This is why using only a PPTC for protection of USB ports is not enough - Page 1
Forum section: electronics / general

author: vstoykov
author_rank: Newbie
timestamp: August 18, 2021, 05:38:55 am
content: The overloading and short circuit protection of computer USB power line is not adequate to protect against short circuit on magnetic USB charging cables.I used a magnetic cable to charge the phone and the magnet stuck to a screwdriver without me noticing. After a while I noticed a burnt smell. I removed the screwdriver and tried to disconnect the cable from the computer, but it was very hot and I burned my fingers. I tried with paper - the same effect.I tried with a small pliers - the USB connector was stuck because of overheating, the plastic parts were soldered.I turned off the computer just in case (if by chance there was a short circuit that continues to heat up).I had to disconnect all the other cables and move the computer to a more convenient location.After a very strong pull, I finally managed to pull out the connector. Together with part of the other connector (the plastic parts were soldered to each other by the heat of the short circuit).I inserted two pieces of wooden stick (from ice cream) to prevent short circuit (the 4 wires originally attached to the plastic part are between the two sticks now).This is the second magnetic cable I have a problem with. On the first one, only the magnetic connector managed to melt. Maybe because the short circuit was shorter.I expected a short circuit to simply turn off the power. But apparently the protection is stupid (maybe with a just a PPTC). Or the resistance of the cable is not small enough to activate the short circuit protection.In my opinion the right way to protect the USB ports is to turn off the power in the event of a short circuit. And (or) to have a temperature sensor that stops the power when the temperature of the ports rises to 70-80 degrees (not to allow temperature to become a few hundred degrees and melt the plastics).

author: Raj
author_rank: Frequent Contributor
timestamp: August 18, 2021, 05:56:41 am
content: Holy cowBTW, correct me if I'm wrong, but it seems your cable was trash... the usb port at the computer could have melted, only if the contact resistance was not ideal there.(to have a 10ohm or just a tiny bit lower resistance at a short circuit is odd. usually it should have been in the tenths digit.)Most USB sockets are designed to deliver at least 500mA at 5v. (2.5W!!) now fuses can't go off at exactly that amperage, so actual current at failure scenario is much higher.Mains voltage is more dangerous but still sometimes shorts result in a fire without giving enough time to let the fuses go off. But yeah, imagine buying a low quality cable that crumbles and shorts your precious computer.

author: ataradov
author_rank: Super Contributor
timestamp: August 18, 2021, 06:02:25 am
content: USB specification also defines how cables should be designed. This cable is not USB-compliant, and it would never pass official certification.

author: vstoykov
author_rank: Newbie
timestamp: August 18, 2021, 06:06:27 am
content: I suspect it's not because of the contact resistance between the USB connectors, but because of the high resistance of the wires in the cable. They heat up the connector and melt the plastic.Why PPTC do not limit the current - I don't know. Maybe the short circuit current is not big enough because of the high resistance of the wires in the cable.I am reading that USB ports are usually protected with a PPTC and not with advanced circuit monitoring the current and temperature of the connector.Another hypothesis - there is a PPTC integrated into the cable's connector and it got too hot for some reason. But this is less probable.Quote from: ataradov on August 18, 2021, 06:02:25 amWhy? Because there is a possibility of a short circuit or because of the high resistance of the wires?

author: ataradov
author_rank: Super Contributor
timestamp: August 18, 2021, 06:15:05 am
content: Quote from: vstoykov on August 18, 2021, 06:06:27 amI don't know why USB IF they chose to define it that way. But USB only allows cables that are terminated by USB plugs or captive on one end (like in keyboards).Here is a quote from Cahper 6 of the specification (USB 2.0 in this case, but it did not change with new revisions, new types of plugs were just added):QuoteNote that extension cables are also explicitly prohibited by the specification, since they allow uncontrolled total segment length, which is also prohibited.

