Cylindrical core coils: inductance calculator

[PeterMcMonty]

Hello,

since I need to wind some coils for a project, I made some calculations using a well known formula that I found on web (see the attached Inductors Formulae.doc), but suddendly I though that a spreadsheet is more useful than tampering with pocket calculator.

Then I did it (see Coil_calc.JPG).

The first sheet allows you to insert coil winding data (fields in green): no. of turns, coil support diameter, wire size, number of paralleled wires (close wound) and turns spacing. On yellow and orange fields you'll find computed results.

In the second sheet (see Res_by_turns.JPG) you'll find a table that, given the data entered in the first page and the number of turns from 0,25 to 51 in 0,25 turns step, returns the following parameters:
- coil inductance
- coil lenght
- total lenght of wire

The file is an excel spreadsheet that you'll find in the zip file Cylindrical coil calculator.zip attached here.
The sheet cells are protected (except those in green where you enter parameters) to avoid accidental modifications, but you can unprotect and modify the sheets as you like.

I'ts not a very big contribution, but I hope it is a tool that can save you some time in designing coils.

PeterMcMonty

[PeterMcMonty]

I made three cylindrical coils and used my spreadsheet for calculations. Here the results:

wire: 1.12mm encu
coil diameter: 20mm

L1 = 6.5 uH 21.75 turns from spreadsheet 26.25 turns actually (+20.69% turns)
L2 = 12uH  35.5 turns from spreadsheet 40.5 turns actually (+14.09% turns)
L3 = 16uH  45 turns from spreadsheet 54 turns actually (+20% turns)

coils measured with Marconi Universal bridge TF2700.
(view: http://www.radiomuseum.org/r/marconiins_lcr_bridge_tf2700tf_270.html)
(manual: http://bama.edebris.com/download/marconi/tf2700/TF%202700.pdf)

So, be careful: take results as a starting point, add some 25% more turns then measure and decrease turns until you get desired value!

[Parmin]

Induction value based on theoretical calculation are assuming for perfect circumstances eg. conductivity of copper, actual diameter of each loops, temperature and etc.
Naturally the value should be slightly different from the actual values.
The more turns you have the larger the difference for the fault indeed added on each turn.

[PeterMcMonty]

yes, you are quite right! This is true expeciallly with "empirical" formulae, as it is the case. Real world always differs from theoretical calculations.

Nevertheless, it's good to have a staring point from some formula!