There are several USB 3.1 ports in my mother board. According to specification USB 3.1 can handle up to 100W. What actual power PC motherboard can output? What if I connect 8 USB 3.1 with 100W (800W in total) power while power supple can output only 400W?
Is it safe to use high consuming device with week power supply?
33 Answers
USB can only handle 100W if the port is a USB Power Delivery Port. Power Delivery is an "extension" to USB, it does not state that every port supports it.
These ports are typically only set up on USB Chargers that are suitable for charging laptops or other devices. It is not supported by default by every port.
The maximum output of a "normal" USB port as would be fitted to a computer is 5V @ 900mA. See Wikipedia USB_3.0: Power and charging
USB 3.0 ports may implement other USB specifications for increased power, including the USB Battery Charging Specification for up to 1.5 A or 7.5 W, or, in the case of USB 3.1, the USB Power Delivery Specification for charging the host device up to 100 W
You will almost definitely not be able to use a computer to charge a laptop or other high current device.
3Either your motherboard will block an unsafe condition, or your power supply will suffer. If your PC was bought in a store and you didn't upgrade it afterwards then the system is designed NOT to over load the power supply.
The motherboard manufacturer programs the USB system to have maximums that fall within your motherboards ability to handle it.
We would have to either contact your motherboards manufacturer or read the manual to see what is allowed.
It is likely that only 1 or maybe 2 ports will actually support 100w.
Further the motherboard manufacturer has a stated recommended power supply size, and not having that could melt the wires on the power supply.
So if you have a 400w power supply something approx 1/2 of that isn't for 5v. So maybe 200w of that is suitable for 5v. The other is 12v, now if your USB device is 20v at 5a then the motherboard up converts the 12v, and draws even more amps in the process. This assumes the motherboard even has the circuity to do this. At 12v to 20v at 5amps will result in a 9-11 amp draw from the 12v system.
Power is not magically generated. You can not draw more power from the power supply that it's providing without breaking it.
If your motherboard did allow this, which it probably doesn't, the minimum recommend power supply for it would be more like 800w or more.
There is a somewhat complex means in which USB devices draw power, and two ways in which a computer protects USB ports from damage due to devices drawing too much power.
The chances of damage to your computer or ports because of too much power draw are small because your computer has circuit breakers or fuses, much like you see in a home's 120 volt AC distribution panel only much smaller. Unlike a circuit breaker in a 120 volt distribution panel the circuit breakers inside your computer are made to reset themselves, they don't have a handle to flip but instead removing and restoring power will reset them. Like any circuit breaker they can sometimes fail to act swiftly enough to prevent damage, but they are better than nothing.
The circuit breakers are a brute force means of managing power, they limit power by just turning everything off. That's a good way to prevent damage but also a good way to damage valuable data.
The primary means USB power is managed is with "bookkeeping" and each device asking nicely for power before taking it.
USB devices are allowed 5 volts and about 100 milliamps of current, a minimum allowance that has changed over time. Low power devices like a mouse or keyboard stop there. Devices that want more power can use USB 2.0, USB-BC, USB-PD, or some other protocol to ask for more power. If there is not enough power available then the request is denied and, presumably, the device will respect that denial. This is safe because the USB host will know to keep a reserve in the bookkeeping for new devices to make these requests. There will not be enough ports to run out of power by plugging in devices that take this small amount of power without asking first.
The USB ports on a computer must be able to supply some minimum amount of power to meet the USB spec. USB 3.1 requires at least 7.5 watts, allows up to 15 watts, and anything beyond 15 watts requires use of some other protocol like USB-PD. If the ports don't meet this spec then they can't claim to be USB compatible. Failure to comply means the manufacturer gets drowned in lawsuits. There will be wailing, gnashing of teeth, rending of garments, really biblical proportions of legal troubles that leave all that bear witness to be left in madness to the point of being unable to speak. Well, maybe not exactly like that.
One thing that prevents too much power from flowing also is that there's only 5 volts at the port unless a request to raise the voltage is granted. This voltage is too low to do much harm even if there is a short circuit. The USB spec allows 100 watts supplied only at 20 volts, if there's not 20 volts then there's not 100 watts. If something tries to take more power anyway then the circuit breakers open and there's no voltage. No voltage means no power.
If you plug in a laptop that would charge from a 100 watt USB-C power supply into the USB-C port on another computer they will "chat" with each other for a split second, the request for power is denied, and you might get a note on the screen that no power is flowing. Or there's a problem, lawyers get involved, and masses of people are left naked, mute, and catatonic in their wake. Or something.
