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Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
J. Aaron Holmes wrote ...
EDIT: I would also add that the use of filter *inductors* on the HV has been largely discredited through numeroous studies around protecting NSTs. See the TCML archives and Terry's papers. In some cases, it was suggested that the inductors did more harm than good. Hence the Terry filter and its RC-ness.
To this day I don't understand how the claimed ''filter'' benefits anything.
I find it very interesting that, when people run into failure in any sort of power electronics they don't understand, way too many times they attribute it to ''voltage spikes'', no matter if such explanation makes sense.
The nst itself owns probably much more leakage inductance, parasitic capacitance and resistance then we may add with a practical filter.
Plus, while heavy currents are flowing and gap is on the transformer is actually shorted.
Only way I could ever think to kill a NST would be resonating it with tank capacitance.
I thought that could be easily avoided by using a LTR cap and properly set safety gaps.
I don't see how those resistors and caps help though. I can make a 100pf HV cap out of some plastic sheet and foil, or some PCB. But what is that compared to NST's winding capacitance?
I myself can't get that much HV mov's, but if resistors are there to protect them, wouldn't it be better to put safety gaps *before* resistors, and caps after? So the resistors limit current that can be thrown back into safety gaps while transformer is directly protected?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
The NST has stray capacitance as well as inductance in its secondary windings. These form a kind of transmission line or filter, and the result is that when a sharp transient with a lot of HF content hits the winding, the voltage isn't evenly distributed. Most of it appears across the outermost turns, which get overstressed and can break down.
A Tesla coil spark gap connected across the transformer generates hundreds per second of extremely sharp transients, with content right up into the VHF and UHF bands. Terry's filter removes the high frequency content from them, so that the NST insulation can cope better with the low frequencies that are left.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Steve, can you a bit better define the term ''sharp transient with lot of HF''? :p
NST would see some high frequency voltage once gap is off, but only then....
So how do resistors help in that case, as current is tiny (impedance of the NST is much higher than that of resistors). Wouldn't it in that case just be enough just to stick some capacitance on the output together with safety gaps?
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Questions about my First Tesla Coil
