RE: [TCML] Terry Filter

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

Oh don't get me wrong - the cap value in a Terry filter is not at all critical from a
frequency point of view.  One can double or half the cap value with (probably) no
impact to effectiveness.  There is no filter resonance involved here.  The R-C filter
is a low pass filter, so it attenuates stuff above a certain frequency.  For a filter
using 500pF & 1K parts, the -3dB knee frequency is about 300KHz.  We typically
choose the same cap & resistor values regardless of our coil's frequency.  But this
isn't the frequency that we're trying to stop.  It's not the coil's resonant frequency,
but something else much higher in the MHz range that we're trying to block with the
R-C filter.  See http://www.laushaus.com/tesla/protection.htm for the long story.

Your mica caps could be used if you series enough to get down to ~500pF, but
that's a waste of large and expensive mica's.  Unlike tank caps, filter caps can be
any type - doorknob, mylar, ceramic, disk, polypropylene, all are fine, so use whatever is cheap, looks nice & available.  Just be aware that using too high a cap value will waste power (see link above).

Regards, Gary Lau
MA, USA

> -----Original Message-----
> From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
> Behalf Of Dan Casaregola
> Sent: Friday, November 06, 2009 12:47 PM
> To: Tesla Coil Mailing List
> Subject: Re: [TCML] Terry Filter
> 
> NST is Gardner.  Ok on 8 mov's per side for 15kv/30ma.  Was not aware that
> cap size was critical (rc resonance) .  i haven't purchased the caps yet,
> but I do have .01 @ 2500 wv micas, and was hoping to use them. I guess I'll
> have to calculate the original resonance freq, then see how many .01 I would
> need and hope the total voltage will be adequate.  Thanks for help.
> ----- Original Message -----
> From: "Lau, Gary" <Gary.Lau@xxxxxx>
> To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx>
> Sent: Friday, November 06, 2009 4:37 AM
> Subject: RE: [TCML] Terry Filter
> 
> 
> What kind of NST are you using?  For a 15kV NST, only -8- MOV's should be
> used per side.  See http://www.classictesla.com/download/NST_Filter.jpg
> 
> The number of MOV's used must change as the NST voltage changes, since the
> number determines the clamp voltage.
> 
> While the number of caps could be varied to maybe save a few pennies when
> using a lower voltage NST, the cap uF rating would need to be increased, as
> you always want the resulting string to have the same capacitance, so that
> the R-C filter will have the same high frequency cutoff characteristics.  So
> don't change the caps.
> 
> Regards, Gary Lau
> MA, USA
> 
> 
> 
> 
> > -----Original Message-----
> > From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
> > Behalf Of Paul Brodie
> > Sent: Friday, November 06, 2009 1:02 AM
> > To: Tesla Coil Mailing List
> > Subject: Re: [TCML] Terry Filter
> >
> > My drawing shows 9 MOV's on each side. Why the discrepancy?
> > Paul
> > Think Positive
> >
> > ----- Original Message -----
> > From: "Dan Casaregola" <ag2z@xxxxxxxxxxx>
> > To: <tesla@xxxxxxxxxx>
> > Sent: Thursday, November 05, 2009 6:38 PM
> > Subject: [TCML] Terry Filter
> >
> >
> > I'm in the process of building a filter for my 15k/30ma NST, and the
> > drawing
> > I have shows the number of MOVs required for the stated voltage, but it
> > only
> > shows 6 MOV. either side of center tap ground.  I remember reading
> > somewhere
> > that the number of caps should be the same as the number ov MOVs.  Is this
> > correct?  Thank you  Dan

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