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RE: [TCML] Terry Filter Resistors
For those coming into this thread late, it began with someone asking if a TO-247 cased non-inductive resistor was suitable for use in a Terry filter. Then someone suggested that Terry had specifically chosen the wire-wound units for the inductance that they exhibit. I said that the inductance was not a factor in choosing the wire-wound units; only their voltage stand-off and power rating (and rough value) was important.
I had also done a good deal of MicroSim circuit simulation to compare R-C, L-C, and R-L-C low pass filter behavior when shock-excited with a spark gap. I remember finding that the R-L-C circuits would still exhibit a significant, damped resonance at their natural frequency, unless the R value was considerably higher than 1K, so much that it would be excessively lossy. So the waveform seen at the NST side was ultimately cleaner if a simple R-C circuit was used, with no inductance. At the time I assumed that the inductance of wire-wound resistors truly was negligible, so I ignored it. But if it really is in the multi-mH region, perhaps a second look is warranted.
There was a different thread several months ago where the practice of using just a choke to each transformer HV bushing was defended as a means of keeping HV transients from damaging the transformer. But I think when bypass caps are added to create a 2-pole low-pass circuit, it changes everything, and the addition of inductance is not a desirable thing. I maintain that a simple 1-pole R-C circuit is very capable of performing the required filtering.
Regards, Gary Lau
MA, USA
> -----Original Message-----
> From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
> Behalf Of piranha
> Sent: Thursday, January 29, 2009 11:06 PM
> To: Tesla Coil Mailing List
> Subject: Re: [TCML] Terry Filter Resistors
>
> Hi,
>
> It's a 1000 ohm resistor in series with an inductor. Feed it a sine
> wave at various frequencies and measure the current and maybe even the
> phase. With two points you should know the inductance. They don't make
> multilayer wire wound resistors ;) One would have to drop on on the
> concrete to see about how it is wound but probably about 50mH.
>
> To really see the inductive part you would have to run the frequency
> high enough to stand out against the 1000 Ohms.
>
> Xl = 2 x pi x F x L
>
> 2000 = 2 x pi x F x 50mH so a frequency of 6 MHz should do. But that
> is high enough that the Medhurst capacitance could mess things up...
> That is probably what all this is about :D
>
> I hope you get it figured out :D
>
> But remember, those caps too that swamp out the stray capacitance. At
> say 300kHz, the odd effects would be nil. It's a second order low-pass
> filter. Not much should get through it all unless I messed it up =:o
>
> Terry
>
>
> bartb wrote:
> > Hi Ed, Gary,
> >
> > WOW! I think your right. That sure puts a damper on my multi-layer
> > technique! Would work in multilayer mode, but if a 1K 1/2W carbon is
> > measuring 184mH, then yes, all bets are off and a good lesson learned
> > with the 27XT (and maybe any LCR).
> >
> > Excellent test Gary!
> > Take care,
> > Bart
> >
> > Ed Phillips wrote:
> >> I also have a 27XT multimeter. If I take a 1.0K Ohm, 1/2W carbon
> >> comp resistor (i.e. NO significant inductance) and try to measure its
> >> inductance with this meter, it reads 184mH. I don't think so!
> >>
> >> Bottom line - don't try to measure inductance of resistors with this
> >> meter!
> >>
> >> Regards, Gary Lau
> >> MA, USA"
> >>
> >> Measuring any kind of coil with this type of meter is sort of a
> >> gamble unless you know what you're doing - the measurement technique
> >> is quite an indirect one. Works OK with reasonable coil Q but not at
> >> all suitable for measuring the inductance of a WW resistor. To do
> >> that I'd break one and try to figure out the number of turns and then
> >> calculate it. I don't think even a good impedance bridge would give
> >> useful results.
> >>
> >> Ed
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> >
> >
>
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