If you build or repair radio equipment, you may come across an inductor value not commonly stocked by electronics suppliers. Unlike other components which need industrial processes to produce, you can make decent inductors in your home workshop. An inductor is simply a coil of magnet wire wound around a cylindrical plastic form. Magnet wire has a thin coating of electrically insulating varnish. Since the varnish is thin, you can wind very tight coils with magnet wire. Some simple calculations will tell you how many turns of wire the inductor needs for a given inductance value.
Calculate the number of turns of wire the inductor needs from the formula: L equals (d² multiplied by n²) divided by (18 multiplied by d added to 40 multiplied by l).
Where L equals inductance in microhenries; d equals the diameter of the coil form in inches; n equals the number of turns; and l equals the length of the coil in inches.
For example, if you need to wind a 50 microhenry coil, 1/4-inch long on a 1/8-inch form, the equation becomes: 50 equals (0.25² multiplied by n²) divided by (18 multiplied by 0.125 added to 40 multiplied by 0.25) or; 50 equals (0.0625 multiplied by n²) divided by (12.25) or; 9,800 equals n² which gives n equals 99 turns.
Insert an inch of wire down into one of the form’s holes. Wind the coil tightly around the form for the number of turns you’ve calculated.
Cut the wire at the spool, leaving an inch of slack. Feed this end of the wire down into the other hole in the form. The form should now have the coil and two wire “legs” protruding down about an inch.
Touch the hot soldering iron tip to the ends of the wire, burning off a half-inch of varnish. Wipe the wire ends with a paper towel to remove the varnish. The ends of the wire should be clean, shiny and ready to solder.
Prepare a dab of epoxy and spread a thin coating on the surface of the coil. This prevents the coil from working loose and unwinding.
For inductors with values much greater than 1 millihenry, use 30-gauge or finer wire.