Replacing unmarked SMD resistor - Page 1
Forum section: electronics / repair

author: Bicurico
author_rank: Super Contributor
timestamp: January 05, 2025, 10:48:45 pm
content: Hi.Die to a stupid mistake I had a short and it burned a SMD resistor. The PCB's top layer has a hole, but I found the traces and it should be possible to replace the burnt resistor and rebuild two traces.I have a good board and I measured the resistor in circuit. It has 6 ohms give or take. Actually all resistors of the same size measured between 5 and 6 ohms. Unfortunately they have no markings, as opposed as other resistors on the board, which are flatter. These have the typical 3 digit codes.I assume that the burnt resistor was a thick type carbon resistor.My questions to the pro's:Can I replace this SMD resistor with a normal 6 ohm resistor, not a SMD One, for easier soldering and availability?Does it make sense to measure a resistor, as opposed to a capacitor, while in circuit? I would get it should be ok.How can I know what the thickness of the wire should be? The trace that burned was fairly thick, but then it had a hole to connect to the middle layer (still working), which is tiny.Thanks!

author: Swake
author_rank: Frequent Contributor
timestamp: January 06, 2025, 08:46:00 am
content: You cannot trust an in circuit measurement of a resistor to have its real value. Whatever is in parallel with that resistor is going to mess up the reading. You'll have to remove the good resistor to measure it out of the circuit. This really is the only right way.You can generally replace an SMD resistor with a through hole one, but it is not going to be an easier soldering at all.Make pictures of both good and bad boards. Chances are the things you're looking at are not resistors.

author: Bicurico
author_rank: Super Contributor
timestamp: January 06, 2025, 10:47:56 am
content: I attached two pictures.I would suspect this to be either a resistor or a fuse.

author: Swake
author_rank: Frequent Contributor
timestamp: January 06, 2025, 11:12:29 am
content: That is a capacitor.Finding the value is going to be tricky unless you are willing to unsolder the good one and have a good C-meter. The value might not be all that important given how it is connected with the 4 legs of that chip nearby. What is that chip? I can't read from the pictures. I think it is a MOSFET and that this is part of some power supply.

author: kripton2035
author_rank: Super Contributor
timestamp: January 06, 2025, 11:13:35 am
content: it looks more a capacitor than a resistor ...

author: Bicurico
author_rank: Super Contributor
timestamp: January 06, 2025, 01:56:09 pm
content: Shouldn't the specific function powered by this line work even in the absence of the capacitor, if it is a capacitor?This leads to one of pins of the boards connector, which powers a different module and now it doesn't anymore.I would expect a capacitor to only work as a filter?This power line is switched on and off, according to the module being needed or not. I can try to make a continuity test on the good board, but I am definitely not going to mess with the good board.Attached is a better picture of the IC (sorry it is not better quality - I read IOR P925, but a search did not produce any datasheet).EDIT: Here is the datasheet:https://www.alldatasheet.com/datasheet-pdf/pdf/68234/IRF/IRF7416.htmlThe red arrow points to a similar component like the one that burned. You guys think that is a capacitor?Thanks!

author: inse
author_rank: Super Contributor
timestamp: January 06, 2025, 02:06:09 pm
content: I strongly suspect it’s a ferrite bead as it appears to be the only connection of the MOSFET (SO8 component)

author: Bicurico
author_rank: Super Contributor
timestamp: January 06, 2025, 02:07:36 pm
content: What would be its function?Edit:Act like a fuse?Better question: How can I fix this? Could I just solder a wire redoing this without the bead?

author: inse
author_rank: Super Contributor
timestamp: January 06, 2025, 02:20:15 pm
content: Damping HF noise, improve EMC performance.Hard to verify the properties as this is a lossy inductor without a distinct value.Might be specified as “100Ohms at 100MHz“ or the like.The current capability is also relevant, desolder from another board and try to measure DC resistance as good as possible.Then get into distributor catalog and apply filters for size and DCR (and current if known) and pick parts on best guess.

author: inse
author_rank: Super Contributor
timestamp: January 06, 2025, 02:26:41 pm
content: Quote from: Bicurico on January 06, 2025, 02:07:36 pmSkipping that part might either impact EMC performance (maybe don’t care) or the switching behaviour of the MOSFET leading to oscillations which cause power dissipation (MOSFET getting hot).Sure you can give it a try without and observe stability and temperature.

author: Bicurico
author_rank: Super Contributor
timestamp: January 06, 2025, 02:34:16 pm
content: Could I buy just any ferrite bead, like this one:https://www.digikey.pt/pt/products/detail/murata-electronics/BLM21PG601SN1D/24763138?Problem is that the part costs 0.11 Euro and shipping is 18 Euro.What alternative would you suggest?Thanks!

