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Question concerning choice of NST for Spark Gap Tesla Coil

I have been designing a spark gap tesla coil, so far trying to use what I have to save cost. I currently have a solid-state NST and I have read that they don't perform properly for Tesla coil applications (I imagine sudden current draws create back emf that is bad for the transistors?). I have disabled overvoltage protection by cutting one of the wires as shown. Ultimately I would be curious as to what problems arise with using a solid state NST with a SGTC before I look into getting a proper NST. NST photo.jpg
 

shortbus=

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From what I know of why they aren't recomended is because they are solid state switching supplies. Where as the old type was a linear supply.
 
I'm not sure what you mean as linear supply for the old NSTs as those are all transformers with a shunt put in to limit current. However the fact that the new NSTs are switching power supplies would mean that the waveform would have plenty of messy harmonics, I don't know if that is the main culprit. I would wonder if that could be compensated for by using some sort of band pass filter to have a truly sinusoidal waveform?
 

rjenkinsgb

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I think the two basic factors are:
A standard transformer has no fast protection, while any well-designed electronic equivalent will have protection circuitry.

The frequency - tens or hundreds of KHz for an electronic one vs. 50 / 60Hz for a basic transformer.

Looking at some schematics for tesla coils using neon transformers, the capacitor bank has to charge to peak voltage within the time of one half cycle as there is no rectification. It also has the primary in series with the capacitors, adding inductance which also limits the charge rate.

Once the spark finishes, the primary is open circuit again so not damping the secondary.

I just do not see how that can work with a high frequency electronic transformer; I think you would need to add high voltage, high speed rectifiers and let the cap bank charge over a number of cycles until the voltage builds to where it can bridge the spark gap?

Some of the schematics I found:


Or this, which could work with conventional or electronic transformers as it uses rectification.
 
Thanks for that assessment. I've found a deal on a traditional NST but once I've finished the typical setup I'll look into a HV rectified setup with the solid state NST and see how well that works. Removing the overvoltage stuff may have been sufficient for removing protections (NST continues to operate when shorted) but I guess I'll see.
 

unclejed613

Well-Known Member
Most Helpful Member
you could also look at designs for spark gap transmitters, as the tesla coil primary circuit and spark gap transmitters have a lot in common. for instance, feeding high voltage DC to the primary tuning capacitor, and having the capacitor discharge through the primary winding across a rotary spark gap is a very common method to excite a tesla coil. you can get some good info from [here]. be aware some old schematics use zigzags for coils, so some schematics may look like there are resistors when they are actually inductors and transformers.
 

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