coil inductance measurement

[green]

I'v been comparing the calculated vs measured inductance for some spiderweb coils I've been making. A 4 inch coil with 30 turns is off by 15 percent. Another wound on the same form with 36 turns less than 1 percent. Calculating inductance using resonant frequency with a .01u capacitor in parallel with coil. Resonance around 100khz. If I wind a coil three times with the same piece of wire how close would the inductance be? I could be counting turns wrong or is it something in my measurement. Plan on trying winding a coil 3 times with the same piece of wire, but if there is some thing I should do with measurement I would like to do it first. Been measuring SRF with the same test without the .01 capacitor.

[Chet]

Hi Green


I have had similar experiences. The tightness of the winding has some effect on the final inductance. On Spider wound coils if you pull and push the wire down tightly as it is wound it comes out differently than if just wound quickly and loosely. I always start with more turns than calculated and remove one at a time to get close to 300uh.


Using the same fixed capacitor of known value is the best method. Without a fixed capacitor the frequency change/percentage can vary considerably by random effects such as stray capacitance.


Have a good day,
Chet

[green]

Another experiment, 8 inch ID. Coil tester: (.01uf cap) 91240 hz, 304uh (.1uf cap) 28740 hz, 307uh. PI coil driver: (.1uf cap) 28090hz, 321uh. Professor Coyle calculator: 8 inch diameter, #28 magnet wire, 23 turns, 249uh. The coil driver had about 1 foot of Teflon coated twisted pair added to the coil. Coyle calculated 26 turns for 300uh. Started with 28 turns. Had to remove 5 turns. The coil tester and Coyle both had the same value for the 4 inch 36 turn coil.

[Chet]

Hi Green

Some of the Crystal Radio coil calculators are designed and tested on small air core or slotted plastic forms with 7, 9 or 11 slots. They may lose accuracy in calculating larger diameter coils. Each turn on small coils contributes less inductance than each turn of a larger coil. Each turn on a 4” coil is approximately 8uH. Each turn on a 8” coil is approximately 13uH. The larger coil is influenced more by the dielectric materials near each winding and the spacing of each winding. The number of crossovers in the weave and the width of the plastic or air former will have more influence on the inductance and capacitance in the larger coil.

Your coil probably has closer magnetic coupling than the plastic former the Professor Coyle calculator is designed for thus providing a higher inductance with fewer turns.


I verified your frequency and inductance values with a Inductor Capacitance and Inductance Estimator at this address; http://www.qsl.net/in3otd/inductors.html then reverse computing the frequencies from the LC combinations.
The tool estimated the coil self-capacitance to be 86pF. This number may need adjusting depending on how your twisted pair and scope probe are / (or not) connected.


I think the accuracy of the tool might improve with smaller capacitors for example 500pF and 1000pF.


Have a great day,
Chet

[Hayden]

You see, this is why God had Sencore make the "Z" meter Just hook it up and mash the button All I try to know about math is that ought from ought is ought and tails from tadpoles is frogs!