon to avoid an future failure in the same carbon tracks, it is afterall a conductive path provide along the horizontal axis of a high voltage winding.

I am sure you will have a great performing Tesla coil with your 942C caps, I have yet to see bottle caps outperform those ;)

Re: Coil done but not working well with new caps
crashstudio, Sun Aug 09 2015, 08:36PM

[quote]
[quote]
[quote]

I am sure you will have a great performing Tesla coil with your 942C caps, I have yet to see bottle caps outperform those ;)
[/quote1439151801]

Since I'm winding a new secondary I decided to update the power source too by adding another 15kv 60mA NST in parallel for 120mA. TeslaMap suggests an LTR of 34.3nF. The MMC calculator suggests 2 strings of 9 to = 33.3nF at 18kv peak. The peak voltage of 15kv is 17.12kv. Is that a safe enough range or should I go with 3 strings of 13 to = 34.6nF at 26kv peak? And what do you believe the best capacitance value would be?
If I go with 3 strings of 13 I'll have to purchase more caps to provide tap points of less value. Right now I have 40, I'll need 48 to provide a total of 4 taps. 34.6, 32.1, 30 and 28.1. According to the calculator.
Thank you for your help

Re: Coil done but not working well with new caps
brandon3055, Mon Aug 10 2015, 03:47AM

Just a couple notes on tuning. First off you don't need to calculate the exact primary winding in order to tune the coil. Just pick a starting point and slowly adjust in one direction about 1/8 of a turn each run until it starts to break out then fine tune it for the largest streamers. If you don't find the resonant point go back to where you started and adjust in the other direction.

Also I'm pretty sure coils will ark to ground no matter how out of tune it is so remove the grounding ring.

Lastly add a breakout point to make it more obvious when you are getting close to resonance. You can remove it when you are done (but you may need to adjust the tuning a little when you remove it).

I have only built one moderately successful coil and it was a couple years ago now so I don't claim that this it the best way to tune a coil but it worked well for me.