Did some measurements on some parts express inductors I had. Thought the information might be of some interest to others. I used a .3mH #20awg coil, a .25 mH #18awg, and a .2 mH #18awg.

I use the Ls/Rs model for an inductor. I ran the 3 pieces I had from 20 hz out to 100 Khz.

I also compared the inductors free air and lying on a 2 sided clad pc board.

First, the inductance. Note that all are flat to 500 hz. The units lying on the copper started reducing inductance noticeably at 1Khz, this is a consequence of the copper conductivity trying to prevent the field from penetrating the pc board. The stopping of the penetration by a conductive surface of permeability 1 will always lower the inductance. Steel of course, would increase the inductance.

Note that even the air coils exhibit inductance drop. That is a consequence of the proximity effect on the current path within the wires. The proximity effect is changing the apparent diameter of the coils. If you find an online coil inductance calculator, try changing the diameter in small increments. Smaller diameters produce lower inductances. This indicates that the current is moving to the inside edge of the copper wires.

Next, resistance (I used a log vertical scale for ease of seeing). All three coils in air extend out to 2khz before the wire resistance starts to rise. Both 18 guage coils have doubled their series resistance by 5 Khz, the 20 guage doubles by 9 Khz.

Notice what happens as a result of the copper clad board proximity. The series resistance increases an order of magnitude by 6 Khz for the two 18 guage coils (about 2 ohms), and the 20 guage does it above 10Khz but below 11.

This is a light copper clad, btw, not very thick.

I expect this to be much worse for a #10 or #12 solid, I'm winding a 10 guage coil with instrumentation wires to detail exactly what the proximity effect will do to the resistance.

jn

I use the Ls/Rs model for an inductor. I ran the 3 pieces I had from 20 hz out to 100 Khz.

I also compared the inductors free air and lying on a 2 sided clad pc board.

First, the inductance. Note that all are flat to 500 hz. The units lying on the copper started reducing inductance noticeably at 1Khz, this is a consequence of the copper conductivity trying to prevent the field from penetrating the pc board. The stopping of the penetration by a conductive surface of permeability 1 will always lower the inductance. Steel of course, would increase the inductance.

Note that even the air coils exhibit inductance drop. That is a consequence of the proximity effect on the current path within the wires. The proximity effect is changing the apparent diameter of the coils. If you find an online coil inductance calculator, try changing the diameter in small increments. Smaller diameters produce lower inductances. This indicates that the current is moving to the inside edge of the copper wires.

Next, resistance (I used a log vertical scale for ease of seeing). All three coils in air extend out to 2khz before the wire resistance starts to rise. Both 18 guage coils have doubled their series resistance by 5 Khz, the 20 guage doubles by 9 Khz.

Notice what happens as a result of the copper clad board proximity. The series resistance increases an order of magnitude by 6 Khz for the two 18 guage coils (about 2 ohms), and the 20 guage does it above 10Khz but below 11.

This is a light copper clad, btw, not very thick.

I expect this to be much worse for a #10 or #12 solid, I'm winding a 10 guage coil with instrumentation wires to detail exactly what the proximity effect will do to the resistance.

jn

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