USB power and testing confusion - Page 1
Forum section: electronics / beginners

author: msknight
author_rank: Regular Contributor
timestamp: October 27, 2023, 07:44:41 pm
content: I’ve been spending some time worrying about my historic pile of USB cables. To a degree I’ve gone around in such circles that I’m not exactly sure what I’m worrying about any more, or whether I can actually check for the presence or absence of what I might or might not be worried about. That’s how confused I now am.Things were straightforward before USB C. All I had to worry about was whether the cable carried data or power, and did I think it was thick enough to charge my phone or device.When fast charging first came in on USB A, I was a little concerned but not overly worried. With USB C, however, it’s gone a little bonkers.If I understand correctly, a chip has to be present in the cable for the devices to unleash 5A, otherwise it limits to 3A. For some older cables a resistor in the power line was used for devices to detect lower power cables but the dangerous ones have no such resistor. However, I find myself concerned over whether my bundle of historic cables can handle 3A (because my charging units are USB A and need converters or A-C cables – I mostly use these -https://www.amazon.co.uk/gp/product/B0BKSGRRQ8)From what I’ve worked out, short of hooking up a cable and pumping 3A down it, and pointing a thermal imaging camera at it, there’s no easy way to determine how, “safe,” it is.I’ve watched videos on who knows how many USB testers and while they are all doing a great job of showing off the features of the various testers… they are not bridging that gap for me… in other words, what should I be testing a cable FOR in the first place, and how to evaluate a cable’s strengths and weaknesses, and work out whether I should probably bin it or not.I mean, I could spend a lot of cash on equipment, but I’m still relatively clueless on how to use it, or interpret what it’s telling me.This is where I currently am. Lost in the wilderness and needing pointing somewhere please.

author: EPAIII
author_rank: Super Contributor
timestamp: October 28, 2023, 09:06:55 am
content: Interesting documents. They will take some time to digest. Thanks!

author: msknight
author_rank: Regular Contributor
timestamp: October 28, 2023, 02:56:34 pm
content: Thanks for the detail, but I'm still in the dark about how, and what, I can tests my cables for to be, "safe," and I'm no longer sure what, "safe," actually is.

author: nightfire
author_rank: Frequent Contributor
timestamp: October 29, 2023, 12:56:09 am
content: Basically it boils down to not overheating the materials. (As in every aspect also in other fields of energy transmission)So you could simply take your cables, pump about 2.5A-3A through, and test if they are only somewhat warm to the touch. If you can touch them with great comfort, its ok.Usually some USB device with Quickcharging etc. should be able to drive at least 2A through it. Or some el cheapo USB load from ebay etc. can help with drawing some Amps.

author: radiolistener
author_rank: Super Contributor
timestamp: October 29, 2023, 07:47:30 am
content: this is more about voltage drop than safety. Higher current leads to higher voltage drop. So the cable needs to be able to deliver voltage with no significant voltage drop. If voltage drop is significant it may leads to fail powered device because it is not intended to work with low voltage.You can test the cable by connecting it to lab PSU with some fixed voltage and 5-10 Amps current, another end of cable connect to a dummy load to get 5-10 Amps trhough cable. Then measure voltage drop between PSU and dummy load.The main problem is to find good quality USB connectors which can handle high current with low voltage drop.

author: msknight
author_rank: Regular Contributor
timestamp: October 29, 2023, 09:53:44 am
content: I have some USB dummy loads coming, but when I look to some of the testing equipment for USB cables I'm starting to come to the conclusion that I might as well ditch the old cables and buy new ones, of quality... because some of my cables go back decades and are just so, "unknown," if you know what I mean.And as for the various C/Thunderbolt cables and what they carry, or don't carry... it looks to be a bit of a confusing mess. It used to be a case of keeping a pile of cables and taking a gamble as to whether it supported data or not.