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Published: 23 April 2015

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Agreement on the dimensions and notations
Different programs (or different versions of this software) are using different methods of measurement and notation of coil sizes. In order to avoid confusion and mistakes on this page details the nuances of this issue.
First of all it concerns a singlelayer coil. Numerical methods of the inductance calculation are based on on the assumption that the coil is wound by infinitely thin wire. The real thickness of the wire is taken into account in the future through adding corrections, however, the initial assumption leads to the fact that all dimensions in the original calculation formulas are measured from center to center of wire:
 D  diameter of the winding
 l  length of winding
 p  the pitch of the winding
 d  wire diameter
 d_{i}  the diameter of the wire in the insulation
For all empirical formulas this rule is retained. Often in articles on this site it is possible to meet the concept of the formfactor of the coil. This is the ratio of the length to the diameter of the winding is l/D. Coil with a large l/D ratio are called the solenoid.
To simplify measurements for desiners, in the program Coil32 the coilformer dimensions and nominal diameter of wire (no insulation!) are measured and then on the basis of these measurements, the program calculates the required dimensions "from center to center", which are used in numerical algorithms and in empirical formulas. This applies to both singlelayer and multilayer coils, as well as other types of inductors. All dimensions and notation is clear from the images in the program. As an example, for singlelayer coils see figure below:
 D_{c}  the diameter of the coil
 l_{c}  the length of the coil
 p  the pitch of the winding
 d  nominal diameter of the wire
 d_{i}  the diameter of the wire in the insulation
Thus, if we are talking about the size of a coilformer upon which the coil is wound, we will use the notion of the coil diameter and the coil length, if we are talking about the coil, we will use the concepts of the winding diameter and winding length, measured from center to center of wire.
It should be noted that because the insulation thickness is the parameter is not zero, even when winding a coil to a coil, the winding pitch p is always greater than the nominal wire diameter d. This is taken into account in the program.