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Curiosity kill the computer port. But why? I bought a 4 port USB 2.0 hub. I removed Vcc and Gnd from the Type A connector of the hub. Then I powered the hub and inserted a USB stick into one of the hub's ports. Then I inserted the hub's Type A connector into a computer (only D+,D- connected). I was curious to see if the computer could see the USB stick using only power from the USB hub. Seems like the computer kinda/sorta wanted to see the USB stick by the normal audio you hear when plug a usb device in. But the audio came on and off.

In the end, I damaged the usb port of the computer. I tried another USB port and damaged it too. In theory, seems like this should have worked. The only thing that could have damaged the computer is the D+, D- lines of the Type A connector.....never thought I could damage a port but maybe because I used a different power outlet for the usb hub than the computer, maybe that's why it was damaged? Voltage on D+, D- must have been slightly higher to damage it?

Am I thinking right or wrong? Any ideas?

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  • \$\begingroup\$ Is your computer a laptop or a desktop? If a laptop, does the power supply have a ground pin or no? Does the power supply for your USB hub have a ground pin, or only a live and neutral? \$\endgroup\$ May 30 at 2:35
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    \$\begingroup\$ "I removed Vcc and Gnd..." So pins that expect a specific bias can swing to any potential, either that caused by circuitry, or static-induced. Why did you think that was a good idea? \$\endgroup\$ May 30 at 2:39
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    \$\begingroup\$ The ground is there to make sure that both devices agree on what voltage they're going to call "zero". If they're off by even a few volts, you'll violate the absolute max on the data lines and risk burning them up. \$\endgroup\$ May 30 at 2:40
  • \$\begingroup\$ usbkill.com \$\endgroup\$
    – mkeith
    May 30 at 3:38

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You killed two USB-Ports on your computer.

Why? I don't know, why you did it (I'd be really interested in your reasoning to do it) but I can tell you how you killed it.

If you look at USB-Connectors closely you might notice, that GND and Vcc contacts are longer tha the data pins. There is a reason for that. It's to make sure, that GND and Vcc are connected before the data pins make contact to mitigate the risk of damage to the host and the devices.

By disconnecting GND you took a way the potential reference. D+ and D- are 2.25V to 2.75V referenced to GND. By taking away GND you're taking away the reference. Now it's pure luck if GND on your USB-Hub and on your PC are close enough together to not damage one or the other or both. GND of your PC is probably the same as Earth of your house wiring via the earth pin of the power cable of the PC (the PC case is connected to that Earth pin and GND of your Power Supply is connected to the case to reduce EMI problems).

Your USB-Hub is probably NOT connected to earth (USB Hub Power supplies normally don't have an earth pin) so GND of your HUB is "floating". The power supply has a weak capacitive coupling from primary side to secondary. So your HUB is probably around 1/2 mains voltage. But it's really high impedance, so most probably you won't feel it if you touch it. But it seems it is enough to kill the electronics on your PC mainbaord.

So how to prevent that?

Don't disconnect GND (unless you know, what you're doing and why you're doing it).

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  • \$\begingroup\$ Thank you @Transistor. I've updated the post following your suggestion. \$\endgroup\$
    – kruemi
    May 30 at 6:20
  • \$\begingroup\$ Brilliant answer! Yes, the hub power supply does not have a ground! That's why it died. Had it had a Gnd reference and both the computer and hub had been plugged into the same power outlet, my suspicion is that it would have worked. \$\endgroup\$ May 30 at 11:51
  • \$\begingroup\$ But now you ask, why did I disconnect Vcc and Gnd? I did this because I have a problem at work and needed to experiment. You see, I have a usb 2.0 port on a custom pcb that barely produces enough power to read usb sticks. Consequently, I wanted to see if I only used the hubs power, would that work? Unfortunately, I didn't consider that the hub's power didn't have a ground reference. :-( \$\endgroup\$ May 30 at 12:06
  • \$\begingroup\$ @kevinzamora If the hub is somehow ground referenced than still a lot if things could go wrong. Ground loops are a realy thing and they can be nasty. For your quest to disable the supply from the PC you could just disconnect Vcc. This still could have side effects but should not destroy your Motherboard. I think your lession should be: never disconnect a GND connection unless you're absolutely sure about what you're doing. \$\endgroup\$
    – kruemi
    May 30 at 12:21
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    \$\begingroup\$ Thanks for giving back. I'm not an EE but diddle every once in a while. I don't mind blowing things up as long as I learn something....now I am going to watch some youtube videos on ground loops :-) \$\endgroup\$ May 30 at 12:45

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