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Need help with my first NST powered SGTC

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hsieh

Mon Feb 14 2011, 08:46AM

Registered Member #1412 Joined: Thu Mar 27 2008, 04:07PM
Location: Taipei Taiwan
Posts: 278

This is my first NST powered SGTC.But I can't get any spark.

My previous SGTC is powered by ZVS driven flyback and produce some sparks even without any tuning.

my NST is rated 240V input 12KV 30ma output.But I only have 110V at home.
my capacitors are three 2700pF 27KV capacitor
secondary diameter is 6cm secondary height is 72CM ,1800turns

Coronafix

Mon Feb 14 2011, 11:57AM

Registered Member #160 Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938

If you are only putting in 110V then you are only getting out 5500V. Try getting a step up transformer to run it.
At full rated voltage your capacitor size should be about 1.4 x 6.6nF as per the formula C=1/(2piZf) for resonant charging of the capacitor. 1.4 will stop the charge reaching 2x the charging voltage.
What sort of capaitors are you using? They don't look like they are pulse rated.
Your secondary ratio is not recommended, ie 12:1.
Try cutting it in half and adding a top load.
Have you used a TC calculator at all?

This will recommend the number of turns you need to tap at on your primary.
Add a safety gap to save your NST.

quicksilver

Mon Feb 14 2011, 04:21PM

Registered Member #1408 Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679

You might consider several options. First determine if your NST is putting any spark at all. Try the standard you are using (110Vac) and with a chicken stick or some insulated safety device, determine IF your NST will give you any arc as is. Go directly from output pole to pole & even if you can get a 1 or 2cm arc you will know that you don't have a problem with the NST itself.
Then obviously make a determination as to how you want to proceed as there are several ways to jump up your mains to 240 OR if you are getting some spark you could simply down-size the unit. Frankly 6cm x 72cm is a tall thin secondary coil form and will take some tuning to make it preform well. It will be much easier to use less than the full power of that NST to tune it.

Coronafix's advice is critical in regards to your capacitor choice. Reality is that generally one needs to make a MMC from several appropriate caps. Pulse-rated appropriate capacitors in single unit size are very expensive. However getting the correct rated capacitance is a must. Research in this issue is important as you may have a market for caps in your area that will work well and cost less than The UK, US or AUS; you may get lucky!

Your spark gap - EVEN if using a "down-sized" NST will wear too quickly and be very loud. Look at "multi-gap" designs. They are easy to make, will reduce noise and will last.
If the NST will preform with your standard home energy you may elect to use it as is. 5500Kv can get some good results as it's the unit as a whole; the tuning of that unit (the TC) that will make a difference. A coil badly out of tune cannot be made more efficient by feeding it more energy.Unquestionably, the quality of the unit as a whole is much more valuable than a large power source.

By using JAVATC or another "Tesla calculator", you will have a very good idea of where appropriate tuning changes need to be. A tall thin secondary coil is a challenge to tune to optimum in a sense rather than a wider one; in that Top-load capacitance becomes a bit more sensitive but it can be done.

Ideally, one takes all issues into consideration regarding availability of parts before actual building - but using what you have is also a great way to learn! In that it pushes you to find the best "middle ground" for what you have.

hsieh

Tue Feb 15 2011, 03:49AM

Registered Member #1412 Joined: Thu Mar 27 2008, 04:07PM
Location: Taipei Taiwan
Posts: 278

So what capacitor should I use?Can I use polypropylene caps?
Electronic component store shopkeepers here sometimes don't know the correct name of components.

Dr. ISOTOP

Tue Feb 15 2011, 04:01AM

Registered Member #2919 Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652

You should use polypropylene pulse caps, preferably in an MMC configuration if you can afford it.
You can also use a commercial pulse cap (e.g. the Maxwell 36xxx series), but those are *really* expensive

quicksilver

Tue Feb 15 2011, 04:28PM

Registered Member #1408 Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679

OK see attached PDF document for the general standard of MMC TC caps. The company is what many if not most people use. These are pulse rated and hold up very well. Use the general rule of 1x1.4 of voltage overall. So that if you're buying 2000v caps to build a MMC that your finished cap bank should be (in general (X1.4) of what you are using. That's another reason for using less input

It's cheaper overall.

Now the important issue is what capacitance should the finished group of caps be? For this you need to do some computational mathematics & it's easiest via a program like JAVATC (see link in past) or programs like WinTesla. Because you have a tall thin secondary [with a LOT of windings] if you went with what your NST puts out when you use 240 it may not be tunable at all..... 12Kv is just too much (I tried it so I could give you SOME idea of where to start).
It MIGHT work at full strength if you did some adjustments aside from tuning but I see that as not too easy...... It will preform quite well when in tune with a well made MMC and a NST: even at 5.5Kv - you will be surprised.

If you went with about half that (let's say it works with your common house 110v) & you get 5.5Kv you'd be in the area of 6nf total (capacitance) depending on what type of top load and a primary of 12 turns .25" tubing apart about .5", etc - You see I can't say exactly only you could because you have to "project" what size primary you'll have and what type of top load. If you simply found two 6" brass, copper or steel bowls and made a sphere from them - THAT diameter would need to be input to get a solid number for your capacitors. But if you want a number figure on buying about 12-15 2000v caps of this: (942C20P15K-F) type 1.5 uf as they give you a great deal of flexibility when building the cap bank (MMC). So to answer your questions..... you have to make further decisions.
]942c.pdf[/file]

You tell the shop keeper you want these. IF he can't get them tell him they need to have the same specs as these (see the last number for 2000v 1.5 uf) to preform & withstand the same levels. {NOTE} best price in USA is around 3 dollars. Do not pay more than 3.75. There ARE cheaper types but not as good. but see if you can get these for better price than 3 US dollars: that would be excellent. These last, they take incredible abuse; they are worth it.

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