July 27th, 2011

those thingies at the end of power cables

I wanted to make a joule thief (a voltage booster) for an experiment and noticed that I required a ferrite bead. I went to the electronics store but they didn’t have ferrite beads. This led me to google the term, upon which I discovered… that’s what those cylindrical components at the end of commercial electronics cables are! In a little plastic jacket.

In this particular application (power cables) they are just a simple cylinder of ferrite (metal alloy) around the cable, but they have an important and fascinating function.

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 is the cables connecting the devices. These cables act as nice, long antennae for the signals they carry. They broadcast the signals quite efficiently. The signals they broadcast can interfere with radios and TVs. The cables can also receive signals and transmit them into the case, where they cause problems. A 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.

Explanation via howstuffworks.

Imagine how much interference there’d be in an electronics store or computer lab without ferrite beads choking the RFI transmission.


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