Help to identify SMD fuse - Page 1
Forum section: electronics / repair

author: onesystem
author_rank: Regular Contributor
timestamp: September 19, 2025, 12:13:34 am
content: Please see the attached image:Fuse in question - the SMD fuse with package size 1206 with a blue coating on top. Picture is of the working circuit, I have two others that have MAX3157 fried on them, with the fuse blown. I tested other components around the circuit - all tested good. So it looks like I just need to replace the fried MAX3157s and the fuses.I looked at datasheet and consumption of the IC is 85 mA maximum 25 mA typical. So the fuse range would be 125-150% of max, which is 106.25 - 127.5 mAThe other SO-8 IC on board does not consume anything. It is a protection IC for the MAX3157 in this case.I would like to replace it with the identical fuse, if possible (with this nice blue color on top). If not possible, I will just order same package size parameter-equivalent from digi-key.Also - what fuse type is it? Slow blow? Fast blow? How would I find that out? Underbelly of the fuse has no markings on it - just plain ceramic.MAX3157 datasheet:https://www.analog.com/media/en/technical-documentation/data-sheets/max3157.pdf

author: thm_w
author_rank: Super Contributor
timestamp: September 19, 2025, 12:55:56 am
content: You have a working circuit, so you can measure the resistance of that fuse and then blow it to get a closer value.Is that fuse going to ground? its not really clear.But I wouldn't even bother, and would throw in a regular 250mA or 500mA fast blow.I mean it didn't even protect the MAX3157 so..

author: onesystem
author_rank: Regular Contributor
timestamp: September 19, 2025, 05:02:26 pm
content: Thanks! Yes the fuse is connected to ground. Yes I can measure the fuse blowing point - very good idea. I did not think with a controlled burning of one to figure out the value.What happened to that circuit - is that RS-485 comm got affected by lightning - which in turn I presume shorted out the die in MAXIM IC - which in turn blew the fuse.I looked on digikey and looks like there are no fuses with the same "finish" - it is probably an obsolete version. Circuit was built around 2008. So I will use a current active part. Just as a side note - darn those MAX3157 are expensive! 20 USD retail, or about 10 USD direct from manufacturer.

author: Kean
author_rank: Unknown
timestamp: September 20, 2025, 10:48:40 am
content: Have you tried tracing the circuit around that section?As it connections to PCB ground, I suspect F1 is a fusible resistor with a low value and intended to provide common ground between RS485 and the rest of the PCB, but allow for some voltage difference in ground between the equipment which is common on factories or between buildings. This would be similar to RG as shown in the MAX3157 datasheet, and easily damaged by lightning even with other protection components like the USB6B2 or nearby TVS.If that is the case, then I don't think its value will be critical at all. I'd probably use a 10R surge rated (aka pulse withstanding) resistor like KOA Speer SG73P2BTTD100J. These are green, but if you look further you might find an alternative in blue

author: coromonadalix
author_rank: Super Contributor
timestamp: September 20, 2025, 12:03:26 pm
content: would F1 be for inverted polarity ? with Z6 ? inverted it shorts out and blow the fuse, the real ones is the green one CP1 and the other next to it ?seems you have 3 voltages lines with inductors in serial L1 L2 L3 ? is that it ?

author: onesystem
author_rank: Regular Contributor
timestamp: September 20, 2025, 05:22:06 pm
content: Quote from: Kean on September 20, 2025, 10:48:40 amI have attached a better photo of the section of the circuit board with MAX chip desoldered.Resistatnce of the F1 component is 0.043 ohms, so I suspect it to be an actual fuse.PCB GND → F1 → Z6 → max3157 pin 14 (Cable Ground) and pins 19 & 20 (setting RS485 A and B impedance to 1.5kΩ)PCB GND → F1 → Z6 → R4 → L1 → Sheild pin of RS485 connectionPCB GND → F1 → Z2 → Sheild pin of RS485 connectionPin 25 of max3157 is connected to PCB GND directly.Quote from: coromonadalix on September 20, 2025, 12:03:26 pmL1 - as per above, is on Shield pin of the RS485L2 and L3 are A and B RS485 lines, going to USB62, and to MAX3157 from there.

