Re: MOV lightning arrestor in place of safety gap...MCOV?

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "J. Aaron Holmes" <jaholmes@xxxxxxxxxxxxxxxxxx>

Thanks Gerry.  That makes sense.  Some rise time
figures on the datasheet seem to support your
interpretation.  I guess I'm wondering now which of
these ratings suggests how I would use one (or many)
of these devices to protect a transformer in Tesla
coil service.  I'm not sure how the arrestor and MCOV
ratings correlate to the ratings of ordinary
off-the-shelf MOVs such as what you'd find at DigiKey.
 Anybody know?  Should I just go with the transformer
HV / MCOV?  For my pig that would be 14760 / 2550 =
5.7 (6 to be safe).  For my PT that's 15600 / 2550 =
6.1, so six is probably fine there too (barely
exceeding MCOV for each unit, and use would be
intermittent anyway).  I've got 13 of these, so
putting six in series isn't a big deal.  They look
cool, too :D

Unfortunately, my RMS test below probably doesn't give
me an accurate picture of when these things start to
clip the transformer output.  I need an HV probe for
the scope!  Time to head out to Fry's and blow my
weekly Tesla allowance, I s'pose :)

73,
Aaron, N7OE

--- Tesla list <tesla@xxxxxxxxxx> wrote:
> Original poster: "Gerald  Reynolds"
> <gerryreynolds@xxxxxxxxxxxxx>
>
> Hi Aaron,
>
> I'll take a stab at the Front of Wave rating.  My
> guess is that this device
> takes some time to turn on and that a lightning
> strike has a fairly fast
> leading edge.  Therefore it would be possible for
> the voltage to get higher
> that what the device would normally clamp to.
>
> Gerry R.
>
> >Original poster: "J. Aaron Holmes"
> <jaholmes@xxxxxxxxxxxxxxxxxx>
> >
> >
> >A few extra tidbits of info just in:
> >
> >I mailed Cooper Power Systems about my parts, which
> >are discontinued parts made by Kearney Co. (Cooper
> >bought them out some time ago, apparently).
> Anyway,
> >they pointed me to an equivalent production model:
> The
> >235-35 "VariSTAR UltraSIL", which is another MOV
> >arrestor.  I just got the datasheet for the 235-35,
> >and it has a whole bunch of additional ratings that
> >don't make a whole lot of sense to me, however one
> is
> >rather interesting:  The "Front of Wave Protective
> >Level" rating.  For the 3kV arrestors, this rating
> is
> >11kV (peak, not RMS).
> >
> >Intrigued by this 11kV figure, I got out the
> alligator
> >clips and dusted off my 15/30 NST and wired one of
> the
> >arrestors across it.  I ran the variac slowly up
> from
> >0V.  At about 80V in (about 10kV out), the current
> >spiked and the transformer sounded like it was
> really
> >working hard.  I brought the variac back down
> slowly.
> >At just under 60V in (about 7.5kV out), the current
> >dropped sharply and the transformer was silent.
> >
> >...anyway, so these things together seem to suggest
> >that the ratings of these lightning arrestors
> (i.e.,
> >the 3kV rating and 2.55kV MCOV rating) don't tell
> you
> >a whole lot about how you'd use them to protect a
> >transformer in a Tesla coil.  Seems like I might
> only
> >need two or three of these instead of six or seven.
> >The MCOV (Maximum Continuous Operating Voltage)
> rating
> >appears simply to mean that, above MCOV, leakage
> >current may lead to overheating.  But maybe this
> isn't
> >a problem for intermittent TC service?
> >
> >Anybody care to interpret all this baloney for me?
> I
> >know this is borderline OT...
> >
> >73,
> >Aaron, N7OE