RE: [TCML] pole pig protection

["Lau, Gary" <Gary.Lau@xxxxxx>]

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.
> >
> > _______________________________________________
> > Tesla mailing list
> > Tesla@xxxxxxxxxxxxxx http://www.pupman.com/mailman/listinfo/tesla
> >
> > _______________________________________________
> > Tesla mailing list
> > Tesla@xxxxxxxxxxxxxx
> > http://www.pupman.com/mailman/listinfo/tesla
> >
> _______________________________________________
> Tesla mailing list
> Tesla@xxxxxxxxxxxxxx
> http://www.pupman.com/mailman/listinfo/tesla
>
>
> _______________________________________________
> Tesla mailing list
> Tesla@xxxxxxxxxxxxxx
> http://www.pupman.com/mailman/listinfo/tesla
_______________________________________________
Tesla mailing list
Tesla@xxxxxxxxxxxxxx
http://www.pupman.com/mailman/listinfo/tesla