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Re: [TCML] Chokes and Terry filters
Certainly no new science in using RF chokes to stretch waveforms across the
time dimension. It's been in ham radio books since the 1915 era.
A series inductance serves to limit the peak current in a waveform. As the
rapid risetime in current increases with a typical 250 kHZ coil, the current
traveling thru the inductor MUST produce a magnetic field which tends to
delay/resist the flow of peak current in a circuit. This stretches out the
pulse risetime and "chokes" off the rapid pulse by stretching it in time
This procedure is very common with Marx Impulse Lightning Generators in
which engineers want to slow down the rapid rise time of a capacitor
discharge. Any resistor or inductor this effect --- that's why companies
such as Resonance Research Corp and HiPotronics use resistors and inductors
in Marx generators -- they slow the rapid nanosecond risetime of energy
discharge caps down to a typical 1.5-50 waveform (1.5 uS with 50 uSec decay
time) and 200-2000 uS switching surge risetime.
In a Tesla coil circuit the rapid oscillations (typical 200-250 KHZ) must be
retarded BEFORE they can reach the xmfr windings. Terry filters work well,
however, for over 50 years I have always just used a nice air core choke to
produce the same effects. Now, I use both a Terry filter and a pair of air
core chokes to slow down rapid transient risetimes which can be destructive
if reflected back into the xmfr windings.
In over 50 years of which 40 of them I just used an air core choke, I've
never lost a NST. That speaks for itself.
I have the empirical data to support my claims but unfortunately, due to
business time constraints, just don't have time to type it all up.
On Mon, Jun 16, 2008 at 8:43 AM, Lau, Gary <Gary.Lau@xxxxxx> wrote:
> DC, as long as you suggest using chokes or inductors in protection
> networks, I'm going to have to refute that. This is IMO a Tesla coil myth
> that needs to be put to bed, so here goes again, with deliberate clarity and
> 1. Chokes or inductors cannot "clip" anything. Clipping suggests a
> non-linear circuit element, like a spark gap, MOV, or zener diode.
> Inductors are linear components. The MOV's and safety gaps in a Terry
> filter don't require any additional components to clip excessive or nasty
> voltages. You would be hard pressed to find an MOV application note
> suggesting the addition of chokes.
> 2. Adding an inductor in series with the resistors in a Terry filter turns
> the R-C filter into an R-C-L filter. This will only create a new L-C
> resonant circuit with the filter caps and expose the NST to yet another
> source of RF.
> 3. You describe the reason for using chokes in very vague terms, without
> ever actually defining the nature and source of the nasty thing that needs
> to be filtered out. Allow me. I believe there are two things that need to
> be addressed.
> 3a) Low frequency (60/120 Hz) high voltage excursions. These can be caused
> by many things: A static gap that is set too wide, a mis-wired (i.e. open)
> main gap, an RSG that is turning too slowly and allows mains resonant rise,
> a sync RSG who's phase is mis-adjusted and misses a firing, a sync RSG with
> a too-small cap value, etc. The properly set safety gap of a Terry filter
> is capable of shunting such excursions without the need for inductors. The
> MOV's do the same thing; they're just there as a tamper-proof Plan "B" in
> the event that the safety gap was not properly set.
> 3b) There is a seldom discussed source of VHF (in the tens of MHz) voltage
> spikes that occur every time the main gap stops conducting, that produces a
> transient of 2X the peak capacitor voltage. The full explanation is lengthy
> and is detailed on my web site at
> http://www.laushaus.com/tesla/protection.htm The good news is that these
> transients are low in energy content and are easily attenuated by the R-C
> filter in the Terry filter. Again, adding an inductor is not needed.
> If there is some other source of nasty things that need to be filtered out
> that I have overlooked, please bring it to my attention. If you use the
> word "kickback", please define exactly what that means - what is its source,
> what determines its frequency and peak voltage, etc. Sound engineering
> practices require sound engineering analyses to back them up. Otherwise I
> consider it to be only so much hand waving. Empirical data in the absence
> of rigorous circuit analysis can be valid, but there needs to be a
> controlled study of failures in IDENTICAL coils with and without chokes,
> before any conclusions can be reached.
> Regards, Gary Lau
> MA, USA
> > -----Original Message-----
> > From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
> > Behalf Of DC Cox
> > Sent: Sunday, June 15, 2008 4:52 PM
> > To: Tesla Coil Mailing List
> > Subject: Re: [TCML] Terry Filter Caps Suffering From Early Death
> > Yes, contact me off-list and I will email the data to you.
> > I use a pair of inductance chokes next to the output going over to the
> > gap. This tends to clip the most nasty of the spikes allowing the Terry
> > filter to despike the rest of them.
> > I also use one additional cap in each series leg of the Terry filter to
> > raise the overall potential value of the cap string.
> > Dr. Resonance
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