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Registered Member #135
Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735
EVR I don't think you've considered the portability/power limit aspect of this design.
The NST offers the power, weight, and Maximum Power Draw allowed. No PT is going to do that.
This has to be an entierly self contained unit capable of being run off an outlet at maximum draw without tripping breakers. That is why we use an NST in this design.
Now sure, I have a PT sitting out in the garage right now, but you know what, it tripps out the breakers. And I might add that I am building a 5kw stepdown for the PT now.
Long story short, we are going the efficiency route with what we have available, we are pulling maximum power without tripping breakers, a PT or pig exceeds our space allotment (footprint), weight is an issue, and the NST is the right answer for this situation.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
As a bona-fide NST destroyer I think an NST is not a good choice for a reliable SGTC because;
1) An NST is not meant to operate at continuous high voltage they produce high voltage to start the neon tubes but a much lower voltage during operation operating mainly in reactance-limited current mode due to the flux shunt the output characteristic of an NST into it's intended load is equivalent to a high voltage with large series inductance e.g. a 1500V 60 mA NST 'Ideally' gives 15 kV @ 0mA, 60 mA @ 0V, 7.5 kV @ 30 mA etc. (i.e. max output is 7.5kV x 30 mA, NOT 15 kV x 60 mA) 2) Operating at or near resonant loading for best TC performance puts a continuous high voltage stress on the insulation made worse by operating at high temperature due to high I^2.R heating.
So, A question to all coilers - Who has a SGTC powered by an NST that has been running for a long time? ANYONE ?
Registered Member #50
Joined: Thu Feb 09 2006, 04:07AM
Location: Vernon, B.C, Canada
Posts: 324
1) An NST is not meant to operate at continuous high voltage they produce high voltage to start the neon tubes but a much lower voltage during operation operating mainly in reactance-limited current mode due to the flux shunt the output characteristic of an NST into it's intended load is equivalent to a high voltage with large series inductance e.g. a 1500V 60 mA NST 'Ideally' gives 15 kV @ 0mA, 60 mA @ 0V, 7.5 kV @ 30 mA etc. (i.e. max output is 7.5kV x 30 mA, NOT 15 kV x 60 mA)
Does that mean the calculations for my MMC, for a 900 watt NST are wrong? (calculated for 15KV @ 60mA)
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
Michael, no it doesn't mean your calculations are incorrect If you look at Christopher's BEAUTIFUL coil he is a using slightly larger than resonant (LTR) capacitor value as you have been reccommended. I was (evidently wrongly) explaining why I consider NST's in SGTC unreliable.
Christopher - you really have done some beautiful work - kudos to you man!
Registered Member #135
Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735
If you talk with any of the engineers that design NST's like I did with Franceformer, you will know that:
The transformer is designed to output full rated current in a short circuit condition with no terminal voltage.
The transformer does not strike the tube at a higher voltage then drop down. The older core-and-coil transformers are not capable of this task. Rather they are designed (insulation class and all) to operate at half-wave rectified voltage levels. This means that a 10kvac NST is designed, insulated and all, for 7070vac. The reason behind this is that the luminous tube is actually a diode. So the engineers insulate transformers based on the fact that the source will be running continuously at 7070 vac at a particular current (determined by how many and how long the gas tubes are).
You will also know that a transformer (any transformer) will not likely output its nameplate value under no-load. So if you go and measure a 15/60 with no load, you should not be surprised to find its output 10-20% higher then the nameplate value. This should be obvious of any transformer if you consider the losses (core and copper) do not truely affect performance until a load is present.
If you want you can even go to the transformer article in our wiki, where I have a worked example showing how you can extrapolate your transformer data and actually find the 'ideal source' before the losses. this is very helpful in design and performance.
Registered Member #50
Joined: Thu Feb 09 2006, 04:07AM
Location: Vernon, B.C, Canada
Posts: 324
Due to the lack of large power resistors in my neighbourhood, could I not wind some chokes in combination with my capacitors, like gregs filter or just chokes? If so how many uH or better, if someones already done this, your specs. (transformer : 15KV 60mA NST)
Due to the lack of large power resistors in my neighbourhood, could I not wind some chokes in combination with my capacitors, like gregs filter or just chokes? If so how many uH or better, if someones already done this, your specs. (transformer : 15KV 60mA NST)
Hi Michael,
Due to a parts ordering goof, I have some 50W 500ohm resistors here. They will get pretty warm but nothing bad. If you want a couple, mail me your address and I'll send you two free.
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NST filter
