Crappy StarTech USB 2.0 hubs - Page 1
Forum section: electronics / projects

author: Renate
author_rank: Super Contributor
timestamp: October 08, 2020, 01:54:24 pm
content: I bought a couple of StarTech ST7200USBM 7 port USB 2.0 hubs (fortunately not at the MSRP of $105!)https://www.startech.com/en-us/cards-adapters/st7200usbmPhysically, they are nicely built with a metal case that mounts.Although I do hate those stupid little green Phoenix screw terminals that plug in.(And for which you can't buy a simple crimp plug.)Well, after a month one blew up.They can be externally powered and this one was getting its 12V (unit spec is 7-24V although the LM2596 maxes at 40V).The PCB inside was nicely fried.Well, the more I looked, the more pissed off I got.These units take in external power, feed it into two LM2596 ADJ set for 5.25, steer it with Schottkys, feed it to two big PTCs and feed two banks of ports.There is no per-port current limiting or ANY kind of current sensing for banks or ports.That's pretty disappointing for a "professional" hub.I guess that they had an engineering change order because they replaced two capacitors with resistors.I've always thought that the two were different animals.Besides stealing bypass capacitance from two of the ports (one in each bank) why couldn't they stick their stupid resistors elsewhere?It's hard to tell exactly what happened, but the LM2596 are not blown through, they still deliver the right voltage.Did the SMD USB protector short? Did the "C7" (470 ohm resistor) short?It's all molten now and the trace is burnt through.

author: Renate
author_rank: Super Contributor
timestamp: October 08, 2020, 05:32:50 pm
content: Well, it must have been either an internal failure of the LM2596 or a problem in the feedback divider resistors.Apparently it did blow 12V out to three USB ports (despite showing the correct 5V now).One device even had its USB cord all heated and deformed.I'm still looking at the three devices to see how much is blown.They couldn't have put a crowbar or throw a zener on the output of the LM2596?

author: Ctrlocti
author_rank: Regular Contributor
timestamp: October 10, 2020, 08:28:05 am
content: why so many 0ohm resistors? I can't tell the silkscreen, are they also supposed to be caps? looks like a "c111" altough I doubt there's a hundred caps on that thing.I'm troubleshooting a USB device right now and there's so many things pissing me off about it. It's not a cheapy either. for example, they replaced the ferrite beads after teh D- and D+ lines with 0ohm resistors. The silkscreen clearly says FB. The board is designed with a bunch of bypass caps and tvs diodes? nah don't put them just leave the pads empty. I don't understand it.I feel you, it's infuriating

author: oPossum
author_rank: Super Contributor
timestamp: October 10, 2020, 08:33:40 am
content: CH1 - Choke 1 - a CMC (common mode choke)

author: Ctrlocti
author_rank: Regular Contributor
timestamp: October 10, 2020, 09:08:53 am
content: Quote from: oPossum on October 10, 2020, 08:33:40 amoh you are right! But that's a 0ohm resistor, right?

author: oPossum
author_rank: Super Contributor
timestamp: October 10, 2020, 09:12:03 am
content: Yes. They chose to not populate the choke, so something has to complete the circuit.Not uncommon. Cheap power supplies often have jumpers in place of the CMC(s) and missing caps.

author: Ctrlocti
author_rank: Regular Contributor
timestamp: October 10, 2020, 09:16:29 am
content: Quote from: oPossum on October 10, 2020, 09:12:03 amRight. I'm dealing with the same thing on a another USB board I'm troubleshooting. 0ohm resistors after the D+ and D- lines where the silkscreen calls for ferrite beads. All the other FB pads are populated with (presumably) FBs except those two. I don't understand why those two were specifically replaced with 0ohm resistors. It looks intentional but I'm not understanding the decision.90% of the bypass caps to ground (after the aforementioned FBs, which are placed) are also missing. As well as all the tvs diodes.

author: oPossum
author_rank: Super Contributor
timestamp: October 10, 2020, 09:29:57 am
content: The choice of what components to populate would ideally be based on testing the RF emissions for compliance with applicable standards and laws. Some of the parts may not be needed to meet specs. It can also be cost reduction without regard to RFI.

