Ferrite beads (L1 in the photo) filter high frequency power supply noise by converting it into a tiny amount of heat. Power supply noise can cause various problems for many parts, especially in analog audio and display circuits.
Ferrite beads are simple, but choosing one can be confusing because they’re not commonly used by hobbyists. Most designs will still work if you omit the ferrite bead(s), but beads are so cheap there’s no reason to sacrifice the added reliability they provide. We describe how we pick ferrite beads for our projects after the break.
A ferrite bead is rated for current, impedance, and resistance; see this Mouser listing for an example. Unless a datasheet or circuit requests specific bead characteristics, we choose a bead rated for sufficient current, and ignore the impedance and resistance values.
If the bead is for a power supply, we determine the maximum possible current the circuit will use and find a bead rated for double that amount. Last week we calculated the the Bus Pirate’s worst-case current consumption as 525ma, so we looked at beads rated for at least 1000ma. We used this one, which is rated for 1500ma and costs 10 cents.
Sometimes a ferrite bead is used to filter the power supply for one specific part of a circuit. We used a dedicated bead to filter the LCD bias voltage on the DIY digital picture frame, and with the ENC28J60’s ethernet transceiver on the web server on a business card. These parts only consume a few milliamps, so we used a smaller 200ma ferrite bead ($0.11).
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