Posted on 01/11/2010 6:26:02 AM PST by pelicandriver
Wouldn’t it be determined by the resistance of saliva? 5a=5v/1ohm
This raised my eyebrow as well because the voltage can also be 12vdc at very low amperage and I can’t see how it can do any damage unless the child had one end in her mounth and tried to plug the other end into a 120vac wall socket.
Helluva lot more than 5 volts involved here I would imagine.
Step one ... don't leave things on the floor that are plugged into the wall.
Sad case ... I hope the child recovers completely.
I was thinking the child’s weight could also have been a factor. But I don’t know....
My little ones can’t get enough of the computer peripheral cords. We promptly take them away but they are fascinated by them for some reason. Hard to blame someone for a kid getting a usb cord.
That said, this story has to be total fluke, bs, or both. It’s not too far flung to think someone else has gotten liquid (coffee, soda, etc.) onto a desk and usb cable. No way this “danger” is possible in today’s hyper-litigious society.
The USB controlling specification defines the limits put in place on the USB interface, for reasons having to do with this exact scenario.
The USB 1.x and 2.0 specifications provide a 5 V supply on a single wire from which connected USB devices may draw power. The specification provides for no more than 5.25 V and no less than 4.75 V (5 V±5%) between the positive and negative bus power lines. For USB 2.0 the voltage supplied by low-powered hub ports is 4.4 V to 5.25 V.[30]
A unit load is defined as 100 mA in USB 2.0, and was raised to 150 mA in USB 3.0. A maximum of 5 unit loads (500 mA) can be drawn from a port in USB 2.0, which was raised to 6 (900 mA) in USB 3.0. There are two types of devices: low-power and high-power. Low-power devices draw at most 1 unit load, with minimum operating voltage of 4.4 V in USB 2.0, and 4 V in USB 3.0. High-power devices draw the maximum number of unit loads supported by the standard. All devices default as low-power but the device's software may request high-power as long as the power is available on the providing bus.[31]
So, lets put some numbers to this. The default starting point is 5.25 Volts (maximum) with a limit of no more than 500mA (unless the child's mouth perfectly micked a USB 3.0 interface and communicated a software packet requesting additional power (sorry, no human's mouth will do that).
Power equals voltage multiplied by current. Thus, 5.25 Volts x .05 Amps = 2.625 Watts.
2.625 Watts is NOT going to do the damage to an infant that this article states. It's far more likely the infant stuck an extension cord in his mouth - I believe the parents are lying about the USB cable, as the math just doesn't work.
Actual title is: “Shocked toddler is now stable”
I think pelicandriver’s title should be kept in brackets for explanation, though...
“Shocked toddler is now stable [Toddler burned by USB cable]”
I’m fairly sure that if this sort of damage was done the power supply failed... My phone is connected via a micro usb charger which is plugged right into the wall.
Yes, the output should only be a few volts on the USB connector end, but if the converter failed for some reason, it certainly could deliver far more.
This child’s heart stopped, that’s not remotely possible with proper output on a USB delivering proper juice.
A properly functioning USB port/cable should not deliver that sort of power. It seems that either 1)The cable in question was a ground path, and the child touched some electrical source. Or, 2) A failure caused the USB cable to be energized at line voltage, and the child touched some grounded object.
Ummm...was the other end of the USB cable inserted in a computer or a 120VAC outlet....?
ping
There’s too much circuitry between the power supply and the USB bus for a malfunction to create that much juice without frying the rest of the motherboard first. I can’t think of any peripheral cable on a PC that could do that much damage, except maybe an ethernet cable plugged into a Power-over-Ethernet switch that also runs a phone, but then you’d only get a big shock like that if ringer voltage were sent, and PoE is rare in private homes.
Yes, you are correct - Ohm's Law applies.
However, the USB has LIMITED power output to no more than 0.5 Amps; thus the absolute maximum power the infant could be exposed to is no more than 2.625 Watts (5.25 Volts x 0.5 Amps).
2.625 Watts will NOT do this kind of damage.
Like you said, must be a malfunction.
No, very likely not a malfuction. Very likely a false accusation and an outright lie.
The USB power is carefully monitored by several chips, and the USB hub is directly connected to the South Bridge chip. There is practically no way that this sort of power could possibly make it through to the USB connector, as there are power regulation circuitry that is in place SPECIFICALLY designed to prevent this exact scenario.
This case was definitely not only considered in the USB spec, it’s also a key component to the motherboard designers. The power supply makes 5 Volts, and then several other devices are in circuit that are in charge of monitoring and shutting down the power if the power demand is above expected levels. A maximum expected level would be 500 mA. At this point a circuit would detect that maximum power has been exceeded and the port would be shut off, with a critical power draw note in the system log.
Unless the parents can provide a motherboard that beats every concieveable failure analysis scenario, I cannot see how this is remotely possible.
To put this in laymans terms, this is like a mid-air collision between a Boeing 747 and a VW Mini-bus. It just aint’ gonna happen.
The USB was plugged into a laptop computer on the floor that had been turned off and was plugged into the wall socket to recharge.
I’d betcha the toddler stuck the CHARGER plug into her mouth. 100W of DC power.
No way a USB did this. IMHO.
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