USB and the Myth of 500 Milliamps

The standards are, as usual, "minimum" requirements; a standard USB port should be able to supply up to 500mA after enumeration and 100mA before, while a device should not draw more than those values respectively. However, since it costs literally nothing for manufacturers of hosts to allow more, they have gone up to several amps. The situation is trickier on the device side, since it does cost more to limit consumption (and it might not even be possible for some types of devices.)

I came across this interesting thread a while ago with lots of technical details (and the usual trolling and arguing), where it seems manufacturers never went below allowing 1A from PC USB ports:

https://www.eevblog.com/forum/projects/simplest-way-to-get-5...

One of my favorite intern projects was cutting up power traces on USB hubs and connecting them to a giant bus bar so we could put multiple amps through them without tripping fuses on the host machine. Testing some very non-compliant hardware.

> If you’re designing a universal port, you will be expected to provide power. This was a lesson learned in the times of LPT and COM ports, where factory-made peripherals and DIY boards alike had to pull peculiar tricks to get a few milliamps, often tapping data lines. Do it wrong, and a port will burn up – in the best case, it’ll be your port, in worst case, ports of a number of your customers.

I'd never thought about this. Way back then, mice connected via the serial port. It seems like they got their power from the RTS or CTS pin. And that pin isn't 0V/5V, it uses negative and positive voltages, -5V/5V on the low end, more likely -12V/12V according to stuff floating around on the internets. Seems complex. Might research this properly later.

Shoutout to Arya who imo has written some of the most interesting articles on Hackaday. Was really interesting to read through!

I currently build commercial and b2b audio products, and have had to explain far too many times, that .. yes indeed .. there are only 2 USB ports on your laptop which will work properly with the device .. and yes .. I know you have 5 USB ports .. only 2 of them will properly power the device .. yes .. the specs do state you need a powered hub, most of the time .. grr ..