RE: [TCML] pole pig protection

["Jim Calvin" <jcalvin@xxxxxxxxxxxxxxx>]

Gary,
	The division ratio will not be the same at all frequencies.  The RF
choke's impedance increases with frequency (hence a low pass action) also I
am fairly certain the inductance of the HV winding is not 1000 henries.  In
fact, the impedance at the HV winding should be the impedance at the low
voltage input times the turn ratio (1/65).  This of course is going to be
the source impedance plus the ballast impedance.

-----Original Message-----
From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On Behalf
Of Lau, Gary
Sent: Wednesday, September 10, 2008 9:51 AM
To: Tesla Coil Mailing List
Subject: RE: [TCML] pole pig protection

Correct - a choke in series with the HV winding does create a voltage
divider.  But -

1) The division ratio is constant for all frequencies, 60 Hz as well as the
tank frequency.  Filters usually imply that the attenuation is
frequency-dependant.  We want a filter that attenuates high frequencies
more-so than low frequencies - a low-pass filter.

2) Let's say that the HV winding L1 has an inductance of 1000 Henries.  If
we put a 10 mH inductor L2 in series with this, the voltage seen by the HV
winding is L1/(L1+L2):
1000 / (1000+.01) = 0.99999 of the unfiltered value.  No difference.

Regards, Gary Lau
MA, USA

> -----Original Message-----
> From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
> Behalf Of Jim Calvin
> Sent: Wednesday, September 10, 2008 8:53 AM
> To: 'Tesla Coil Mailing List'
> Subject: RE: [TCML] pole pig protection
>
> As a point of fact I have destroyed a pole transformer last year with this
> very same coil.  So they are not indestructible.
>
> Gary, you are right, an inductor by itself is not a filter, but in series
> with the HV winding of the transformer forms a voltage divider with an
input
> impedance of greater than 5Kohm (at 90Khz).  I have not put a scope on it
> with my HV probe so I can not say for certain it is helping.
>
> Can you email me this filter DC.  This is exactly what I am looking for.
> My email is jcalvin@xxxxxxxxxxxxxxx
>
> -----Original Message-----
> From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
> Behalf
> Of DC Cox
> Sent: Tuesday, September 09, 2008 11:52 PM
> To: Tesla Coil Mailing List
> Subject: Re: [TCML] pole pig protection
>
> I disagree with some of this.
>
> If there were zero oscillations then we would not have to use Terry
filters
> even on NST circuits as the spark gap would be shorting out the
oscillations
> there also.  If you throw a scope on the circuit you can see some
> oscillations coming back from the tank circuit directly toward the xmfr
---
> yes, even thru the shorted spark gap (conduction resistance is
> constantly changing and not a fixed value so it is not always at zero
> value).
>
> and yes, one should employ a simple air core inductor and also a resistive
> filter (12 pcs 600 Ohms in parallel, 200 watts each) even on pole
> xmfrs.  again, if it is scope you can see the non-60 Hz oscillations.
>
> pole xmfrs are testing usually for a few single shots at 125 kV BIL (basic
> impulse level) using a Marx generator, however lightning oscillations tend
> to
> be short-lived and certainly not continuous as with a LC tank circuit.  I
> know many experimenters who do not use a simple filter but these may be
> the same guys who drive around without auto insurance!
>
> the filter is simple and will save your $300 pole xmfr, plus the $250 it
> cost to ship it if in another state.  The filter costs approx $30  ---
> $300/$30 risk/reward ratio.
>
> I have the filter circuit that I use if anyone wishes to contact me
off-list
> I will email it to them.
>
> better safe than sorry and out a few hundred bucks.
>
> Dr. Resonance
>
>
>
>
> On Tue, Sep 9, 2008 at 6:59 PM, miles waldron
> <mileswaldron@xxxxxxxxxxx>wrote:
>
> > These comments are in response to Gary Lau, who writes on this thread.
> >
> > Gary is a very nice person. The following opinions are mine, and for the
> > record I agree completely with everything Gary has said. Someone please
> let
> > me know if I have missed the point entirely, and if that is the case
