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RE: [TCML] Terry Filter Resistors



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

 
> > -----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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> > Tesla@xxxxxxxxxxxxxx
> > http://www.pupman.com/mailman/listinfo/tesla
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