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Trying to fix a board that had a SMD inductor. it was labeled L1 on the silk and there was no marking on the part, and I have no schematics. ...or there may have been markings, but someone tried to fix the board before me and plastered the thing on solder. no way I can salvage that part... pic included at the for laughs. (the only part I could read the labeling was R3. probably because it is the less dense than solder so it floated :)

enter image description here

[L1 is right next to C6 there on the right. under that unsightly blob of solder.]

It is used on V+ from the broken USB connector, and then goes to those Resistors/Caps on the right side of the picture and then to the ICs.

I'm going to solder wires and trhu holes compoenents to substitute the damaged ones (because I have nothing SMD here)

Can I just bypass that if I use a USB cable with a ferrite core? ... I confess I'm not really sure what a SMD inductor does there. I always assumed USB had a standard frequency on the power line. is that not the case?


Edit: found the schematics for the USB chip this board uses. The inductor is the one marked L1 on the evaluation board. http://www.nxp.com/documents/user_manual/D12PCKitMan.pdf there is not spec there. BOM just says "ferrite bead"

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    \$\begingroup\$ Can you please promise the EE.SE forums that this person gets to never hold a soldering iron again? Please? \$\endgroup\$ – Asmyldof Jun 10 '15 at 3:23
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    \$\begingroup\$ haha i'm doubting he even used a soldering iron. that was probably done with a ironing iron or something. \$\endgroup\$ – gcb Jun 10 '15 at 3:40
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    \$\begingroup\$ What a poor excuse for a BOM... \$\endgroup\$ – Matt Young Jun 11 '15 at 12:22
  • \$\begingroup\$ @MattYoung agree. and in case anyone arrives here because of that link; the actual board has 68Ω resistors instead of the 22Ω that reference design suggest. Also the reference says 1%... well, for one of them i used parallel 220+100=68.75Ω 5% and it sort of works :) i guess MIDI devices do not need much bandwidth for all that to even matter. \$\endgroup\$ – gcb Jun 12 '15 at 7:08
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Ferrite beads are a common feature on the +5 & GND lines of USB. They're typically "needed" to filter conducted noise in/out via the USB cable. By "needed", I mean to pass national/international EMC emissions/immunity regulations. For a 'quick n dirty' fix, I wouldn't hesitate just to jumper it out. If you really want to replace it, a ferrite bead in the 60 to 600 ohms @ 100MHz range would probably be adequate. The package appears to be 0805, but photos can be deceptive! As it's a USB application, at least 500mA current handling ability. So here's some options for you:

Digikey Ferrite Bead Listing

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  • \$\begingroup\$ 60Ω to 600Ω in a 5V circuit isn't a lot of variation? \$\endgroup\$ – gcb Jun 10 '15 at 3:52
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    \$\begingroup\$ Variance only at 100MHz, which is their purpose - to reduce RF emissions (or increase immunity from 'external' RF sources. But at DC, they're a small fraction of 1 ohm. What ohms @ 10s or 100s of MHz is needed on this particular implementation can't really be known here with any accuracy, because it depends on factors we can't know (slew rate they've designed to, standards they're trying to adhere to, etc). \$\endgroup\$ – Techydude Jun 10 '15 at 4:07

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