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July 3rd, 2024

USB and the Myth of 500 Milliamps

The article explores USB power delivery's evolution, from initial limits to USB-C PD's efficiency improvements. It discusses challenges, current limits, USB2 to USB3 transition, proprietary standards, and standardized mechanisms.

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USB and the Myth of 500 Milliamps

The article discusses the evolution of USB power delivery, highlighting the initial 500 mA limit and the challenges it posed for devices requiring more power. It mentions the enforcement of current limits on USB ports and the complexities faced by both device manufacturers and users. The narrative delves into the transition from USB2 to USB3, where the power limit was raised to 900 mA for higher-speed devices. It also touches on proprietary charging standards like Quick Charge for smartphones. The introduction of USB-C PD is emphasized as a significant advancement, simplifying power consumption by allowing devices to check a port's current capability. USB-C PD enables devices to consume up to 3 A from a port efficiently. The article concludes by discussing the elimination of proprietary smartphone charger standards in USB-C and the increased compliance in power consumption due to standardized mechanisms. The piece provides insights into the historical context and technical intricacies of USB power delivery, showcasing the advancements made to address power requirements effectively.

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Link Icon 5 comments
By @userbinator - 6 months
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...

By @burutthrow1234 - 6 months
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.
By @qiqitori - 6 months
> 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.

By @luyu_wu - 6 months
Shoutout to Arya who imo has written some of the most interesting articles on Hackaday. Was really interesting to read through!
By @aa-jv - 6 months
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 ..