Going to make a coil, some help needed

[Paul Harris]

I'm after making a 436uH circular coil but what I need is a good formula to calc the inductance. Tried a couple of websites but its obvious they're using different formula between them, since the same input yields quite different results.

I have a formula used for RF circuit inductors and it states it has a one percent accuracy with very high Q, however, they also state that the coil length must be greater than 0.67r, not sure if that's achievable with a metal detector coil.

the formula I have is

L = 0.394r^2*N^2/ 9r + 10l

Any input on this would be appreciated. Thanks

[Qiaozhi]

I'm after making a 436uH circular coil but what I need is a good formula to calc the inductance. Tried a couple of websites but its obvious they're using different formula between them, since the same input yields quite different results.

I have a formula used for RF circuit inductors and it states it has a one percent accuracy with very high Q, however, they also state that the coil length must be greater than 0.67r, not sure if that's achievable with a metal detector coil.

the formula I have is

L = 0.394r^2*N^2/ 9r + 10l

Any input on this would be appreciated. Thanks

At the top of the Coils Forum there is a Coil Calculator
See Appendix A in ITMD.

[Matt_Rowe]

Just made my first coil, using the calculator in coils forum, 9 inch, 22 turns of .22 awg insulated wire, copper 300 volt, pvc coated.came out at 1.1 ohms, 280 MH,going on a surf pro. Have to tighten up and wrap

[6666]

Just made my first coil, using the calculator in coils forum, 9 inch, 22 turns of .22 awg insulated wire, copper 300 volt, pvc coated.came out at 1.1 ohms, 280 MH,going on a surf pro. Have to tighten up and wrap

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Just for the future what you have written is milli henry with the M, the unit is u for micro henry, so it would be 280uH

Also if you want a bit more accuracy with the Qiaozhi calculator, when using thick insulated pvc wire like that use the outside diameter of the wire for the wire size .

[Matt_Rowe]

thanks, just tried my new LCR meter also, for the first time, ill get it all straight one of these days.lol

[6666]

What sort of meter do you have ?

[Matt_Rowe]

on the right lower corner it has an L, then the selections are capital H, or mH

LCR meter

[Elliot]

Just made my first coil, using the calculator in coils forum, 9 inch, 22 turns of .22 awg insulated wire, copper 300 volt, pvc coated.came out at 1.1 ohms, 280 MH,going on a surf pro. Have to tighten up and wrap

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Just be mindful that the inductance will increase 10-20uH when you tighten your wire bundle.

[SVEN1]

Just be mindful that the inductance will increase 10-20uH when you tighten your wire bundle.

Exactly. So I wind my coils tight around the form and then cable ties to tighten the bundle. Then check inductance, you may then have to add or subtract a wind to get a base point. Then after adding spiral wrap, the inductance will go up. Then a bit more when wrapping copper tape shielding around the bundle. The tighter the bundle the closer the inductance will be from start to finish. After you build a number of coils you'll be more consistent in your coil building.

[Matt_Rowe]

so far it hasnt moved much when i wrapped with electrical tape, but i am trying to learn the tester and results.
when the dial is put at these on dial, here are the results, i asked the person whom i bought this kit from and this is what they said,
(230uh 1.37 ohms,(pi pro) doesn't have to be spot on. just somewhere within this range plus or minus)
but ive read on forum should be around 300uh.
dial set at 2mH====.316
dial set at 20mH==0.28
dial set at 200mH====00.3
so which one is correct. the number 3 at least is consistent. and 0.28 i assume would be .280, which is close.
I was shooting at 280 because of what was stated in forum that after wrapping, and shield, ect it would go up.

[Qiaozhi]

so far it hasnt moved much when i wrapped with electrical tape, but i am trying to learn the tester and results.
when the dial is put at these on dial, here are the results, i asked the person whom i bought this kit from and this is what they said,
(230uh 1.37 ohms,(pi pro) doesn't have to be spot on. just somewhere within this range plus or minus)
but ive read on forum should be around 300uh.
dial set at 2mH====.316
dial set at 20mH==0.28
dial set at 200mH====00.3
so which one is correct. the number 3 at least is consistent. and 0.28 i assume would be .280, which is close.
I was shooting at 280 because of what was stated in forum that after wrapping, and shield, ect it would go up.

Personally I would use the inductance range that is closest to the value you are trying to measure. In this case that would be the 2mH range.
I also have the same meter, and a Peak atlas LCR40. These two meters are in fairly close agreement. Obviously there is some tolerance to be expected in the measurement. The Atlas claims +/-1.5%, and the Proctor claims +/-2%.

[nickel_n]

Personally I would use the inductance range that is closest to the value you are trying to measure. In this case that would be the 2mH range.

for the record
I did my schooling last century.
No digital meters. Only analogue.
other than the closest range
I remember well the tech prof saying
you take the correct reading when the middle is at 2/3 of the scale