How to determine value of inductor? Not on schematic... - Page 1
Forum section: electronics / beginners

author: desertrout
author_rank: Contributor
timestamp: July 31, 2020, 02:41:03 pm
content: Hi - I'm very new to this, so please bear with me. I'm trying familiarize myself with the circuitry of various old Macintosh computers I'm in the process of restoring. In this particular case I need to replace the SWIM (floppy drive controller) PLCC shown in the attached image, but my question is actually about the two inductors . When looking at the schematics, I noticed that the two inductors at L4 and L5 are shown on the last page in a kind of appendix (see attached).I'm curious to know what value would these inductors have? How would I find out?PS I don't have an LCR meter - it's on the list.I guess I have the same question about the cap at C7... looks like a decoupler on the same +5V rail, the schematic says 0.1 (uF I assume), but what voltage rating?

author: Siwastaja
author_rank: Super Contributor
timestamp: July 31, 2020, 02:50:09 pm
content: OK, so let's start from basics: components do not have just "a value"; they have complex sets of properties, and a part number helps you find a datasheet with the most relevant parameters. Inductors are not an exception.But these guys look like ferrite beads, which is kind of special case of an inductor: typically lossy at high frequency on purpose. If you are analyzing the subtleties of the analog/power performance, or trying to build an exact replica for any reason, you should get similarly rated ferrite beads - they are typically rated with impedance at 100MHz, this typically varies about between 30 and 300 ohms. They are quite AC lossy. Saturation current would be an important parameter but rarely specified; replacing with a smaller saturation current bead might cause it to filter less than expected.But may I ask you, why are you interested in the properties of these inductors? Are you building a replica from scratch or analysing the circuit?Similarly, the voltage rating of C7 is similarly of little interest. More interesting is the dielectric type, and exact graphs. In the end, what matters most in this decoupling use, is the total stray inductance, which is defined solely by package size and PCB routing, so any ceramic capacitor in the same package and at least 100nF, but can be more, performs basically the same.

author: ledtester
author_rank: Super Contributor
timestamp: July 31, 2020, 03:49:30 pm
content: Quote from: desertrout on July 31, 2020, 02:41:03 pmYou can make a guess by considering...- what is the voltage that the cap is decoupling?- what are the voltage supplies present in the system?Just take the maximum voltage that the capacitor is expected to experience under normal operation and add a healthy margin to it. Physical size might limit your selection.

author: desertrout
author_rank: Contributor
timestamp: July 31, 2020, 06:18:39 pm
content: Thanks for the responses. This is a new hobby for me and I'm hoping to learn.My interest is this: these are 30+ year-old circuit boards, and often suffer from corrosion from leaked electrolytes or battery acid, and I would like to have at least some sense of how to determine a suitable replacement of such small passives if the need ever arises, since there is no part number on any of these small inductors nor any reference on the schematic beyond indicating that they are indeed ferrite bead. "Exact replica" isn't my goal, and sometimes can't be since some components are no longer available (for instance the 3.9uF/100V polypropylene cap from a 1988 SE/30 analog board I replaced with film), but certainly wanting to able to make smart but "good enough" choices so that these machines can continue to operate well and safely.If it requires analysis of the circuit, or perhaps removing the component and testing it on its own or in a test circuit, then okay.I'm curious about how stray inductance is a factor in a SMD capacitor?FWIW, most of the others are rated at 16V and 25V. It's your standard sort of PC power system, i.e. +5V, -5V +12V, -12V. This particular cap is on the +5V line, so I expect 16V should be sufficient...

author: Siwastaja
author_rank: Super Contributor
timestamp: July 31, 2020, 08:17:29 pm
content: It's unlikely that corrosion destroys a ferrite bead or MLCC unless it's bad enough that it destroys the PCB and IC legs as well.These components do not fail, so there is no need to replace.Larger package MLCCs may fail (crack as a short circuit or low-value resistor!), though, due to mechanical stress (like board flex), but this isn't relevant for the small (0805 and smaller) 0.1uF IC bypass caps.The purpose of IC bypassing capacitors is to provide low-impedance voltage source for the chip to draw large and short current spikes whenever it needs it. If there is any inductance in series, there is a voltage drop over that inductance (V = L*di/dt) so the IC sees a voltage dip. Even worse, after the current pulse ends, the energy stored in that inductance is released as an opposite overvoltage pulse. Hence the idea of placing these capacitors close, the IC can draw the current from the capacitor with minimum possible series inductance. And being close means, it must be in a small package, otherwise the package itself acts like a distance and adds extra inductance. The exact capacitance of these parts is completely meaningless, as long as it's large enough; they are selected by package size being smallest they can reliably handle in the manufacturing (so they get smaller with more recent designs). Many designs default to 0.1uF. If you are unsure, use this value for the small caps near ICs. Voltage rating is trivial: more than the voltage on that DC bus. MLCCs do not need voltage derating. If you happen to have 6.3V rated parts, they are OK for the 5V bus, but obviously 16V or 25V parts are equally fine.The most difficult part regarding specifications is providing enough capacitance - when that matters, that is - because the true capacitance under DC voltage bias and over temperature and aging variations will varya lot, some only delivering maybe 10% of the nameplate specs, others closer to 60-70%! If you do MLCC replacements for any reason, pick X7R or better dielectric, beware of Y5V and similar. In this case, you can't know the intent and the part choice of the original designer, so you should replace using a better part.But for those small caps next to IC power pins, really, the placement is the most important thing and they already took care of it when designing the PCB, so just use whatever X7R 100nF or 220nF or 470nF in the same case size they used - or bodge a one case size smaller if you can solder it there and that's what you have around.

author: desertrout
author_rank: Contributor
timestamp: July 31, 2020, 08:29:05 pm
content: Wow - very informative - I've got a lot to unpack there, but you've certainly provided a lot of context. It definitely helps my understanding. Thank you!Also, the corrosion does indeed get bad enough to entirely corrode IC pins, pads, traces. Here are two pictures, one from a IIci main board, and its high-res video card (which is worse) from which I've removed the SMD electrolytic capacitors and are about to go into a vinegar bath, showing some of the damage that bathing in electrolytic goo for several years can do - and this isn't even especially bad.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 31, 2020, 09:00:49 pm
content: Well if perfect operation to original specs doens't need to be guaranteed, I would replace any severely corroded ferrite bead with the same size part, maybe specified at 60-120 ohms@100MHz. Really, just the cheapest basic kind you can get. If you can see that it's in series in a power trace (instead of a signal), it's a good idea to look at a ferrite bead specified as a "power line" version, or something with a bit higher current rating (see the distributor websites and compare what's available). It's going to be good enough, very likely. You might even get away with replacing them with shorts.

author: desertrout
author_rank: Contributor
timestamp: August 02, 2020, 01:48:59 am
content: I found this article to be informative in relation to power line ferrite beads:https://www.allaboutcircuits.com/technical-articles/choosing-and-using-ferrite-beads/and this one from the same series about bypass capacitors (noting the importance of inductance and its relation to case size):https://www.allaboutcircuits.com/technical-articles/clean-power-for-every-ic-part-2-choosing-and-using-your-bypass-capacitors/I know I'm just barely scratching the surface, but even this little bit and your assistance helps me understand these boards a little better.