author: Kean
author_rank: Unknown
timestamp: September 21, 2025, 02:11:17 pm
content: Well, OK if it measures 43mΩ then it must be rated for several amps, but would be fast acting.I didn't try to hard to trace the circuit, but it looks like Z6 and C5 are in parallel, and these sit between the MAX3157 isolated ground and F1 which then goes to PCB ground.Z6 is providing the main protection. The marking YM is for a 33V bi-directional TVS (Vishay/General Semi SMD33JCA), which seems appropriate for the 50V isolation of the MAX3157.F1 is just there as a failsafe should there be a major fault condition like a lightning strike.Looking at the datasheet for the Vishay MFU series the 3A 1206 part MFU1206FF03000P500 has a cold resistance of 44mΩ and is rated for 63V.A real Vishay MFU also has a blue coating, but the above part should have a marking of "P".They were serious with protection on this board. Obviously for industrial application, and probably not cheap.

author: onesystem
author_rank: Regular Contributor
timestamp: September 21, 2025, 02:19:43 pm
content: Thank you. Yes, you are totally correct. It is industrial board and it was priced at several thousand dollars when it was in production. Automated Logic Corp is the manufacturer.Thanks on the analysis. I did not want to do it, but I will blow one good fuse I have here under controlled conditions to measure its rating.

author: Kean
author_rank: Unknown
timestamp: September 21, 2025, 05:25:21 pm
content: Quote from: onesystem on September 21, 2025, 02:19:43 pmThat is not a very scientific test, and will be a waste of a good fuse.I'd suggest you study the typical curves and see how much current or time you need to blow a fuse. Even under controlled conditions, you are unlikely to have a decisive result. You would need a large sample size for statistical analysis.But if you go ahead with it I'd certainly be interested in the results.

author: fzabkar
author_rank: Super Contributor
timestamp: September 21, 2025, 07:17:25 pm
content: Quote from: Kean on September 21, 2025, 02:11:17 pmIf Z6 is clamping the Vcc supply of the IC, and if Vcc has a max spec of 7V, then what is the point of a 33V TVS diode? It seems to me that in this case the IC sacrificed itself to protect the diode. I don't get it. :-?The only component that would make sense to me at location Z6 would be a unidirectional 5V TVS diode. A resettable polyswitch in place of the fuse might be a good idea, IMHO.

author: Kean
author_rank: Unknown
timestamp: September 22, 2025, 12:14:46 am
content: Quote from: fzabkar on September 21, 2025, 07:17:25 pmAs mentioned above, the TVS appears to be clamping the RS485 bus ground. The MAX3157 has internal isolation, but it is limited to 50V between isolated and local ground.These components seem to be intended to add extra protection beyond that, but very little can effectively protect from a lightning strike.

author: onesystem
author_rank: Regular Contributor
timestamp: September 24, 2025, 05:58:56 pm
content: Update: Same device model, same PCB revision, but different kind of fuse was used. This time, it had marking! Please see attached photo. It is still 1206 package, marked "P", looking through datasheets Littelfuse series 466 seems to match. 3A, 32V:https://www.littelfuse.com/assetdocs/fuse-466-datasheet?assetguid=dbe9bcd7-6072-4adf-bf5b-d33e52a6b90fSo I will go ahead and use this fuse for the other boards as well.

author: Kean
author_rank: Unknown
timestamp: September 25, 2025, 02:18:05 pm
content: Yes a "P' marking would be a 3A fuse, and the 1206 part I mentioned previously (Vishay MFU1206FF03000P500) would be a better match as it is rated for 63V.The 32V rating for the LF part is a bit low to handle the fault condition after the 33V TVS has caused the fuse to blow. It will likely be absolutely fine until it isn't...