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Comment by _a_a_a_ | Hacker News Reader

Not an EE but 100w, that gives me The Fear. It reeks of trouble. Someone please set my mind to rest.

Many Watts means some combination of Amps and Volts - power is Amps times Volts.

The main risks from each are different though. Simplified explanation for the non-physicist:

Lots of Amps make wires get hot, make connections melt, etc. In general, thin wires (eg headphone wires) are good for 1 or 2 amps safely, and chunky wires (eg. AC cords) are good for 10 or 20 amps safely. The exact amount depends on how much cooling they get - which is why you should never put blankets over power cords. If you screw this up, the outcome is probably a fire, which maybe kills you.

Lots of volts causes electrocution. Thats because your skin is pretty decent at blocking electricity, but when you have enough volts, some can sneak through your skin, freezing up your muscles, stopping your heart which probably kills you.

So - amps and volts have totally different mechanisms of death. USB-C @ 100W is 20 volts at 4.5 amps. The voltage is well below dangerous levels, and the current is pretty safe in the cables specced, but I can totally imagine a few fires.

It's worth noting that about 10x as many people die from amps (electrical fires) than volts (direct electrocution).

By the way... The USB-C standard goes up to 240 Watts.

Thats 48 volts (fairly safe IMO) at 5 amps (only marginally safe IMO, on that connector design).

There are laws in many countries capping voltages, but not on currents - which is why the standard is pushing the safety envelope there!

The standard has no way to detect damaged cables (ie. baby chewing through the cable), nor connections getting hot (ie. an old dirty connection heating up and catching fire). But I suspect a future revision will allow both more volts and more amps, and will add those safety features - I'm hoping on the next version being 500 volts and 8 amps, allowing it to replace all household outlets.

Hopefully they come with auto-update firmware where if I plug a gen 15 cable into a gen 14 device and a gen 12 local power delivery unit the cpu in the cable will update the key in the PDU and get key material from the device to ensure that everyone's up-to-date on their licenses.

That would enable a new generation of USB worms.

"It would unleash an entire new generation of recurring revenue generation opportunities as innovators could develop exciting new technologies and industry leadership while protecting intellectual property rights holders"

"blah blah blah worm security blah blah"

-- same sentence different language.

(and -- I'm entirely trying to be tongue-in-cheek -- the notion of little dynamically updated microcontrollers everywhere in my house, getting bricked or asking me for a micropayment to turn a light on is a pkd story come to life)

Eloquent that, thanks.

> The standard has no way to detect damaged cables

Of course it does. All it needs is to ask the other device what voltage is on the other side, calculate resistance off it and trip or drop to lower power mode if it deems that there is too much power lost in the cable. Still not perfectly safe as say 3W lost over whole cable is far less dangerous than this 3W loss focused solely on dodgy connection but still.

What part of the standard mandates, or even optionally supports that?

Standard USB-PD only requires that you negotiate a voltage and current suitable for both sides (among a preset list). It essentially goes open loop after that; you can't exceed the current limit set, but beyond potentially tripping the PD controller (on the sink side) if the voltage drops too low, there's not really any protection here. It's fully on the PD sink to implement (or, not implement) any sort of protections like that.

I think the parent was saying that it could be done, not that it is being done.

> What part of the standard mandates, or even optionally supports that?

None, which is the fucking problem. Cheap cable can just lie about it's current capacity.

> USB-C @ 100W is 20 volts at 4.5 amps

Clearly one of those numbers is wrong

It is 5A.

* https://imgur.com/a/mjF0coQ

I got a pinprick sized 3rd degree burn from the end of a micro usb charging cable. It was more startling than dangerous, but I now have a little more respect for low powered devices, especially when plugged into mains.

In what sense? Putting that much power down a USB cable? USB-PD keeps the potentially cable-melting amperage capped at 3A or 5A (the latter only if the cable is explicitly marked as 5A capable) and moves more power by increasing the not-cable-melting voltage instead, so 100W is 5A at 20V. There's already laptops which charge at 140W over USB-C, it's fine.

