Can A Computer USB 2.0 Port Run A Device Safely Above 500mA?

Thread Starter

Vegasbob

Joined Feb 18, 2019
15
According to on-line specifications, USB 2.0 ports on any computer motherboard are rated for 5 VDC at 500 mA. I have a device that normally runs at 5 VDC at 450 mA. but sometimes jumps to 5 VDC at 590 mA. The device runs perfectly fine even at the higher amperage. Can I damage my motherboard by running this device or would the device just quit working if I exceeded my motherboard's safe limits?
 

dl324

Joined Mar 30, 2015
8,849
Welcome to AAC!

The USB spec says a USB2 host can provide up to 500 mA, but anything above 100mA needs to be negotiated.

The USB spec also says that all USB host ports should handle overcurrent with a self resetting fuse. If the PTC fuse fails, that could disable that port.
 

dl324

Joined Mar 30, 2015
8,849
What do you mean by "needs to be negotiated."?
The USB specification states that all USB clients must start in a low power mode (one load - for USB2 that's 100mA) and negotiate for more loads from a USB host. If the request is denied, they must operate in their low power mode.

Any device that can't (isn't smart enough) negotiate for more than 1 load, it's considered a USB "decoration" and can't use more than 1 load.
 

Thread Starter

Vegasbob

Joined Feb 18, 2019
15
My device is powered by a USB step up converter which I guess would be considered a "decoration". The converter has only has two conductors for power. It draws over 500mA or 5 loads but has no way to negotiate with the motherboard.

I have been told a motherboard USB port can only provide a certain amount of power. If a device needs more than can be supplied, the device just won't work and will not damage the motherboard. Only way to damage the USB port is to short out the port. Can you tell me if that's true or false.
 

dl324

Joined Mar 30, 2015
8,849
the device just won't work and will not damage the motherboard. Only way to damage the USB port is to short out the port. Can you tell me if that's true or false.
In theory it's true, but the protection employed can fail. Best case, it just disables the port that was abused.
 
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