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The humble USB cable is part of an electrical revolution

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Re: The humble USB cable is part of an electrical revolution

#31

Post by nickff »

em3rgent0rdr wrote: post #14
kabdib wrote: post #3

It's going to be interesting, from a security standpoint. One of the original cracks on the PS3 was via the USB stack. I'd not go plugging my computers or phones into jacks that I don't necessarily control. I can see a market for buffering devices that allow power through, and perhaps do power negotiation for you, but that do not allow data traffic. I believe these devices already exist. though I don't know how sophi…

surely this could be a software fix: tell usb driver to use special mode that is only allowed to use data line to negotiate power levels, before you plug into an untrusted power source.

I believe that this is what Apple implemented with their "do you trust this computer" dialog in the newest versions of iOS.

Re: The humble USB cable is part of an electrical revolution

#32

Post by csmuk »

fab13n wrote: post #29

100W at 5V means 20A. We usually recommend 4 A/m2 max for copper conductor sections, so it would require two 5mm2 wires in the cable here. It would feel more like a rod than a cable IMO. Anyone got an idea how they plan to address this? Higher voltages?

New "USB heating wire!".

I've had USB cables get hot before. The finest being a cheap Chinese USB cup warmer. Why they build such things escapes me.

The losses at low voltages are pretty high as well.

There's a reason that we have mains cables how they are and our current power distribution infrastructure.

Re: The humble USB cable is part of an electrical revolution

#33

Post by nickff »

fab13n wrote: post #29

100W at 5V means 20A. We usually recommend 4 A/m2 max for copper conductor sections, so it would require two 5mm2 wires in the cable here. It would feel more like a rod than a cable IMO. Anyone got an idea how they plan to address this? Higher voltages?

Who usually recommends 4A/mm? Using the wire parameter calculator below, a 2mm diameter wire is capable of carrying 20A, with a 0.1V drop over a 1m (3") cable. A 2mm diameter stranded copper wire is not entirely unwieldy.

http://circuitcalculator.com/wordpress/2007/09/20/wire-param...

Re: The humble USB cable is part of an electrical revolution

#34

Post by sliverstorm »

nickff wrote: post #30
sliverstorm wrote: post #19

Earlier quoted context omitted.

You can make a power-pin-only USB connector, but I believe that precludes requesting high-current mode (i.e. you are stuck in 100mA mode)

Most hosts will provide at least 500mA without any signal from a device. Hosts and chargers vary in their current capability; those capable of providing more than standard USB spec vary in their implementation. In general, high current hosts always provide their full capability, and the devices are the ones which require some signal to indicate maximum current available. Other devices increase their current draw unti…

Ok, so maybe I had it backwards which way the communication goes.

Re: The humble USB cable is part of an electrical revolution

#35

Post by nickff »

sliverstorm wrote: post #34
nickff wrote: post #30

Earlier quoted context omitted.

Most hosts will provide at least 500mA without any signal from a device. Hosts and chargers vary in their current capability; those capable of providing more than standard USB spec vary in their implementation. In general, high current hosts always provide their full capability, and the devices are the ones which require some signal to indicate maximum current available. Other devices increase their current draw unti…

Ok, so maybe I had it backwards which way the communication goes.

I wouldn't say you got it backwards, the communication is bilateral, but the device is the unit which controls current flow; the effect is the same.

Re: The humble USB cable is part of an electrical revolution

#36

Post by kyzyl »

fab13n wrote: post #29

100W at 5V means 20A. We usually recommend 4 A/m2 max for copper conductor sections, so it would require two 5mm2 wires in the cable here. It would feel more like a rod than a cable IMO. Anyone got an idea how they plan to address this? Higher voltages?

Well they're pretty limited in what they can do. Most things you'd plug a USB cable into are not thing you'd want to plug higher voltages into, so that would require device-side voltage conversion which is wasteful in energy, space and complexity.

