Why Does a Power Supply Go Through a Magnetic Ring?

Power supplies can use ring-shaped ferrite beads to reduce noise.

To reduce noise in a system, a power supply cable may be fitted with a ferrite bead. This is a ring or cylinder made from ferrite, a ferromagnetic material. Power supplies also use transformers, which convert the voltage from the electrical mains supply into the voltage required by your computer or other electronic device. A transformer can have a ring-shaped or toroidal core.

About Ferrite Beads

A ferrite bead is a hollow tube, sometimes ring-shaped, which is placed over the cable near the end which connects to the device. Ferrite beads often appear as bumps in a power supply cable, as they're typically covered with the same insulating material as the rest of the lead.

Why Ferrite Beads

All computers and electronic equipment produce a certain amount of radio frequency noise. This can interfere with their own operations and can also affect other equipment nearby. The power lead for an electrical device can act like an antenna, picking up RF noise and transmitting it to the device. A ring or toroid of ferrite, placed around the cable, blocks the RF noise from reaching the device.

Power Transformers

The voltage of the mains electricity supply is too high for computers and electronic devices. To provide the correct operating voltage for your device, you need a power supply. The main component of a power supply is the transformer, which converts the mains voltage into a lower one that your device can use.

How Transformers Work

A transformer has an iron core with a hole in the center. A wire is coiled around one side of the core. This is called the primary coil. On the opposite side, a coil called the secondary coil is wound around the core. The primary coil induces a magnetic field in the core which flows around it in a specific direction. This magnetic field flows through the center of the secondary coil, creating an electrical current through the coiled wire.

Toroidal Transformers

Transformer cores have traditionally been square shaped, as toroidal (ring-shaped) cores were difficult to wind. Recent technological improvements in the machines used to wind transformers now mean that toroidal cores are much easier to manufacture, and these have come into more widespread use. A ring-shaped core saves space and has additional favorable characteristics, including the absence of an external magnetic field and lower power loss.

References

• Light and Matter: Electromagnetism
• Hyperphysics: Transformer
• Electronic Components; K. Padmanabhan

Resources

• Hyperphysics: Ferromagnetism