then
> I
> > am sorry.
> >
> > Terry filter protection for a pole pig is pointless. A pole pig is
> designed
> > to be struck by lightning over and over again during its lifetime. Have
> you
> > ever been hit by lightning? As equipment goes, a pig is typically used
to
> > avoid transformer limitations. You can do just about whatever you want
> with
> > a pole pig, and it will still be there (not hindered in any way) after
the
> > explosion.
> >
> > Miles Waldron
> >
> >
> >
> >
> > -----Original Message-----
> > From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
> Behalf
> > Of Lau, Gary
> > Sent: Tuesday, September 09, 2008 6:25 PM
> > To: 'Tesla Coil Mailing List'
> > Subject: RE: [TCML] pole pig protection
> >
> >
> > Opinions vary on this topic.  Here is mine.
> >
> > First off, in contrast to NST's and MOT's, pole pigs are extremely
rugged
> > and well designed.  They are designed to be undamaged by lightening
> > strikes.
> > While it is not unheard-of, damage to pole pigs is a very rare thing.
> >
> > As to protecting the pig from RF - I assume that your TC circuit has the
> > main gap in parallel with the pig HV secondary.  Please consider that
the
> > ONLY time there is RF oscillating in the TC primary or secondary, the
> spark
> > gap is conducting.  If the spark gap is conducting, it is essentially
> > shorting out the pig's HV secondary, so there's not a lot of RF to
protect
> > against.
> >
> > Now, it's not entirely true that the gap is conducting continuously
during
> > each bang.  Each time the current through the gap passes through zero at
> > the
> > resonant frequency, the gap arc extinguishes briefly for a few(?)
> > nanoseconds.  During this brief time the primary rings with its
> > self-capacitance, in the tens of MHz region.  The initial magnitude of
> this
> > oscillation is Vbang, and since it is in series with the tank cap, also
> > initially at Vbang, the combination of the two in series (what the HV
> > winding sees) is 2Vbang.  This, IMHO, is the thing that needs to be
> > filtered, at least for NST's and less sturdy transformers.  And this is
> > easily and effectively attenuated by the R-C filter in the Terry filter.
> >
> > Simply adding a choke in series with the pig's HV terminal does not a
> > filter
> > make.  The pig's secondary may be viewed as a many thousand HENRY
> inductor.
> > Adding a few mH in series with it accomplishes nothing.  Low pass
filters
> > are constructed of typically two or more elements - L-C, R-C. L-C-R,
etc.
> > Just adding a choke inductor does not achieve any filtering.  You might
as
> > well string together cloves of garlic or St Christopher medallions.
> >
> > I apologize for being so wordy and blunt, but the use of chokes to
> > "protect"
> > transformers is a practice that was once done, just because it "seemed"
at
> > first glance like a good idea, and persists despite strong logic not to.
> >  It
> > has since been shown through rigorous circuit analysis, simulation, and
> > measurement, to be ineffective at best and counterproductive when done
> with
> > bypass caps.
> >
> > Regards, Gary Lau
> > MA, USA
> >
> > > -----Original Message-----
> > > From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
> > > Behalf Of Jim Calvin
> > > Sent: Tuesday, September 09, 2008 3:07 PM
> > > To: 'Tesla Coil Mailing List'
> > > Subject: [TCML] pole pig protection
> > >
> > >
> > > I have a 25KVA pole pig I am using for my large tesla coil (the coil
> > > only uses about 5-6KVA).  I am using a hand wound RF choke to protect
> > > the RF from the coil from getting back into the pig.  It is wound with
> > > 16 gauge wire onto a 8 inch sonatube.  I measured the inductance with
> > > RLC meter to be 10mH.  My pole pig has two HV leads with one grounded
> > > and the other connected through this choke to my coil.  I have not
> > > used the coil in awhile (been a busy summer).  I was going to start
> > > using it again but I wanted to make sure this is sufficient to protect
> > > my piggy!  Any advice would be helful.
> >
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