I suspect that the 100W figure is actually just a pulsed maximum specification. The thing is that current ratings for wires are actually specified as a max. continuous current for a given temperature rise in the conductor, per unit length. So blowing 3-4x the current through a conductor for a very short time (think of flashing a bulb or moving a servo) is not a big deal. You just get a transient heat rise. Also, most applications simply don't require 20A. Even microwaves and kettles stay below the 15A residential fuses. (Although they do come close. I once had a shitty basement suite with an underrated fuse. If I ran my toaster and my kettle at the same time the breaker would flip!)

Re: The humble USB cable is part of an electrical revolution

#37

Post by makomk »

fab13n wrote: post #29

100W at 5V means 20A. We usually recommend 4 A/m2 max for copper conductor sections, so it would require two 5mm2 wires in the cable here. It would feel more like a rod than a cable IMO. Anyone got an idea how they plan to address this? Higher voltages?

Yeah, higher voltages. Can't remember off-hand if this tops out at 12V or 20V at the higher charge wattages, but they're definitely not using 5V for them.

Re: The humble USB cable is part of an electrical revolution

#38

Post by nickff »

csmuk wrote: post #32
fab13n wrote: post #29

100W at 5V means 20A. We usually recommend 4 A/m2 max for copper conductor sections, so it would require two 5mm2 wires in the cable here. It would feel more like a rod than a cable IMO. Anyone got an idea how they plan to address this? Higher voltages?

New "USB heating wire!". I've had USB cables get hot before. The finest being a cheap Chinese USB cup warmer. Why they build such things escapes me. The losses at low voltages are pretty high as well. There's a reason that we have mains cables how they are and our current power distribution infrastructure.

The reason those cables get hot is the same reason that you got them so (relatively) cheap: metal is a significant fraction of the cost of a cable. The manufacturer who produced the cable knew exactly what their customer was looking for, and provided a low cost cable with thin conductors.

Re: The humble USB cable is part of an electrical revolution

#39

Post by nickff »

kyzyl wrote: post #36
fab13n wrote: post #29

100W at 5V means 20A. We usually recommend 4 A/m2 max for copper conductor sections, so it would require two 5mm2 wires in the cable here. It would feel more like a rod than a cable IMO. Anyone got an idea how they plan to address this? Higher voltages?

Well they're pretty limited in what they can do. Most things you'd plug a USB cable into are not thing you'd want to plug higher voltages into, so that would require device-side voltage conversion which is wasteful in energy, space and complexity. I suspect that the 100W figure is actually just a pulsed maximum specification. The thing is that current ratings for wires are actually specified as a max. continuous curr…

Almost all your USB-powered devices have voltage converters with varying inefficiencies.

It should also be noted that a switched mode voltage converter can have well over 90% efficiency, even with large changes in voltage.

You should also remember that those microwaves and kettles are getting up to 15A @ 120VRMS continuously, which works out to 1800W. You can verify the actual power output of a kettle by timing how long it takes to boil a liter of water, and calculate power from this time and the specific heat capacity of water.

Re: The humble USB cable is part of an electrical revolution

#40

Post by phaer »

nwh wrote: post #22
jakobe wrote: post #21

Earlier quoted context omitted.

Even worse than USB is Firewire, which, if I am not mistaken, gives direct memory access to whatever device is connected. However, manufacturers are already realising that blindly trusting every USB host is a bad idea. For example, iPhones now ask if you want to trust if you connect to a previously unknown computer via USB. If you chose not to, your iPhone will take power, but won't allow data access. Therefore it's…

> Even worse than USB is Firewire, which, if I am not mistaken, gives direct memory access to whatever device is connected. Both FireWire and Thunderbolt offer DMA. There's publicly available tools that allow you to extract passwords, remove lock screens and find keys in the memory of running devices using either type of port.

Plus PCMCIA and ExpressCard, so most laptops are vulnerable out of the box[1], but there are some ways to mitigate those attacks. See https://en.wikipedia.org/wiki/DMA_attack

[1]...if you plug untrusted devices into your expansion slots or leave your laptop unattended.

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