Daily Archives: March 7, 2023

Using Ferrites in Wire Assemblies

The phenomenon of magnetism is prevalent all over the world, along with related concepts like the magnetic field, electromagnetism, and electromotive force. Although these are complex subjects at a higher level, they are easy to understand. However, these are principles on which electric motors operate, the earth’s magnetosphere shields life, and refrigerator doors remain closed.

The wonderful properties of magnetism also help products and applications like cable assemblies. There are well-known magnets like those made of neodymium, and these are permanent magnets with inherent magnetic properties. They comprise elements of Neodymium, Boron, and Iron. Neodymium magnets are among the most powerful permanent magnet types available. In comparison, there are non-permanent magnets also. Typically known as electromagnets, they derive their properties from the passage of an electrical current.

Other types of permanent magnets are also available. The most popular of these is the ferrite magnets, and industries use them for a lesser-known reason. Used in various forms like chokes, cores, and beads, these inexpensive devices greatly help filter electrical noise and get products to comply with EMI/EMC regulations. Countless design applications use them in different form factors and are available from numerous manufacturers. Ferrite magnets comprise a mixture of iron oxide and ceramic magnets. In doughnut-like shapes, they keep control over signal integrity within bundles of wire. For instance, a data cable carrying high-frequency data transmission,  when routed through the magnetic field of a ferrite, can eliminate unwanted electrical noise, as the ferrite acts as a passive EMI filter.

For a ferrite to be effective, the cable must pass through the center of the ferrite and its magnetic field. Looping and routing the wire multiple times through the ferrite helps incrementally improve the signal integrity. While a majority of cables have their wires passing through the ferrites only once, some designs require them to make as many as three loops to meet design objectives. Typically, there are two types of ferrites available that are suitable for cable assemblies—snap-on ferrites and doughnut ferrites.

Snap-on ferrites are the easiest to assemble. These are passive suppression devices with two halves. A plastic clamshell case holds the two halves as it snaps close around the wire. Available in a wide variety of sizes for different cable diameters and performance types, these are excellent devices that can mix and match various types of ferrite to help pass an aggressive test requirement. However, snap-on ferrites can be expensive and require accurate sizing to match the wire’s outer diameter to create an interference fit. As their design is like a clamshell, it is easy to remove snap-on ferrites.

Doughnut ferrites are simpler, being in the shape of a ring or a doughnut. The cable must pass through the center of the continuous circle of the ferrite before the wires terminate into a connector. The doughnut ferrite is therefore a permanent fixture, unlike the snap-on ferrite that the user can remove at any time. Overmolding the ferrite helps to fix its position on the cable while protecting the brittle ferrite magnet from damage.