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?

 

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?

Have you read the USB 2.0 specification? (an excellent tutorial can be found here). USB 1.0 and 2.0 can supply a max of 500mA.

 

Use USB Y cable. It will have two male ends and one female end.

 

Most USB ports on computers seem to be rather forgiving, I think I may have shorted the 5V on some of mine a couple of times, and at the very worst I had to restart to get the USB port to come back online. I think mother boards have pretty good protection circuit on them. and they also have some sort of +/- tolerance for the 500mA limit. So I would be brave and just try it!
As shteii01 says though, the common way round this is to have a USB splitter, so you take power from 2 USB ports rather than one. This is / was very common for things like external DVD burners and some external hard drives.

 

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.

Did you make this measurement under load or open terminal?

 

Did you make this measurement under load or open terminal?

No load. However, the low voltage, 4.6V, is from an add-on USB 3.0 card.

 

Another reference. Your results may vary. So while it may work on your computer, it may not work on the next. This is important if you may use different computers or wish to share or sell your design.

If your USB Host is a computer running Windows, one of the replies in the reference to which I linked, shows how you can determine the maximum and current current draw of your device.

Using a powered USB Hub between your computer and your device, may also resolve your problem...

Additionally, if your computer has an USB 3.0 port, this may be a solution. These ports are identified with a lightning bolt above the computer USB port. They have a 900mA limit.