How can I choose a ferrite bead if on schematics it's marked as 100uH? On Mouser all ferrites have only Impedance value. How can it be compared to inductance in Henry?

JIC, in BOM it also commented that it can be replaced with 100 Ohm resistor.

Here is a schematics: enter image description here

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    \$\begingroup\$ Potentially stupid question: Are you sure it is actually a ferrite, not a 100uH inductor? If so, then the schematic is basically wrong... \$\endgroup\$ – Jack B Nov 24 '16 at 11:40
  • \$\begingroup\$ @JackB yes, it's really confusing. On schematics it looks like inductor but in BOM it says: "L1,L2, R0125W package, 2 pcs., 100uH, Ferrite bead or 10ohm resistor". So it looks like it's used here to filter HF-noise from PSU. Will inductor also work here? \$\endgroup\$ – everm1nd Nov 24 '16 at 14:38
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    \$\begingroup\$ Without knowing a lot more about the circuit and what is being filtered, it's hard to say what will work. I read your quote from the BOM as meaning 100uH inductor OR ferrite OR resistor. Possibly the intent was to try all three options in testing, and choose the one that worked. Possibly any one would be OK. You might need to contact the designer to clarify. \$\endgroup\$ – Jack B Nov 24 '16 at 15:37

Don't confuse Inductors with Ferrite beads (aka Impeders).

The designers of an Inductor strive to make it as wideband and as low loss as possible. It does get lossy, and at some frequency it resonates, but they are pushed to as high frequencies as possible for a given inductance. Detailed data sheets (not simply shortform data) will usually give you Q values at some frequency, and a figure for minimum self resonant frequency (SRF).

The designers of an Impeder/bead strive to make it never resonate, so it's constructed on lossy ferrite. At low frequencies, generally below 1MHz, it's a reasonable inductor. As the frequency rises, it gets lossy and looks more like a series resistor, with a specified impedance. If you look at the detailed data sheet, it should give you a graph of inductance and loss or equivalent series resistance versus frequency.

You can also get ferrite bead cores, usually with several holes in, through which you can thread your own wire to give you an impeder, rather than an inductor. The detailed data sheet for these will usually give some examples of performance versus turns of wire.

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    \$\begingroup\$ What you're calling an "impeder" I'm pretty sure is what everyone I've ever met, and the companies that make them (Murata, etc), call "ferrite beads". It would be nice to have different words for these devices and for unwound cores, but at the moment we don't. \$\endgroup\$ – The Photon Nov 24 '16 at 16:01
  • \$\begingroup\$ @ThePhoton It turns out you're right, after a quick google round. A decade ago when I was using these things in anger and trawling every datasheet going looking for sweet spot characteristics, the term 'Impeder' was much more current, but today you can barely find it. I'll modify the answer slightly. \$\endgroup\$ – Neil_UK Nov 24 '16 at 17:10

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