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Accurately Calculating Coil Inductance
1843, Geo and all
Coil inductance is one of the most difficult calculations to accurately make for the following reasons.
1. The insulation thickness changes the spacing of the wire. Wire that is closer together (thinner insulation) will have higher inductance.
2. The dielectric constant of the insulation will have a slight effect on the inductance.
3. The frequency of the LC meter at which the inductance measurements are made can affect the accuracy of the results.
Go to the following web site and do the following.
http://my.athenet.net/~multiplx/cgi-bin/airind.main.cgi
I simply enter the Inside Diameter (ID) of my coil as the coil diameter.
I enter the number of coil turns.
I enter the wire bundle diameter as the coil length.
Here is a quick way to measure the wire bundle diameter.
You don't even need to cut your wire to do this.
Let's assume that you want to make 30 turn coil. Wind 15 turns of the same wire around your hand. You will have a small coil of wire. Squeeze it together in the center and place two wraps of electrcal tape around it pulling the wire bundle tight. Measure the diameter of the 15 turn coil which will have 30 strands in the center under the electrical tape. Measure the diameter of the wire bundle and subtract 4 thicknesses of tape from your answer. Enter this number into the coil calculator as the coil length.
Use the 1% answer.
Your coil will be very close to this calculated value.
I use the LC Meter IIB from Almost All Digital Electronics. See my review of this meter kit below with web link to the web site where details of this meter and the assembly instructions are available.
http://thunting.com/geotech/forums/s...ad.php?t=11102
I have purchased commercial coils from 100 uH to 300 uH and even have a 500 uH standard inductor. This meter is right on the values marked on these components within less than 5%. This meter tracks very closly with my commercial Tektronix model 130 LC meter.
After proving that the LC IIB works very well, I removed the Tektronix model 130 LC meter from my bench and no longer use it. The model 130 only goes up to 300 uH and requires putting a known value inductor in parallel with an inductor larger than 300 uH and go to an "S" shaped graph chart to interpet the larger inductor value. This is not very convenient for making coils.
After winding nearly 100 coils, I see no reason to do anything other than enter the numbers the way I described above and make my coil to the desired value.
When you add a coil shield, the coil inductance will go up about 5%.
When I take a round 10.5 inch ID coil that is 355 uH and squeeze it into a long, narrow oval coil that is 3 inches wide, the inductance drops to 310 uH.
So, changing the coil shape does change the inductance, within some limits, about 10% lower as noted from the above numbers. The narrower the coil is squeezed, the lower the inductance will go but not any lower than about 305 uH with the coil squeezed down to 1" wide.
I hope this helps.
bbsailor
1843, Geo and all
Coil inductance is one of the most difficult calculations to accurately make for the following reasons.
1. The insulation thickness changes the spacing of the wire. Wire that is closer together (thinner insulation) will have higher inductance.
2. The dielectric constant of the insulation will have a slight effect on the inductance.
3. The frequency of the LC meter at which the inductance measurements are made can affect the accuracy of the results.
Go to the following web site and do the following.
http://my.athenet.net/~multiplx/cgi-bin/airind.main.cgi
I simply enter the Inside Diameter (ID) of my coil as the coil diameter.
I enter the number of coil turns.
I enter the wire bundle diameter as the coil length.
Here is a quick way to measure the wire bundle diameter.
You don't even need to cut your wire to do this.
Let's assume that you want to make 30 turn coil. Wind 15 turns of the same wire around your hand. You will have a small coil of wire. Squeeze it together in the center and place two wraps of electrcal tape around it pulling the wire bundle tight. Measure the diameter of the 15 turn coil which will have 30 strands in the center under the electrical tape. Measure the diameter of the wire bundle and subtract 4 thicknesses of tape from your answer. Enter this number into the coil calculator as the coil length.
Use the 1% answer.
Your coil will be very close to this calculated value.
I use the LC Meter IIB from Almost All Digital Electronics. See my review of this meter kit below with web link to the web site where details of this meter and the assembly instructions are available.
http://thunting.com/geotech/forums/s...ad.php?t=11102
I have purchased commercial coils from 100 uH to 300 uH and even have a 500 uH standard inductor. This meter is right on the values marked on these components within less than 5%. This meter tracks very closly with my commercial Tektronix model 130 LC meter.
After proving that the LC IIB works very well, I removed the Tektronix model 130 LC meter from my bench and no longer use it. The model 130 only goes up to 300 uH and requires putting a known value inductor in parallel with an inductor larger than 300 uH and go to an "S" shaped graph chart to interpet the larger inductor value. This is not very convenient for making coils.
After winding nearly 100 coils, I see no reason to do anything other than enter the numbers the way I described above and make my coil to the desired value.
When you add a coil shield, the coil inductance will go up about 5%.
When I take a round 10.5 inch ID coil that is 355 uH and squeeze it into a long, narrow oval coil that is 3 inches wide, the inductance drops to 310 uH.
So, changing the coil shape does change the inductance, within some limits, about 10% lower as noted from the above numbers. The narrower the coil is squeezed, the lower the inductance will go but not any lower than about 305 uH with the coil squeezed down to 1" wide.
I hope this helps.
bbsailor
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