Ferrite bead - again - Page 1
Forum section: electronics / beginners

author: Kwaq
author_rank: Newbie
timestamp: January 29, 2021, 12:12:29 pm
content: As this is my first post on this forum, I would like to greet everyone.I have some basic knowledge about electronics, bit more in computer repair. I usually don't attempt to component level repairs if I don't see obvious reasons, like bulging capacitors.This time client brought me a mainboard that was mechanically damaged - few components broken off, couple scratches, but otherwise in fair condition. MB is ASUS X570-PLUS. Capacitors were fairly easy task, while I didn't have original specs, because couldn't find "MIL" manufacturer (?), I've replaced them with best of the type I could find from Panasonic. I do, however, have one broken ferrite bead I don't know what to do about. At that moment I've put a link in its place, board works but I would like to repair it as close to original specs as possible. Ferrite bead don't have any markings, ASUS Service won't help with board repair and I don't have that broken component - however even if I did, I don't have any means of measuring it...Is there any way of recognizing those components without board schematic or measuring them on-board?What could be best solution? Put there a random ferrite bead (I'm thinking 7A 60-70\$\Omega\$) or leave a link?

author: Terry Bites
author_rank: Super Contributor
timestamp: January 29, 2021, 12:43:30 pm
content: Tricky. You need to know what current its expected to carry at the very least. I'd scour the catalogues for one the same physical size and hope for the best.

author: TimNJ
author_rank: Super Contributor
timestamp: January 29, 2021, 01:58:36 pm
content: Sometimes ferrite beads don’t behave as you might expect, so it’s hard to say whether leaving it out completely or substituting a random bead will be better.If you had any sort of VNA/SA + tracking generator, you could measure the impedance with respect to frequency, and then get an idea about the material properties.My guess is that it’s a NiZn material, but just a guess.Oh another way to check is to check the resistance of the material. NiZn should show high resistance, MnZn show low resistance. This is due to resistivity of the bulk ferrite material. Make sure your multimeter probes are making good contact if you perform this test.You can buy a few beads of different permeabilities and base material type (NiZn or MnZn) and try to match by ferrite resistance method. This is a little flawed, but the general rule is that material resistivity goes down as permeability goes up.

author: gnuarm
author_rank: Super Contributor
timestamp: January 29, 2021, 08:48:52 pm
content: I'm not clear what the picture is of since you say you don't have the part. Is this a random device on some other board?I was recently educated that ferrites on the power cord are more for damping resonances than for presenting an inductance. I don't know if this is the general rule or not. Seems the ferrite material is lossy meaning the material dissipates heat from the magnetic flux changes and so reduce their amplitude. This would only be a significant impact on a resonance which is the accumulation of energy over many cycles. The ferrite absorbs energy faster than it enters the resonance preventing the large amplitudes. If that is the case here, it is purely a matter of suppressing EMI. Omitting it would add some EMI but probably not enough to present a problem to the device. It could irritate hams in the area.The chance of it being an otherwise functional part of the design is remote in my opinion. That bead is too little inductance to do much really. If the unit is working I think the only issue is how much it radiates or conducts through a cable. What does the signal trace go to? Any chance it is on a power line from the PSU?

author: TimFox
author_rank: Super Contributor
timestamp: January 29, 2021, 09:00:01 pm
content: There is an important difference between ferrite beads and ferromagnetic cores on inductors. A ferrite bead has a hysteresis curve (flux density B vs magnetic field H) enclosing a relatively large area. From magnetic theory, if you go around the loop by applying a periodic H field (amp-turns/meter in rationalized MKS units) from a periodic current around the material, every cycle will dissipate an energy per unit volume equal to the integral of BH around the hysteresis loop. (Note that B is volt-sec/m2.). This is a loss of power when the frequency is high, heating the bead, while there is negligible loss at low frequencies (cycling slowly around the curve). Good inductors use materials with narrow hysteresis to reduce this loss.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 29, 2021, 10:57:40 pm
content: I'd just put in a, say, 50-100 ohm ferrite bead, look for similar dimensions, yeah, NiZn material probably. They may use a material code instead of the composition, have to check several points in the catalogs to cross-reference it all, yeah bit of a pain if you don't know all of these things to begin with.Let's see... Laird brand, like a uh, 28L0138-80R I guess, something around there?https://www.digikey.com/en/products/detail/laird-signal-integrity-products/28L0138-80R-10/1015875?s=N4IgTCBcDa4BwBkAMBGAzHAtHJAlEAugL5ARelatives vary by impedance (80R) and length, YMMV.Mind, there's no shortage of types and manufacturers, you'll easily get lost shopping through them all...Ferrite beads aren't great filters (as detailed by others above), I would be a bit surprised if its use is critical -- it might even be just a "feels right" thing and not actually useful. Alas, us designers aren't immune to superstition, by any means... The most likely outcome is higher RF emissions -- not great, but probably not going to affect function, either. More to the point, there's probably other things in the machine that are aging, which affect EMI/RFI more than this one part does. It's like complaining about a pinhole leak in your car's exhaust pipe, when the muffler is already rusted to hell... (Of course, not having seen the "car" in this analogy, I have no idea whether that's actually the case.)Tim

author: Kwaq
author_rank: Newbie
timestamp: January 30, 2021, 04:18:35 pm
content: Attached photo shows that bead as it should be, it's from undamaged board. Diameter is 3.5 mm, lenght 6 mm, wire gauge 0.7 mm.As to whether it is important or not, my thoughts is - nowadays everything seems to be directed and designed by accountants. If someone decided it should be there and it wasn't removed for "economic reasons", well... On the other hand, given the board price it might be as well just forward thinking from engineers. With board being multilayer and cramped it's hard to tell which circuit it is on. As it is near RAM and chipset maybe it helps filtering switching noises? Maybe it allow higher stability during overclocking?We'll see - because of time pressure I had to return it to client, and read answers about measuring resistance afterwards... Left it with just link and obviously explained to the guy current board state. He'll be testing it with various benchmarks and tests to check if everything is in order. If something new pops up, I'll post it here.Nonetheless, thank you all for answers so far!

author: Siwastaja
author_rank: Super Contributor
timestamp: January 30, 2021, 04:31:07 pm
content: Accountants removing every excess part is kind of a myth.These consumer computer gadgets have many thousands parts and the product design cycles are extremely quick in this highly competitive market. Any accountant would have to work through all the parts within weeks or just days and actually have the engineering knowledge whether the part could actually be removed.I'm quite sure even the cheapest consumer gadgets have several "just in case" parts or non-optimal design choices. Of course when sold in tens of millions, it does make sense to optimize the BOM cost down by a few cents, but the point is, optimizing a few centstakes timeand even if it's net positive regarding to designer's labor cost, it won't be net positive anymore when looking at the increased time-to-market.So simple rules-of-thumb regarding bypassing and noise filtering pretty much live on the low-cost market as well, because such components are not expensive enough to warrant the long optimization cycle.