author: sandalcandal
author_rank: Unknown
timestamp: October 10, 2020, 11:04:14 am
content: Power circuits aren't always designed with the intent that 100% of pads need to be populated. Having extra unpopulated pads gives options for parts and design flexibility: not sure you got enough bypass but don't have the time to respin the PCB if it ends up not being enough? got some nice high performance MLCC at a good price but not sure your supply will stay that way? just put some extra pads that can be populated if need be, the space and costs of doing so is usually negligible for the flexibility it provides. For ultra compact devices like smartphones you're unlikely see unused pads but for cheaper commodity devices like a USB hub I can't say I'm surprised. Not that this is always the case and scraping margins down can often go awry... but unpopulated or bridged pads do not necessarily imply bad design.In Renate's case, the expectation of a cheap consumer grade item isn't applicable given it's price tag and literal "industrial grade" marketing. Lack of per port overload protection is disappointing but not an advertised feature. oPossum has already pointed out the 0ohm links are replacing some chokes which probably don't have much to do with the failure.As discussed inthe other thread, the blown component is probably an ESD protection TVS diode. Some of the outputs got fried too according to the presence of melted USB cords. Both these suggest a high power sustained overload. The main SM regulator is also apparently fine but might want to see if it stays that way under a dummy load. So perhaps this is an overload due to back feeding of power into the hub? Might be useful to know what was plugged in.Another more likely cause though. I did a quick search on LM2596 failures and found it appears to be vulnerable to input transients which cause it to short input voltage to output voltage without leaving much evidence it has failed:https://e2e.ti.com/support/power-management/f/196/t/732739andhttps://electronics.stackexchange.com/questions/245674/lm2596-random-shutdown(edit:should have linked the answer I was looking at) So I'd suggest checking to see if you can find any transient protection on the input. Loose wires and an intermittent connection while under load could easily cause some spikes.I'd also suggestbootlace ferrulesif you want reliable connections from a wire to screw terminal.

author: Renate
author_rank: Super Contributor
timestamp: October 10, 2020, 12:58:23 pm
content: Quote from: sandalcandal on October 10, 2020, 11:04:14 amThere was a bunch of stuff plugged in, none of them connected to anything external at the time, no sources of voltage anywhere near.Quote from: sandalcandal on October 10, 2020, 11:04:14 amThis. I went looking but didn't find anything. I did suspect it though.I already had tried this with a 10 ohm on the output of the 2596 and repeatedly, erratically connecting/disconnecting the input power.I couldn't get it to blow 12V through.Edit: Neither of those two references is an exact 1:1 copy of the problem.I'm kind of wondering more just power on latch up.The 2596s are STD2596. The units are made in Taiwan. Could they be fake?

author: sandalcandal
author_rank: Unknown
timestamp: October 10, 2020, 02:32:06 pm
content: Quote from: Renate on October 10, 2020, 12:58:23 pmI was looking atone of the answers in the stack exchange linkthat also cited potential issues with transients but looking again that's probably less hard evidence and more my confirmation bias.10ohms=500mA might not be enough of a load unless you're sure the load when it failed was less than that. At least some hard start/stop testing gives a bit more faith but insufficient load potentially won't create the required transients.Searched for the STD2596 and foundthe datasheetpretty easily. Its made by the Taiwanese-Chinese company Semtron and it does appear to be an LM2596 clone, so I guess it's a real fake? Its not a genuine TI for sure. At this point I think my guess is as good as yours or anyone else's on how legit and well implemented it actually is unless someone with experience using this particular chip can be found. I hazarded a look into the Chinese internet (a single baidu search and google translate) and came across this forum threadhttp://club.szlcsc.com/article/details_4196_1.htmlalas nothing useful but complaints about the struggles of being a domestic Chinese manufacturer pressured by the expectation of absolute rock-bottom prices while surround by a sea of substandard clones and counterfeits. It is interesting to see the perspective from "the other side" though.Apart from loading it up a bit harder and giving it a few more tugs, the only other things I can think of trying to do to get a definitive answer would be really involved like transplanting it onto a test circuit to characterise it more or decapping and comparing the silicon to the genuine article. Faulting into a latch up state with the input shorted to output and dumping 12V across but surviving and functioning after a reset seems plausible. Someone with a more in depth understanding than me of this IC or the style of control loop it uses with its fault condition vulnerabilities could maybe answer better. Really would be just be speaking from speculation rather than experience for anything more. I think it might be fair to blame cheap the LM2596 substitute used at this stage tho.

author: Renate
author_rank: Super Contributor
timestamp: October 11, 2020, 01:03:20 pm
content: Quote from: sandalcandal on October 10, 2020, 02:32:06 pmYes, the load on the 3 ports was much less than 500 mA. (There are two 2596 and two banks.)Well, after repairing one device and putting in an order for a new Atmel ICE, I decided not to risk life and limb with these treacherous devices.I ripped out the steering diodes and connected an external 5V from a reputable supply directly in (bypassing those horrid green Phoenix connectors).The 16 AWG zip cord goes through a strain relief mounted in the former connector hole.Compression screw terminals might not be a bad idea on a DIN rail equipment cabinet, but it's a horrible idea on free standing equipment.There is no strain relief on the cable. A bit of jostling could create a flakey connection (or even a short).I think that that was the actual instigator of the melt down.

author: Renate
author_rank: Super Contributor
timestamp: November 17, 2020, 12:39:21 pm
content: And another thing...There's an internal .1" jumper for enabling incoming USB Vcc (through a Schottky) to outgoing USB Vcc.The shipped and default position is connected through.StarTech doesn't document it and doesn't encourage changing it to disconnected (power only from internal power supply).I think that I know why.The power-on-reset to the USB hub chip is a 4.7k, a chip cap and a discharge diode.I've had a few times where the chip did not power on correctly.I'm not sure what the value of the cap is, but I replaced it with a 1 µF tantalum.So far things are working solid. We'll see.Still, there's no real undervoltage detection.I think that I'll throw a MC34064 5 V (4.6 V) TO-92 undervoltage chip on it.