Help with part ID - Ubiquiti 8 port POE switch - Page 1
Forum section: electronics / repair

author: Johncanfield
author_rank: Regular Contributor
timestamp: April 23, 2019, 04:06:37 pm
content: This is a Ubiquiti 8 port managed switch with 150 watts of POE available for the ports (48V and 24V.) All of a sudden it stopped working - dead, I have several of these switches (and other Ubiquiti wireless products) and this is the first failure ever.So as you can see in the image some magic smoke escaped on the part - there's eight groups of these parts so I assume each part cluster is for POE on each one of the eight ports.I've tried searching for "LF 100 33" and "LF!", etc with no luck. I don't even have a clue what this part does.The power supply seems happy - it's putting out about 53VDC - good enough to regulate down to 48V and 3.3V, etc.Any help is most appreciated(I've replaced this switch but it would be fun to get it repaired.)

author: PedroDaGr8
author_rank: Super Contributor
timestamp: April 23, 2019, 04:51:31 pm
content: That is likely a fuse, the LF is the logo for Littlefuse. Being a fuse, it's very unlikely that it is the only component that failed.While I didn't find your exact model, I found a related model:https://www.mouser.com/ProductDetail/Littelfuse/1812L050PR?qs=PWhpLWeW8wetL6yrmAPPxg%3D%3D&gclid=CjwKCAjw7_rlBRBaEiwAc23rhu_Pd_8qdgTb0lVF-SicEtwFyVGfgk2-vziWxW7FPg3kEl0xlNfhSRoCbYMQAvD_BwENote the data sheet and chip logos at the end of the dayasheet. From that I can ascertain that your model is a 1A 33V fuse from Littlefuse. Beyond that, we would need dimensions to size it and drill down further.Sent from my LG-LS998 using Tapatalk

author: mariush
author_rank: Super Contributor
timestamp: April 23, 2019, 04:55:32 pm
content: That looks like a current sense resistor, with two big terminals and 2 smaller ones. Could be a fuse, as poster above says.Check diode above it, D248 and the IC on the left could be dead, q148desolder resistor, check fuses and see if switch starts.if not see if that q148 is dead or shorted, causing protections to kick in and shut down the switch.maybe see if ic gets more hot (if shorted that happens) or see if switch starts if you desolder IC.

author: PedroDaGr8
author_rank: Super Contributor
timestamp: April 23, 2019, 05:01:34 pm
content: I think I got it:https://www.mouser.com/ProductDetail/?qs=PWhpLWeW8wfvE3Z3mhFkLg%3D%3DThis looks identical to yours, just confirm dimensions.Based on it's voltage rating, my guess is that something dumped the full 50+ V into the fuse and killed it. I would look both upstream and downstream for shorted transistors and the like.Sent from my LG-LS998 using Tapatalk

author: Johncanfield
author_rank: Regular Contributor
timestamp: April 23, 2019, 05:42:28 pm
content: Ah-soI did have a search hit for Littlefuse but I had no idea that was the profile for a fuse but it does make sense. I'm impressed with Ubiquiti's engineering/design - protecting every POE port. Now the challenge is to find out why the magic smoke was released.Thanks, I really appreciate the help.

author: Johncanfield
author_rank: Regular Contributor
timestamp: April 25, 2019, 10:39:01 pm
content: I removed the fuse and it’s good, not opened. I checked resistance of the components around the magic smoke and they match the other ports. I think I’ll throw the board into the junk box.