Bumps at the end of computer cables

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Have you ever thought why there exists a tiny little cylinder on your laptop charger? Today we are going to reveal the secret behind it.

In a typical computer system found in a home or office, you normally see these "bumps" on the mouse, keyboard and monitor cables. You can also find them on power supply wires when a device (like a printer or scanner) uses an external transformer.

These bumps are theoretically called as ferrite beads or ferrite chokes. Here is how they look like:

Bumps at the end of computer cables

Their goal in life is to reduce EMI (electromagnetic interference) and RFI (radio-frequency interference). You can see these beads in the following photo:

A ferrite bead is simply a hollow bead or cylinder made of ferrite, which is a semi-magnetic substance made from iron oxide (rust) alloyed with other metals. It slips over the cable when the cable is made, or it can be snapped around the cable in two pieces after the cable is made. The bead is encased in plastic - if you cut the plastic, all that you would find inside is a black metal cylinder.

What would happen if the charging cable was not designed to have the little cylinder? We all know that an electrical current passes through a charging cable in order to power up any device battery. Due to this electric current in the cable, it creates radio energy within itself and emits it in the form of waves outside the cable.

This phenomenon is similar to how an antenna works i.e. converting an electrical power into radio energy and vice versa. Just like an antenna, this cable transmits and receives signals to or from surrounding gadgets with same capability. Due to this, the electric power gets used in unnecessary function and hinders charging the actual device.

In addition, these signals can interfere with other electronic devices resulting in noisy signals such as a nearby radio gives distorted voice or does not tune up to a channel.

Computers are fairly noisy devices. The motherboard inside the computer's case has an oscillator that is running at anywhere from 300 MHz to 1,000 MHz. The keyboard has its own processor and oscillator as well. The video card has its own oscillators to drive the monitor. All of these oscillators have the potential to broadcast radio signals at their given frequencies. Most of this interference can be eliminated by the cases around the motherboard and keyboard.

Another source of noise are the cables connecting the devices. These cables act as nice, long antennae for the signals they carry. They broadcast the signals quite efficiently.

The ferrite bead has the property of eliminating the broadcast signals. Essentially, it "chokes" the RFI transmission at that point on the cable -- this is why you find the beads at the ends of the cables. Instead of traveling down the cable and transmitting, the RFI signals turn into heat in the bead.