author: Alti
author_rank: Frequent Contributor
timestamp: August 18, 2021, 08:38:14 am
content: Because of the reasons pointed out by ataradov I suggest OP should change the misleading title.Maybe: "Random cable + USB port = meltdown".

author: vstoykov
author_rank: Newbie
timestamp: August 18, 2021, 09:46:33 am
content: When designing protection for USB ports, it should be borne in mind that users can insert any charging cables for phones, including magnetic ones, which potentially do not meet the USB specification.Also damaged cables can be inserted my accident.The USB port should have a thermal sensor that shut down the power when get heated. And also a reliable overload and short circuit protection (not just one PPTC on the +5V).Therefore, the conclusion is that the specification should be changed to reflect reality, and not to try to bring reality in line with the specification.

author: Raj
author_rank: Frequent Contributor
timestamp: August 18, 2021, 09:59:45 am
content: I've literally seen my mains 15A socket blowing up, after a guy with his floor grinding machine (with locally built motor) stalled. It did trigger the MCB but it wasn't enough.

author: Ian.M
author_rank: Super Contributor
timestamp: August 18, 2021, 10:53:38 am
content: The USB 2.0 standard says:QuoteThis would apply to the host's root hubs, so even with a compliant motherboard host port, there is the possibility of 26.25 watts of dissipation into an overload before the over-current trips. Fractionally over an ohm of cable resistance would be enough to prevent a shorted cable reaching the worst case 5A trip current permitted by the standard. If the motherboard ports are non-compliant, well the PSU 5V rail can probably pump out 20A to 40A, so how much current can the motherboard's narrowest power or ground trace carry on the way to the port in question before it fuses?Polyfuses typically have a trip current somewhere around 5/3 of their rated holding current so if the motherboard manufacturer has used a polyfuse in excess of 3A with no additional over-current protection, it cant be USB 2.0 standard compliant.Ideally the motherboard should have individual smart over-current protection for each root hub (usually a pair of ports) or even per port, but that costs significant money and is difficult to explain to average (and below) IQ consumers, who will bitch that their non-compliant USB HDD, CD/DVD drive, desk fan, coffee warmer or other tchotchke don't work on their high end motherboard but it 'works fine' on their friends/relatives 'box store' PC.Therefore you are risking your PC motherboard any time you plug in a non-compliant cable or device direct to one of its ports, or to a down-stream bus powered hub. Front panel ports are not much better than rear ones, unless they are on a compliant self-powered drive bay hub, as there is still a risk of motherboard damage if the USB header and cable connecter cant carry 5A, but at least its easier to replace melted USB A sockets!Anyone who's developing experimental USB stuff should have either a compliant self-powered hub between their device and their PC to protect the PC, or an externally powered USB isolator. Unfortunately many cheap hubs with external power jacks are *NOT* USB standard compliant and many commit the cardinal sin of back-feeding upstream Vbus from their PSU, which can also fry your motherboard. If you encounter such a hub, disconnect its upstream port Vbus pin!Assuming the O.P isn't expert in multi-layer board rework with all the kit required to replace the connector assembly without wrecking the board, what should they do now?Well if they don't mind being one port down, I'd advise cropping the remains of the contacts as far back as possible, carefully straightening the stubs and filling the port with non-conductive, non-corrosive glue. Hot glue would do at a pinch, but I'd prefer a clear or white epoxy (avoid grey ones as they are often metal filled).If they cant tolerate being one port down, the only practical option for the average user with ordinary soldering skills is to take a short USB A M-F extender cable, cut it to get a female A socket on a short length of cable and solder the wires to the remains of the contacts in the b----red port. Then shim between them to ensure they don't touch, secure the trailing A socket to the back panel so there is no strain on the wires and test it. If it works, back-fill the b----red port with epoxy to anchor the wires and permanently insulate the connections.TLDR: *DON'T* use non-compliant charging cables or random USB tchotchkes that you haven't personally checked for wiring integrity and maximum current draw on your PC. That's what USB 'charger' wallwarts are for!