Replacing SMD ferrite bead on motherboard, what value do I need? - Page 1
Forum section: electronics / repair

author: rf473
author_rank: Newbie
timestamp: January 11, 2023, 09:19:26 am
content: Hello,I am trying to repair a motherboard that has no power on the PS/2 keyboard port. The problem is caused by a damaged ferrite bead. It does not have continuity across its terminals and when I temporarily bridge it with a wire, the keyboard is working again. The circuit leads to the 5V pin of the keyboard port, apparently, its purpose is to filter electromagnetic interference from the power signal.I want to replace this ferrite bead, something I have never done before, and I have no idea about the correct parameters I should choose. Here is an example for an SMD ferrite bead:Inductance: 0.047 µHQ: 20Self-resonant frequency: 320 MHzResistance: 0.15RRated current: 300 mAEdit:As someone below pointed out this is wrong, these parameters are for an inductor, not a ferrite bead. A ferrite bead has parameters such as: Impedance at 100 MHz, max DC resistance, max current. Example: 200Ω, 0.25Ω, 500mA.Could anyone please suggest which parameters I need? Attached is a photo of the motherboard with the ferrite bead marked in red.

author: DavidAlfa
author_rank: Super Contributor
timestamp: January 11, 2023, 12:10:27 pm
content: Not really required, you can short it out and will work fine.There was a old hack in where you removed the ferrite on the USB data lines and the speed greatly improved, I guess they filtered too much.

author: rf473
author_rank: Newbie
timestamp: January 11, 2023, 12:37:27 pm
content: Yes, I have bridged it with a wire for now and it's working. But I still want to replace it, it is a vintage motherboard and for me it's important to retain its original condition.

author: wraper
author_rank: Unknown
timestamp: January 11, 2023, 12:58:20 pm
content: Quote from: DavidAlfa on January 11, 2023, 12:10:27 pmIt will still work if filter on data lines is replaced with jumpers but improving the speed is an utter BS. There is no data rate reduction mechanism in USB, it either works at maximum speed or malfunctions and device disconnects.

author: SeanB
author_rank: Super Contributor
timestamp: January 11, 2023, 02:06:25 pm
content: Easiest is to grab a ferrite bead from another dead board, or simply put in a zero ohm link in place of it, it will still look like a SMD part, but will slightly degrade EMC performance, which the case is doing a better job of degrading anyway.

author: DavidAlfa
author_rank: Super Contributor
timestamp: January 11, 2023, 04:59:36 pm
content: Quote from: wraper on January 11, 2023, 12:58:20 pmI know it sounds silly but it was true!I can't remember the device, the speeds instantly went from ~20MB/s to 34.Apparently it had too heavy chokes, so some packets had to be sent several times.I'm not sure if that statement is true, "Works perfect or doesn't work at all".https://www.totalphase.com/blog/2020/07/about-the-usb-protocol-common-usb-bus-errors-and-how-to-troubleshoot-them/Quote

author: wraper
author_rank: Unknown
timestamp: January 11, 2023, 05:09:32 pm
content: Quote from: DavidAlfa on January 11, 2023, 04:59:36 pmIt must be flawed testing method. Transfer depends a lot on a USB drive, file size and some other things. If there were enough CRC errors to cut speed almost in half, USB drive would lose connection all of the time.

author: rf473
author_rank: Newbie
timestamp: January 11, 2023, 05:41:24 pm
content: I have read in some manuals about decoupling circuits that I found online. I did not understand 99% of it, but their examples mostly used values in the range of 200-600Ω at 100 MHz. Should I just buy some 400Ω ones and hope for the best? If that doesn't work, I will probably just replace it with a 0Ω part as suggested by SeanB.

author: Traceless
author_rank: Frequent Contributor
timestamp: January 11, 2023, 06:25:55 pm
content: Measure FB17 and the one next to it. If they have the same/similar value use that as a reference for your replacement. Side-note: C304 right below FB17 seems to be in pretty rough shape you probably want to replace that one too.

author: rf473
author_rank: Newbie
timestamp: January 11, 2023, 06:47:35 pm
content: Quote from: Traceless on January 11, 2023, 06:25:55 pmDo you mean measuring its resistance? This is the only thing that I can think of. Do I have to desolder it first? I have tested it and the resistance is about 0.25Ω. That would correspond fairly well to a ferrite bead range of 200-600Ω (smaller ones have a lower resistance, bigger ones have higher resistance).

author: wraper
author_rank: Unknown
timestamp: January 11, 2023, 06:58:46 pm
content: Most of the time engineers place such ferrite beads just in case without ever finding the best value or figuring out if they are really needed. So there is no much reason finding an exact part.QuoteFerrite beads come with different current ratings and DC resistance within the same package size and impedance rating.

author: rf473
author_rank: Newbie
timestamp: January 11, 2023, 07:16:53 pm
content: No, it does not need to be an exact replacement. If it resembles what it originally did in that circuit, it's fine.QuoteI found that the ones with a rating of 500mA correspond fairly well to these values. PS/2 has a maximum current of 275mA, so I figured it could be one with such a rating.

author: Traceless
author_rank: Frequent Contributor
timestamp: January 11, 2023, 07:59:36 pm
content: Quote from: rf473 on January 11, 2023, 06:47:35 pmSince it is a ferrite bead you should measure the inductance. For that you will need at least one of those relative inexpensive component testers (link to Dave's review of one below, there is also a thread about those here in the forum), make sure to get one with SMD pads like the one in the video. Alternatively an LCR meter would do but is much more expensive. To measure the bead with such a tester it would of course need to be desoldered.

author: rf473
author_rank: Newbie
timestamp: January 11, 2023, 08:21:28 pm
content: Quote from: Traceless on January 11, 2023, 07:59:36 pmThanks. It seems that I need to get me one of these, also for testing capacitors. I cannot desolder SMD components yet, but equipment is already on its way.

author: wraper
author_rank: Unknown
timestamp: January 11, 2023, 08:44:24 pm
content: Quote from: Traceless on January 11, 2023, 07:59:36 pmYou cannot measure inductance of a ferrite bead and get a meaningful result which will help you to find a replacement. Their inductance is not rated for a good reason. Just use any ferrite bead with sufficiently low DC resistance to not drop too much voltage at max current, and everything will be fine. Anything beyond that is a waste of time and effort which won't give better results.

author: wraper
author_rank: Unknown
timestamp: January 11, 2023, 08:53:39 pm
content: Quote from: rf473 on January 11, 2023, 09:19:26 amThese are specs you get for normal inductors, not ferrite beads.

author: Bud
author_rank: Super Contributor
timestamp: January 11, 2023, 09:04:49 pm
content: Quote from: rf473 on January 11, 2023, 07:16:53 pmUse it and call it a day.

author: tooki
author_rank: Super Contributor
timestamp: January 12, 2023, 05:34:57 pm
content: Quote from: wraper on January 11, 2023, 06:58:46 pmTell me about it!I spent half the day yesterday modifying a board for someone to remove and bridge all the SMD ferrite beads he added to his design “just in case” which then actually caused his board to be more susceptible to oscillation. (DC-4GHz RF amplifiers, four channels, with two stages per channel, in close physical proximity, despite being expressly warned by our RF expert that this is likely to oscillate… I didn’t design it, just did the assembly for him.)