Setting Safety Gaps and Saving NSTs and MOVs ;-))

[Terry Fritz <twftesla-at-uswest-dot-net>]

Hi All,

	I notice that a number of MOVs strings have gotten fried since may
protection filter has become popular:

http://users.better-dot-org/tfritz/NSTFilt.jpg

Even though DigiKey has 1704 of them in stock, those MOVs aren't exactly
free at $1.47 each ;-))  Although, far cheaper than a blown NST!!

One should never blow the MOVs.  The regular safety gaps should take most
of the load off the MOVs.  The MOVs are sort of a "last resort" protection
when everything else screws up.  Normally, the safety gaps fire just above
the NST's rated voltage and the MOVs conduct at a still higher voltage.  

It goes like this:

a.  The NST puts out 15000 x SQRT(2) = 21213.2 volts peak normally and
"safely".

b.  The safety gaps should be set a bit higher to act if that voltage goes
higher.  We'll say 22-24 kV.

c.  14 x 1800 volt MOVs conduct at ~25.2kV.  The MOV voltage is not
adjustable so you can't mess up :-))  The MOVs are very fast and very
stable.  They will not let you down!  However, continuous overload will
heat them up (fairly fast) and they will eventually melt down and become
permanently shorted...


Of course, this beautiful system assumes the safety gap are set right which
is perhaps NOT a great assumption judging from the MOVs (and formerly) NSTs
frying... 

So how does one set the distance of the safety gaps?  Here is how!!!


1.	Remove everything (primary cap, primary coil, odd wires) from the NST's
output terminals. 

2.	ONLY connect the safety gaps (or the protection filter circuit with the
safety gaps on it) to the output of the NST.  Nothing else should be connected.

3.	Set the safety gaps to about 1/8 inch (really close).

4.	Turn up the variac power until they arc.  Let the gaps arc for a few
seconds to burn off the dust and such and to "prime" them.

5.	By trial and error, set the gaps to a distance "just barely large
enough" that the safety gaps just do NOT fire with the variac at 120 VAC
output.  Thus, the transformer is putting out it's full rated open load
voltage and the gaps are set just a tiny bit higher.

6.	Your done!  The gaps are set.  DON'T go messing with them!!  If they
fire a lot now, something else is wrong.

Of course, be sure to remove all power and unplug things when you go to
adjust the gaps so you don't get electrocuted!  =:O    You will be going
from full high voltage to "hands on" a lot so be super careful!!

Cheers,

	Terry

BTW - If you REALLY want to set gaps right, just try and find a 50kV
high-precision switching high-voltage DC power supply like the one I just
added to my toy box today!!  He he he!! ;-))

http://63.225.104.218/test/TeslaCoils/Misc/NewToy.jpg

Looks like my old MOV string turns on at 25.58 kV.  If I just tap the dial
it over currents.

Thanks Mike for cluing me on in this deal.  The power supply is like new
and runs perfectly ;-)))