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Finally Killed my NST

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Hazmatt_(The Underdog)

Fri Mar 10 2006, 08:17PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

I just wanted to take a moment to thank everyone for their input and trying to help me with the capacitor issue side of my Tesla Coil.

I have been trying to quantify where my loss in performance was and had thought that the major issue was in my Cx bank because the Corona dope insulated the screws of my doorknobs. Well I just found out this morning that one of the NST terminals is practically dead.

The transformer itself is a Jefferson Luminous tube transformer rated at 12,000VAC 60mA
and worked well with a 16nF capacitence. Increasing the capacitence to 26nF felt wrong, but I did so anyway. I probably should have trusted myself more in my calculations.

I had even gone through Terry's figures yesterday and found the proper capacitor to be 20nF for my system, but its a bit too late for that now.

So now I get to unpot my NST and spend a lot more time on this then I have. It's a real shame because this is the second 720VA transformer I've blown, and my 300VA Allanson survived no problem.

I hope to have it fixed soon and maybe remove some shunts to bump it up to 80mA, but for right now, time is a total loss.

vasil

Fri Mar 10 2006, 08:47PM

Registered Member #229 Joined: Tue Feb 21 2006, 07:33PM
Location: Romania
Posts: 506

Did you have a safety gap?

Hazmatt_(The Underdog)

Sat Mar 11 2006, 04:18AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

Safety gap, limit resistors, power control.

It's not exactly clear when I lost my power, but I'm fairly certian it was when I changed my bank to 26nF.

>>>Update<<<

I did a quick calculation for my 12,000 30mA Allanson and found the Cres=.0066uF
Csystem=Cresx1.6 = .010uF and worked well.

My only regret is that I didn't have the chance to test the 20nF for my 12,000 60mA transformer. But at least now I know my exact operating value.

Part Scavenger

Mon Mar 13 2006, 03:48AM

Registered Member #79 Joined: Thu Feb 09 2006, 11:35AM
Location: Arkansas
Posts: 673

Hey! Is that a tar filled NST? If it's just shorted inside, and not dead, try putting it in the oven at 100 degrees (F) for two hours. Then every hour after turn it up until the tar gets watery (don't boil it, it makes things worse). If you do it right there is almost no stink, but it does expand somewhat and tends to run down the sides. Use a cookie sheet or something to catch it. I fixed my NST that way. Oh, and make sure to pry off the top so you can see it get watery. The guy who's site I found that info on said to just shake it every hour to see if its watery, but if it starts to boil, it lets off gas. I'd pry off the top to avoid pressure buildup. Mine took like three hours to fix, but works fine now. I never did get all the crap off the sides, but I'll bet some sort of acetone, paint thinner or something would take it off.

Hazmatt_(The Underdog)

Sun Mar 26 2006, 02:22AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

Well, I started the autopsy today. I used the chisel method because cracking off large chunks of the tar was going to be cleaner in the end as opposed to melting it out. I know this because I've done it both ways. I was fairly careful with the chisel. Only had a few oops moments, and it was a crummy chisel to begin with, so no biggie.

Here are the pictures of the autopsy. The last showing how damaged the coil is. I am not convinced this is reparable, but I do want to find what gauge of wire was used. I am saving the primary and may do a rewind for the secondaries. I am considering this because I might be able to get a 10 lb-er of #32 for a steal. :P We'll see.

Oh forgot to mention! Xylene is an awsome solvent! Eats tar like butter. But I think warm Xylene would be even better, trouble is doing that safely. So I's sticking to using a paint brush and thick gloves.

GimpyJoe

Sun Mar 26 2006, 03:46AM

Registered Member #316 Joined: Mon Mar 13 2006, 01:30PM
Location: Marietta, GA
Posts: 212

Is that last pic of one of those "oops" moments with the chisel or did the secondary burn up that badly?

teravolt

Sun Mar 26 2006, 04:47AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

If you use RF inductors for a choke on each terminal in series with your tesla in conjunction with a safty gap this should not happen. Each choke if large enough in value will block high frequencys from the tesla. If you remove windings from the bad coil and put it back to gether with the ciols in the right polarity you could repot it with candle wax. It may put out 6000+4000. If you want expieriment you can make a 6000 60ma out of a 12000 30ma by reversing one winding. Multipule NST transformers of the same voltage can bephased and paralleled in this way to increas current and power. good luck.

Hazmatt_(The Underdog)

Sun Mar 26 2006, 07:06AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

This is a secondary burn through. The carbonization is such that in the local affected area its a total loss.

I know I could take the winding apart to the point just before the burn but the trouble is that at this point it wouldn't really 'get me there'. I need to see what wire guage they used and see if I can do a total rewind.

I am aware of chokes, believe me I considered that to a very large extent. I had a safety gap, limiting resistors of 1.5K on each leg. What caused this is very simple. I was trying to verify 'accepted' values for capacitors and trying to see what their effets were on performance.
My best performances were with 1.6-1.7xCres on my rotary. 2.6xCres was too much of a load on the transformer and the output diminished. I think at one time I had 28nF in the tank and that may have caused the demise right then and there. I should have used 20nF, I know that now.

I was getting all set up to measure terminal current now that I had my potential divider and the Tek 2430A, but something just wasn't right with the output and loss in performance, and there it was.

I want to build an equidrive system now but first I have to get the plastic situation corrected. My feedthrough plan failed miserably, now I have to go back and re-do it! setbacks..setbacks

>>>>>>>>>>Update<<<<<<<<<<

I have started work on the possibility of a rewind. As you can see I've lost a whole third of the coil. Its 35AWG from here on will be replaced by 34AWG and I must put both coils back on the old transformer and see exactly what the ratio is between them and determine how many turns to put on. This will prove to be somewhat of a challenge because of the core design and its build in gaps.
Once I know exactly how many turns then I will see about my paper selection and what kind of margins I have to have to make it possible. It's going to be tight to be sure.

I am also planning on building an 'ugly can' for this transformer and sealing it in oil. I doubt any non-hermetic insulator would seal effectivily, but its worth a shot. To a larger extent I don't have the metal forming tools do roll a bead on the steel, this will be the greater challenge. And also I would probably have to make it such that I can break the seal and add oil if need be, so it would have to be gasketed.

Acrylic would probably be my best bet anyways. I might look into putting the transformer in a nice plexi box.

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