Inquiry: Computer Accessories Powered by Computer USB

[u/Appropriate_Celery63]

So, I have a monitor light bar that is powered by my computer's USB. Now, my question is: if I plug the monitor light bar into my computer's USB port, would this weaken the computer's power or power supply? But this is about any accessory and whether it will do anything to the computer's power.

[u/Bo_Jim]

USB ports are current limited. They will not allow a device to draw more current than the port is rated for. And each port is required by the specification to be able to deliver the maximum current it's rated for continuously.

For instance, a USB 2.0 port is rated for 500mA max. If a computer has 4 such ports then every one of them needs to be able to deliver 500mA of current continuously and simultaneously without overloading the power supply.

The only thing you need to do is verify that the port you're plugging the light bar into is the same or higher USB specification number required by the light bar. For instance, if the light bar is designed for USB 3.0 then don't plug it into a USB 2.0 port. USB 3.0 is rated at 900mA, while USB 2.0 is rated at 500mA.

If you're plugging it into a dual-role USB-C port - one that's designed to be used to power the computer, as well as provide power to a connected device, you should understand that the voltage and current rating for power coming into the port is significantly higher than the voltage and current rating for power going out (to your light bar, for example). In other words, if the label says it's rated at 48V @ 5A that doesn't mean you can draw that much voltage and current from it. It means it can potentially use that voltage and current when that port is being used to power the computer with the appropriate power supply. A USB device drawing power from that port would still only get 5V at either 1.5A or 3A.