[SOLVED] USB 2 Port Max Amperage

[VegasBobby]

I have a device that operates at 5V and about 500mA. The computer USB 2 port I want to plug the device into only has a voltage of 4.6V. If I plug my device into this port, my amperage would need to increase to 543mA to obtain the same power or Watts. Would this happen? That is, would my USB 2 port provide 543mA or would my motherboard restrict the amperage to 500mA? Could this cause my USB port to fry even though it's providing exactly the same power? Also, for design work, what percent of the rated amperage should one design to? Is 90% acceptable?

 

[clutchc]

I wouldn't chance it. There's something wrong with your system if it is only providing 4.6V. How did you test that? And what kind of device are you wanting to plug into it?

 

[VegasBobby]

I tested the voltage using two different digital volt meters. I figured not supplying 5V was common. Maybe I was wrong. I'm using the port to power a 5V to 12V buck step up converter that runs an LED light.

 

[InvalidError]

USB ports typically don't cut off at 500mA, they have margin beyond that for inrush, tolerances on the polyswitch or e-fuse, noise, etc. which usually pushes the cut-off well beyond 600mA when over-current protection is actually present.

BTW, USB3 has bumped the baseline current limit to 900mA. I wouldn't be surprised if most boards today use that or higher for all ports to reduce the number of different parts on the board.

 

[compprob237]

The USB spec for low-power devices (up to 100mA draw) is 4.4v-5.25v.
The USB spec for high-power devices (100mA-500mA) is 4.75-5.25v at the upstream connector.

So, with no load 4.7v is within tolerance. You would have to apply a 500mA load, after enumeration permission, and then measure the upstream connector's voltage to get a proper reading. It should be within the above high-power device 4.75-5.25v.
Maximum inrush is 50uC (usually covered by a 10uF cap).

Many older USB devices (both ends) that are not aware of BC/USB Battery Charging 1.0/1.1 (1.5A) or BC 1.2 (5A) will be limited to 500mA internally regardless of the device, port, and cable's current handling.

 

[InvalidError]

You would have to apply a 500mA load, after enumeration permission, and then measure the upstream connector's voltage to get a proper reading

While devices shouldn't draw 500+mA from the port before enumeration and getting ok'd by the host in theory, there are countless controller-less devices that do exactly that. I bet there would be outrage if USB power limits became strictly enforced overnight from the number of dumb USB devices (USB lights, power banks, fans, etc.) that would no longer work.