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Re: [TCML] Terry Filter Resistors



Ok,

I missed most of this... But the resistors were always known to have a little inductance. Tricky to measure since they are resistors. But the NiChrome wire is wound on the ceramic core making a series inductor too. That was supposed to help the filter work actually ;) I never calculated the values or cared much. I hope I did not miss anything. In all cases the "Q" should have been critically damped.

Terry


bartb wrote:
Hi Gary,

The resistor I'm measuring is the resistor that Terry Fritz called out on the Terry filter (L100J1K0). It's 0.75"D x 6"L, 1000 ohms, 100W, 1000V.

Your right, it is a good sanity check, but remember your coil is a single layer air core type. The answer is layers. For proof, I ran multi-layer air core coil equations to find what wire size I would need, turns, layers, etc. at the .75" x 6" dimensions in order to achieve 1000 ohms with 62.7mH.

If I use 39 AWG, 1460 turns with 4.23 close-wound layers at the dimensions above, then I get 1000 ohms at 69mH. I used simple magnet wire for these calculations.

The larger 145mH direct measurement value is possible with more layers, but it requires a slightly larger wire size near 37 AWG. If I use 37 AWG, 950 turns, 10.33 layers, then 1000 ohms at 132mH.

Anyway, you get the point. I basically backward engineered the resistor to see how easy it would be to achieve both the measured and the calculated values. I would need to run a resonance test to be sure which is right. I expect the lower 62.7mH value is close.

Layers make all this possible and is the "only" way it could be done. If it was a single layer, then "extremely" low inductances.

Take care,
Bart

Lau, Gary wrote:
What are the dimensions of these resistors? My 6"x28" secondary coil's inductance is only 89mH, so 145mH seems huge. Granted, the resistor is wound with much finer wire with more turns, but it'll make an interesting sanity check to see how many turns a secondary wound to the same dimensions as your resistor would require to achieve 145 mH.

Regards, Gary Lau
MA, USA

-----Original Message-----
From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
Behalf Of bartb
Sent: Wednesday, January 28, 2009 9:17 PM
To: Tesla Coil Mailing List
Subject: Re: [TCML] Terry Filter Resistors

Hey Gary, Matt,

Confirmed today at 145mH on todays reading (leads). Also measured 992
ohms. Measurements with 27XT.

I think Matt is correct and it's logical. If I do the math (noting that
the 27XT frequency is 1kHz):

Z = 145mH x 6.28 x 1kHz = 910.6 ohms.

XL = sqrt(992^2 - 910.6^2) = 394 ohms (note R-Z due to -sqrt).

L = 396 / (6.28 x 1kHz) = 62.7mH

Still significant, but then these are rather large wire wound resistors,
so not overly surprising.

Regards,
Bart


Gary wrote:
Please confirm "I just checked the resistor in question and it measured 146mH at
1kHz." 146 mH sounds about a million times too high for the inductance of a
wirewound resistor.

Matt wrote:
I think part of the problem is that most LC meters don't measure Inductance
directly. They actually measure the Z of the device and assume Z=XL, and therefore L=Z/2pif, which for most inductors is close enough. For a wire-wound resistor though, this is not a good approximation, since actually Z=sqrt(R2+XL2), and R is no longer negligible. You could use the L?shown by the meter to calculate what Z it actually read by Z=2pifL, then using the known resistance, calculate a corrected XL? from XL=sqrt(Z2-R2)and from that, determine the corrected L by
L=XL/2pif.
Matt D.
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