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I believe USB ports on my computer are rated for 0.5A, 5V.

According to Ohm's law, if I connect a 10 Ohms resistor to a port it will reach it's maximum current limit (5V / 0.5A = 10 Ohms).

What will happen if I connect 1 - 9 Ohms resistors instead?

  • Will the port immediatly shut down?
  • Will the voltage go down to keep the current within limits?
  • Will the USB port get damaged?
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    \$\begingroup\$ Found an answer here electronics.stackexchange.com/a/168858/235256. Basically, it all depends on a USB port. \$\endgroup\$ Jan 15, 2020 at 4:38
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    \$\begingroup\$ Sorry, I can’t answer your question. But, I know that the USB specification states how a compliant USB port behaves in this kind of situation. Google it. \$\endgroup\$
    – user69795
    Jan 15, 2020 at 5:20
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    \$\begingroup\$ At least on my PC I accidentally short-circuited the USB 5V and my PC shutdown and re-booted. It looks that there the full PC 5V amperage is behind it! \$\endgroup\$
    – Oldfart
    Jan 15, 2020 at 7:03
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    \$\begingroup\$ One of my friend's kid inserted a key into the front USB port of the case (i.e. not the ones on the MoBo itself located at the back side). The PC re-booted and the port got unusable. Probably there was a fuse located right on the case's port extension. \$\endgroup\$ Jan 15, 2020 at 8:31
  • \$\begingroup\$ When I shorted one on my PC, it seems to have tripped a resettable fuse. It took weeks before the USB port regained its normal current capabilities. Beyond that, it didn't seem to have caused any damage. \$\endgroup\$
    – Dampmaskin
    Jan 15, 2020 at 13:22

2 Answers 2

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The possible outcomes from overloading a USB port range from temporary shutdown to permanent damage. I've seen cases where the USB 5V supply thrashes between 0 and 12-15V, causing damage to both the host and the peripheral. There's very little consistency.

USB negotiates current available, and USB-C has an analog alternative mostly for charger purposes. However, for the safety of both the host and peripheral, it's a good idea to provide a current limiting mechanism in hardware as well as an UVLO (since there are ports that don't make the spec as well as applications such as audio that can draw high peak currents unpredictably).

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Depends on the computer. I have a Lenovo W530, when the USB 5V line is overloaded, or driven by an external 5V source, it reports a surge current occurred and shuts down the port. Restarting the laptop is then required to restore the port.

I often work with FTDI modules on my laptop for serial comm's to external Arduino type boards, and have taken to adding a series diode from the external board 5V sources to prevent them from backdriving 5V into the USB port thru the FTDI connection.

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