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There seems to be no such thing anymore as a simple formula for the physical implementation of an electronic component. Back in the early days of electronics discovery, frequencies were low and the precision needed for predicting the performance of circuits was equally low. As techniques for component construction and circuit analysis improved, more complete formulas were required to have theory match reality. Simple equations like Wheeler's Inductance of a single-layer air-core - RF Cafe for a single layer coil was good enough. Now, the more complete Wheeler model of Single Layer Coil Inductance Wheeler's Formula - RF Cafe is often more useful (it is the one that produces the result shown in the screen shot above). Be sure to conform to proper units.

Fortunately, some kind souls have seen to it that software is available free of charge that removes the laborious task of typing long equations into a spreadsheet or into a hand-held calculator. In this case, Coil64 (or the Coil32, 32-bit version) is available from Valery Kustarev. He provides calculations for single layer coils with round or rectangular wire, multi-layer coils, multi-layer coils with insulative material between the layers (like as found in transformers) - all on round forms. It also covers rectangular forms. Coils on toroids, flat spirals, and other types are included as well. Pretty nice for a free program! I have not attempted to verify the accuracy of all the equations, so caveat emptor.

The Coil32.net website also provides some theoretical background on coils such as self-resonance, a handbook of common formulas, helical resonators, and inductor color codes. A wire gauge table of dimensions, resistances, diameters, etc., is handy when inputting parameters into the calculator.



Posted January 12, 2023

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