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Toroid Core and Ferrite Bead Winding
Ferrite beads are torid cores. This page contains some general information about winding torroidal cores.


With a torroidal core, whether it is used in an inductor or transformer, every time a wire passes through the hole in the core, it counts as one turn. If you want to make more than one turn, then you have to pass the wire through the core in the same direction each time.

Refer to the picture above. To make a 6 turn inductor, pass the wire through the hole 6 times. If you want to make a 6  turn inductor with a center tap, wind 3 turns, pull the wire away from the core a hold out a loop with one finger, and make turn number 4 by passing the wire through the hole. Twist the loop of wire you had held out with your finger, then make turns number 5 and 6 by passing the wire through the hole. Every time you pass the wire through the hole, it must be in the same direction.


The wire used for the inductor or transformer depends upon the application. Often enameled magnet wire is used. Regardless of the type of wire used, in the vast majority of cases it will be an insulated wire, and the insulation will have to be removed. Some enamel coatings such as Beldon Beldsol will melt away when heated with a sufficiently hot soldering iron, but one tradeoff is that these coatings, in my experience, tend of be softer and scratch more easily, which could affect reliability.

Other coatings such as Heavy Polythermaleze is very scratch resistant, but needs to be scraped away before soldering. I use an X-ACTO knife-like tool (A C.K.S. Cutter G-400) to scrape away the enamel, being careful not to nick the wire because stress builds at the nicks and the wire can easily break off at the nick. Then, using a fairly aggressive solder flux (with a PH of about 4) and some solder, I tin the leads with a hot soldering iron.

For some inductors and transformers #30 Kynar wire wrapping wire works well. This kind of wire is easy to handle and strip.

The picture above is two of the steps in winding a 10 turn center tapped inductor on a 3 millimeter ferrite bead. On the left, is the bead with 5 turns. On the right is the bead with the additional 5 turns, for a total of 10 turns. The wire is Belden 8070 #36 AWG Heavy Polythermaleze. This is the inductor for the LED penlight power supply.

An example of a tapped RF inductor. The wire here Beldon 8055 #30 AWG Single Beldsol. The wires were tinned with a hot soldering iron and no scraping was necessary. This core is a good candidate for Beldsol coated wires because its edges are smooth and it is coated to make it easy for the wire to slip around the edges as it is pulled through, thus less likely to scrape the insulation. For RF inductors, the more evenly the turns are spaced around the core, the better.

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Contents ©2005 Richard Cappels All Rights Reserved. http://www.projects.cappels.org/

First posted in October, 2005

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