MOVs and NSTs

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi All,

Using MOVs with NSTs seems to be catching on :-))

I just wanted to mention how to size them to match the NST voltage.

I use Panasonic ERZ-V10D182 types from DigiKey (PN# P7215-ND).  They are
1800 volt 2500 amp surge devices.  They have up to 6500 amp ones too but
the price gets high.

The turn-on voltage of the 1800 volts MOVs is rated +-10% so that gives a
minimum voltage of 1620 volts.  

The "peak" voltage an NST puts out is the rated "AC" voltage multiplied by
1.414 (the square root of 2).  So a 9 kV NST really has a peak voltage of
12728 volts.  However, if you have a 0-140VAC variac :-)) then count on
another 16.67% for 15850 volts peak.

Each leg of the NST has half that voltage or 7425 volts.

So if you divide 7425 by 1620 you get 4.58 MOVs are needed from each leg to
ground (the NST case).  Always round up, so 5 MOVs will work for a 9kV NST
with a 0-140VAC variac.  The NST can take a more than it's rated voltage
(to a point :-)) so all is well.

For "my" filter*, no other parts need to change for smaller (or larger)
NSTs other than setting the safety gaps properly which is a given.  My
filter can also take higher currents but I over designed it since I know
how you all are :-))

*
http://hot-streamer-dot-com/TeslaCoils/Misc/NSTFilt.jpg

I would also recommend fusing the input of the NST so that if the cores
saturate and the NST starts to go nuclear, the fuse will blow before the
NST.  With a properly set up filter and an input fuse.  There is no known
way to blow an NST.  The NST may die of old age or something but they
usually die do the terrible stress being put on them that the filter and
fuse will stop.

Unlike spark gaps, MOVs always fire at the right voltage and you can't mess
them up unless you really try.  They are super fast and short to a low
voltage (in NST service) if you burn them up, so they are pretty fail safe.
 I originally tried them as an experiment, but the experiment seems to have
worked out very well :-))

Cheers,

	Terry