[TCML] Chokes and Terry filters

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

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 detail.

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 spark
> 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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