Re: [TCML] Terry Filter Resistors

[bartb <bartb@xxxxxxxxxxxxxxxx>]

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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