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I never knew that USB-C extensions are not allowed for a reason
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this post was submitted on 26 Dec 2024
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Anybody care to sum this up for people who can’t watch videos?
So a standard cable needs to be chipped to show its rating to the device, its not that the device can pull what it wants or can get, but the cable itself tells it what it can supply. Extension cables can’t do that, because it doesn’t know what it’s plugged into, and that would be if they even bothered to put a chip in. They instead piggy back off the chip for the main cable. The problem comes when you you have a 240 watt cable hooked up to a cheap 120 watt cable, with the device being told it can push 240, and starts to super heat the extension cable
Brilliant thanks
5 sentences that inexplicably need a 9 minute video to say
Fuck YouTube
Correct, except for your example. Firstly, 120 watt USB c cables don't exist, only 60w, 100w, 140w and 240w. And only plugging in a 100w or higher cable into a 60w extension would be dangerous, since it would allow drawing 5 amps on a cable over an extension only designed for 3 amps. However, as soon as your extension is rated for 100w it is completely safe to use with any USB c cable, even those rated for 240w, as those only operate at a higher voltage but still only allow 5 amps max.
I have also never seen an USB C extension cable rated for less than 100w, so this is kind of a moot point. If 60w usb c extensions exist somewhere, they would indeed be dangerous, but I have never come across one
Soon on Amazon..
1m USB-C CABLE HEATER!! 0.99c
($5.99 shipping)
This sounds solvable, doesn't it? Have the extension cable have a chip saying it can do X at maximum, then compare with whatever is to be extended and communicate the minimum of both upstream. Might not become a sleek cable-like design, but would extend the 240W cable with the extender safely staying at 120W
That's an active extension cable, which is essentially a single port USB hub.
Shouldn't it be possible to only do the negotiation part and otherwise bridge everything? Not having to do anything high-bandwidth actively should keep the silicon costs down.
Yes, and such cables already exist, like this splitter cable:
https://www.amazon.de/dp/B0CRZ6JJ6D (not an affiliate link)
It's not an extension cable, but it does exactly what you are suggesting. It gets the available PD profiles from the charger and then intelligently negotiates a profile that will work best to split the power to the 2 devices connected to it. The charger thinks it's just connected to 1 device, and the connected devices think they are directly connected to a charger.
Doing the same for with a USB C extension would be trivial, but it's probably hard to market such a cable when passive USB c extension cables are available at a fraction of the cost, even if those aren't compliant to the USB standard
I wish there was a clearer explanation or nomenclature for this. With things like cables and converters everything always seems to have a black box layer.
I don’t understand why there are so many PD profiles either. Maybe Cat-1 USB-C, Cat-2 USB-C, etc? Maybe just having a smaller set of voltage-defined profiles that have a safe maximum current rating? Maybe that’s already how it is? I don’t know
There are technical reasons for why so many PD profiles exist.
In fact they were not enough, which is why the USB Standard was extended with the "PPS" extension recently, which let's the attached device freely choose a voltage between 3V and 21V in steps of 20mv, and more importantly it let's the device freely change this voltage without interrupting the charge process. This change makes it possible for devices to bypass their own but in charging electronics and just directly forward the voltage coming from the charger to the device, improving efficiency and significantly decreasing how much the device hears up during charging
Sadly PPS is not found on many devices or chargers yet, and makes the already complicated USB C charging situation even more complicated for consumers
Heh heh heh. Wait till you dive into the world of "That $15 cable costs 12c to make."
Getting $30 cables for $3 with my employee discount was almost the only good thing about working for Best Buy in the early 2000s.
I'm right now in China and those cables cost $0.50 shipped to your address, so not surprised
Well, the source checks the cable using the CC line which doesn’t go through the cable (VCONN). So source only knows the cable directly plugged in. To make the extension cable visible, the sink would be required to check the cable plugged in using VCONN and then the tell max ampere to the source over the other CC that goes through the cable.
2 Problems:
Sink devices normally don’t read or can’t read VCONN as far as I know
No way of detecting if a third cable (extension in the middle) is present and what specs it has
Well I'm glad I know that now.
Pretty much this, thanks for the summary
Going to take a wild guess and say the same reason you shouldn’t chain extension cords. USB can carry over 200w these days.
what if I don't know that either
Chaining regular extension cords isn't a problem by itself, connecting too many things in parallel and exceeding the rated max is a problem (and chaining extension cords "just" increase the risk that ordinary people will decide to connect more than they should, especially because the lowest rated cable in the chain sets the total limit)
The issue of chaining extension cords is that you can physically plug a 10 amp extension cord into a 30 amp cord. If you don't know what amps the device will pull, the 10 amp cord can overheat.
It's an almost identical problem to USB c.
Why in the everloving would your electrical code allow sales of extension cords that can't withstand the whole of the plug/socket rating. If it's an adapter from a higher amperage plug to lower amperage socket you need a fuse.