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What the "L" ?

Have a few inductors, are really degaussing coils re-bent to make nice /planar solenoids / loop antennas / metal detector loops / etc. ((do not forget the "etc" as very important due to generic aspect))

Approx values.

Loop 1: dia=8.5 inches, L=0.11mH, R=1.0 ohm, SRF=940Kc, resonates w/0.1uF at 60Kc for Lcalc 98.5uH. How many turns does it have?

Loop 2,3,4,5: dia=36.5 inches, L=unmeasured, R2=18.38 ohm, R3=8.38 ohm, R4=12.51 ohm, R5=8.20 ohm, SRF=23.5Kc, resonates w/0.1uF at 8.3Kc for Lcalc 3.6mH. I assume they have the same number of turns due to same self-resonant frequency. How many turns would that be?

** Do not let them "DC"ieve you when activated from a battery... ..they can become attractive.
Reply to
Robert Baer
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14

34

About the same size wire in both. Given that 2-5 vary within the set by +/-

2 AWG's or so.

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
Website: http://seventransistorlabs.com
Reply to
Tim Williams

  • Maybe that is what the formulas say, but Loop 1 has exactly 15 turns; hand wound, counted 2-3 times,no mistake. Any idea as to why the discrepancy?
  • Grumble...my AADE is dead, so i drug out the "universal" tester (diodes, bipolars, FETs, capacitors, inductors). DVM Universal ohms ohms inductance Loop 2 18.38 21.9 13.2 mH loop 3 8.38 11.1 2.43mH loop 4 12.51 15.7 7.59mH loop 5 8.20 11.9 2.41mH So, the ASS-u-MEtion of "same number of turns due to same self-resonant frequency" is WAY off: loop 2 is in the wilderness.

Dang. that question of What the "L" really stands out.

Reply to
Robert Baer

I actually calculated 13.7 or something like that.

Real is 15? That's not a discrepancy, that's fantastic accuracy! You realize how shitty those formulas are, right? :-)

The SRF is suspiciously low for anything but a metric shitload (technical term!) of wire. You're also measuring quite close to the SRF, which puts pseudo-helical waveguide modes into play (the shape of the resonance isn't a double pole, but involves trig and hyperbolic functions, arising from standing waves and crosstalk between turns). And, the DCR is quite significant at those frequencies, so it may be hard to call it a resonator at all.

To put it another way: you'd like it to be a simple L + R, but the real equivalent circuit is more like a bunch of R + (L || R) + ..., with C strapped all over there too. So the L + R equivalent depends strongly on frequency.

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
Website: http://seventransistorlabs.com
Reply to
Tim Williams

Redone measurements:

DVM Universal ohms ohms inductance SRF* Rc* Loop 2 18.93 21.9 13.2 mH 17.6 4.0 loop 3 9.87 11.1 2.43mH 18.1 8.5 loop 4 12.78 15.7 7.59mH 17.8 5.1 loop 5 8.59 11.9 2.41mH 17.8 8.5 all series 49.0 54.8 83.0mH 15.4 1.72 *SRF=Self Resonant Frequency, KHz; Rc=resonance with 0.1uF in parallel

Reply to
Robert Baer

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