I had an old firewire PCB and was pulling some parts off of it and came across these little guys that I can't really identify. The PCB designator had a "B" next to them, which I tried looking up but the only reference to that would be a "battery". These obviously look like caps or resistors, however neither measure with any certainty. In resistance mode I get 0.1 ohms, and in cap mode I get out of range. Diode mode it reads 0. There are no markings on them.

I hooked the big one up to my ESR meter and it read 0.24 ohms.

What are these? There's one of the larger and several of the smaller ones on the board.

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1 Answer 1


They are ferrite beads. Inductive at low frequencies, resistive at middling frequencies like ~100MHz and capacitive at high frequencies.

If you have the equipment you can measure them, but they're cheap and easily available.

Here is a typical datasheet.

  • \$\begingroup\$ Thank you so much! Out of curiosity, what would be the way to measure their value? I have a scope and a function generator. \$\endgroup\$
    – LarryBud
    Commented Apr 5 at 1:31
  • 1
    \$\begingroup\$ You could use a known resistor in series and two scope channels with your function generator. \$\endgroup\$ Commented Apr 5 at 2:25
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    \$\begingroup\$ @LarryBud If you're curious what the "proper" equipment for measuring them would be, it'd be something along the lines of this or this. \$\endgroup\$
    – Hearth
    Commented Apr 5 at 3:02
  • \$\begingroup\$ Yes, big bucks to do it properly. But a NanoVNA could give a rough idea. \$\endgroup\$ Commented Apr 5 at 3:44
  • \$\begingroup\$ @SpehroPefhany I'll check that out. Thanks. \$\endgroup\$
    – LarryBud
    Commented Apr 5 at 19:24

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