Paolo:
I wound 10 turns #26 magnet wire on an FT37-43 core this afternoon and
measured it with four different instruments. Here are the results:
1. HP4620 RLC Bridge @ 1 KHz:
a. AUTO mode: Ls=68 uH Q= unknown
b. Ls manual mode: Ls = 16.5uH, Q=2.55
c. Lp manual mode: no null possible, as Q is below minimum of instrument
2. General Radio 1650A RLC Bridge @ 1KHz
a. Ls = 54uH, Q=5.3
b. Lp = 58uH, Q=4.95
3. Leader LC740 RLC Bridge @ 1 KHz
a. 77.1uH Q=2.90
4. HP4342A Q-meter
a. @ 2.5 MHz, Lp=26.8uH, Q < 5 (minimum calibration scale on meter is Q=5)
b. @606 KHz, Lp=27.9uH, Q = 5.2
Remember the conversion between series and parallel inductance if you
wish to compare the Ls and Lp measurements, as it is not safe to say
Lp=Ls for low Q inductors.
As a reference point, this core is FairRite's PN 5943000201, and has a
catalog Al factor of 350nH (+/- 20%).
Hence, 10 turns yields a theoretical inductance of 10^2 X 350nH/turn^2 =
35uH.
These measurements show the difficulty in measuring a material with an
inductance that changes with frequency and also with drive level (to
some extent). Also, the way AL is measured in the industry is with a
coaxial-type test fixture (as I understand it) that forces 100% of the
flux into the core, i.e., there is no leakage inductance. Winding copper
wire onto a core will not necessarily duplicate the manufacturer's data.
I ran a calibration check on my HP4342A a few days ago with a couple of
103-series Boonton standard inductors and found it to be within a couple
percent of matching the standard inductors. On normal type inductors
used at audio range, all three of my RLC bridges usually agree within a
couple percent as well.
So, what we are seeing, I believe, represents the perversity of
ferrites; wonderful materials but sometimes with baffling behavior upon
casual observation.
Jack
paolo gramigna wrote:
Hi all,
I'm arrived at page 53 of the manual, and I'm making my first toroid. I
followed the instructions, and then out of curiosity I tried to measure the
inductance, using my old HP 4260A. I know that I'm not supposed to match the
indicated inductance, and that I should stick to the published turns count;
in fact I was just willing to give a try to the HP 4260A, the least used
instrument in my lab.
Well, it ended up indicating a value of 160 uH, not 18 uH as supposed. So I
tried to measure the value on another bridge I had there by chance, and
ended up with a very similar value.
So I tried the HP 4260A on the solenoidal RFC (the green bodied) and I found
values very similar to what I expected.
I know that i'm doing something wrong, but I cannot understand what's my
mistake....
Cheers,
IK4YNG Paolo
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