RE: Gap Losses (II) Re: Primary Heating

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

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

Hi Alex,

At 07:14 PM 3/13/2002 -0500, you wrote:
>Terry
>Is your NST filter designed for running NSTs at 140Vac. I am assuming it is
>by the few numbers I crunched on the MOVs but I am just checking. I don't
>want to blow up the filter($$$).
>Thx
>Alex Madsen
>
>

This is a very good question.  The 1800 volt MOVs are +-10% tolerance.

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

There are 14 in across the NST so the voltage range is 14 x 1800 = 25200
+-10 % or 22680 - 27720 volts

An NST at 140 VAC in is 15000 x SQRT(2) x 140/120 = 24749 volts (21213
volts at 120VAC).  So the MOVs are NOT designed for 140 VAC input.

Normally the safety gaps should start to fire if you set them at 120 VAC.

But lets suppose you want to play dangerously and go to 140 VAC.  The MOVs
"might" be high enough in tolerance to take it.  You can turn the NST up to
140 VAC and watch carefully if they start to get hot.  The MOVs will burn
up if they have to dissipate substantial power for an extended time.  You
could go to 16 MOVs for a range of 25920 to 31680 but now the top end is
almost 50% over the NST rating!!

Bottom line is, you almost have to test the MOVs for breakdown voltage if
you want to really run at 140 VAC.  You can't just trust the component
tolerances.  On my big coil at 140 VAC in the MOVs start to heat but not
"too" badly.  So they are being stressed along with the NST about "right".

Of course, modifying the original design or operating at over 15kVAC voids
the warranty :o)))

Cheers,

	Terry