Partlist Wednesday: Ferrite beads

Every Wednesday we highlight a component from the updated partlist. This week: Ferrite beads.

We usually use ferrite beads as a power supply filter. You’ll find them on the USB supply in circuits with an FT232 USB->serial converter chip, like the Bus Pirate, and on the ethernet interface of the web platform.

For general purpose digital prototyping we just get something with a sufficiently high current rating and under 330ohms of impedance. When we moved to all 0603 parts, we picked up a few hundred of this 1.5A ferrite bead.

Some circuit might require a more carefully chosen bead, but most of our projects with run fine without the bead at all. If a specific bead is needed, the datasheet will usually list it.

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  1. You might want to clarify that the impedance numbers you cite are at a specific frequency (e.g. 1MHz for the 180ohm part), as otherwise folks may find it a bit confusing … a 180R power supply filter doesn’t sound like a very good idea.

  2. Completely right Mike, I was a bit alarmed at first tooI :)

    It’s also worth nothing that at DC this little guy only has 0.05 ohms resistance, so it will only cause a voltage drop of 100mV at 2A. From a quick look at the datasheet, it looks like the first ferrite’s impedance is spec’ed at 100Mhz.

    A lot of times designers will start throwing ferrite beads of specific impedances at circuits when testing at an EMI lab and failing tests at those frequencies. This application is for “conducted” radiation, to prevent noise from coming into the circuitry through the power supply lines. This all comes into play when performing testing to pass certifications, such as CE mark.

  3. Sorry guys, this piece was a little reckless and unclear :) I get more questions about ferrite beads than anything else, and I wanted to convey ‘go for it, or leave it out, it’s not a huge deal for DIY’. For example SparkFun doesn’t use one at all on their Bus Pirate. You are totally right though, there’s a ton of details an engineer would consider for approval, or if eg analog performance were important.

  4. Eheheh I rimember this:

    I guess it was before the Canon EOS arrival, and maybe before the philips flux looking at solder joints ;)

    BTW, i found really helpful this document by Murata, that highlights the main differences between ferrite beads and inductors:

    as well as this more detailed presentation about ferrite beads applications:

    Finally, this short Vishay note about EMI/EMC gives some general design hints:

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