author: inse
author_rank: Super Contributor
timestamp: January 06, 2025, 02:39:04 pm
content: Quote from: Bicurico on January 06, 2025, 02:34:16 pmHow could I answer this question?Alternative: find a more reasonable seller.P.S. Stupid mistakes need to be punished 😉P.P.S. did you verify the MOSFET and surroundings are OK?

author: Swake
author_rank: Frequent Contributor
timestamp: January 06, 2025, 02:47:34 pm
content: Quote from: inse on January 06, 2025, 02:06:09 pmyes, you're right. A cap would not make much sense in that place. And the color in the new picture goes in that direction too.QuoteIf you can free one leg by removing that connector you can measure the ferrite bead for resistance. it will be DC resistance, but better than nothing.You can find FB's for not much money on aliexpress.

author: Bicurico
author_rank: Super Contributor
timestamp: January 06, 2025, 02:57:05 pm
content: This happened: the device is modular and I inserted the wrong board in one of the slots (stupid me).When I switched the device on, the power board burned visibly (certainly because it shorted the wrong rails) and what seems to have been a ferrite bead was gone.After putting the correct board in said slot, all works, except this board - it is not powered, which makes sense.I believe that only the ferrite bead burned and after fixing this spot, the module will be powered correctly again.All this makes sense to me, as the module is switched on/off by software and I think that is what the MOSFET is doing.So now I wonder if:a) I just short the burned part without the use of the ferrite beadb) I try to find a replacement ferrite bead and try to solder that in place - the question is what specs should I consider for the ferrite bead

author: inse
author_rank: Super Contributor
timestamp: January 06, 2025, 03:01:46 pm
content: Quote from: Bicurico on January 06, 2025, 02:57:05 pmWithout schematics: Jumpin' jack flash, it's a guess guess guess 🤭

author: shapirus
author_rank: Super Contributor
timestamp: January 06, 2025, 03:14:07 pm
content: I doubt that a ferrite bead would measure as high as 6 ohms on the working board. I'd expect it to be near-short.It could also be an inductor.Really the best way would be to desolder it from the working board and find out what it is by measuring R, L, C.Another option (but without guarantees) would be desoldering the parts that look the same near the burned one on the damaged board and testing them. They might be the same, and if they measure the same on the working board (even in-circuit), that could speak in favor of it.

author: kripton2035
author_rank: Super Contributor
timestamp: January 06, 2025, 04:14:52 pm
content: even an inductor would not measure 6 ohms ?

author: Bicurico
author_rank: Super Contributor
timestamp: January 06, 2025, 04:22:26 pm
content: That might have been a mistake, I just measured in-circuit with my Uni-T 61E in Ohm mode. Perhaps there was some oxide layer or dirt. I will measure that again.Please note I measured the OTHER similar looking components (probably/hopefully ferrite beads, also).The good board is in another working device and I don't want to risk breaking it - I already caused two misfortunes in two weeks...

author: shapirus
author_rank: Super Contributor
timestamp: January 06, 2025, 05:15:02 pm
content: Quote from: Bicurico on January 06, 2025, 04:22:26 pmI think a decent starting point would be to carefully desolder one (or better more) of them on the damaged board and measure them out of circuit. If you don't have an LCR meter, then measuring their resistance and capacitance with a general DMM can already tell us something useful.

author: Bicurico
author_rank: Super Contributor
timestamp: January 08, 2025, 10:45:31 am
content: I have some further questions:Imagine I do have a compatible replacement SMD ferrite bead.1) How would you tackle the burned area? Leave it as is or fill it with hot glue or resin?2) How would you repair the broken pad? Try to use lots of flux and apply solder in the hope it will make contact with the hole (which is still contact) or would you solder a new wire from the SMD ferrite bead to the pad of the connector?3) What are the chances that the repair would end up stable and lasting, without risking damage to the board that is powered by this pad of the connector?I know these are vague questions, but I would like to hear your opinions based on experience.Thanks!

author: inse
author_rank: Super Contributor
timestamp: January 08, 2025, 10:53:09 am
content: I would scratch off charred resin, attach the new part with wires for stabilization, additionally you can place a blob of epoxy if you are afraid of vibration.

author: Swake
author_rank: Frequent Contributor
timestamp: January 08, 2025, 11:29:03 am
content: Same hereClean as much as possible. Stabilise and insulate the hole with epoxy.Connect the part with new wires, fixed the part and the wires with small blobs of UV cured epoxy.

author: Bicurico
author_rank: Super Contributor
timestamp: January 08, 2025, 11:58:37 am
content: Thanks to everyone!