It's a lot of energy. And close to signalling wires. Controlled by computers (What you describe is done by a CPU, not some simple foolproof system, I guess?) which can go wrong. In a small plug that can get yanked out easily and damaged.

That kind of thing. Am I being paranoid? It just smells like a house fire in the offing.

There's a lot of moving parts yes, but in practice it's proven to be pretty safe. Issues usually manifest as charging being too slow, rather than going too fast and causing a fire.

Whenever you charge a device you're already trusting a complex computer-controlled system not to turn the lithium ion pack into an incendiary device, trusting a computer not to overload a copper cable is small stakes relatively speaking.

in practice we don't have all that many devices pushing it that much that are not "a vendor charger connected to vendor power supply".

I expect there will be some surprises with cheap cables pretending they can handle full power.

Never mind “pretending they can handle full power”. Remember this story?

https://arstechnica.com/gadgets/2016/02/google-engineer-find...

Unless the other end lies about what voltage they're getting, a cable can't lie about its ability to carry current.

Of course it can. It is not tested by any of the devices, it is returned by cable itself. Cheap cable maker can just program cable to pretend to be more powerful one but not have wiring up to spec:

> Sources Shall detect the type of Attached cable and limit the Capabilities they offer based on the current carrying capability of the cable determined by the Cable capabilities determined using the Discover Identity Command (see Section 6.4.4.2) sent using SOP’ Communication (see Section 2.5) to the Cable Plug. The Cable VDO returned as part of the Discover Identity Command details the maximum current and voltage values that Shall be negotiated for a given cable as part of an Explicit Contract.

Please don't talk about stuff you have no idea about.

higher voltage makes it a lot more sane, imo. if anything increasing the voltage a long time ago would've probably helped us avoid the plague of all these shitty under spec USB cables. the cynic in me says they would've just shifted to even thinner gauge wires, but hey, a guy can dream, right?

No worse than an Apple power cable, and have those ever gone up in flames

Given that Apple users have been charging their laptops this way [1] for more than five years, maybe you are :)

[1] To be fair, it's more like 60-80 Watt for most models, but these days it can be up to 140 Watt, as far as I know.

yep, exactly. there's voltage negotiation up to 48v going on. though in practice probably not quite that high yet

USB power delivery works very reliable. Much better than those 10W USB-A chargers/devices operating outside the USB spec and sometimes shutting down your old Laptop when connecting it, because of too high power consumption.

The size of the pins they're running this over is what makes it scary.

I've had high-end devices (Macbook Air I'm typing this on) give me unreliable connections for basic USB 1.1, and now they're planning to run amps over similarly-sized pins?

My experience with USB-C on laptops has been nothing but terrible.

> now they're planning to run amps over similarly-sized pins?

Now? Between 2016-ish and the re-introduction of MagSafe, MacBooks have been using USB-C charging exclusively!

There's plenty of smaller pins that run MUCH higher amperage through them without an issue (M.2 sockets for NVMe drives come to mind). Granted, those are secured a bit better... but you have to trust the mechanical designers of this interface have actually thought things through here. It's robust.

M.2 is limited to 14W.

According to Wikipedia, each pin is only rated to 500mA. If you look at a pinout (depending on key), there's roughly 9 3.3V pins. 14W at 3.3V over 9 pins would be 470mA per pin.

It seems like comparing the amperage of M.2 to USB-C is indeed a little silly.

It's already ubiquitous, most new laptops have these as primary power plug

Do laptop power supplies scare you?

I guess they don't but here remixing a power supply with other things and I don't like the complexity. See my other answer above if that helps explain my concern.

GPUs are arguably much more complex and have been using much more than 100 W for a while now.

But they're not exactly cheap commodity items like USB cables.

Laptop plugs and wires are significantly beefier than average cable

Average USB cable? Not in my experience. And between my laptop power and USB cables, it's a laptop power cable that's the thinnest.

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