USB 2.0 max current - Page 1
Forum section: electronics / projects

author: ANTALIFE
author_rank: Frequent Contributor
timestamp: October 18, 2024, 06:11:59 am
content: Could someone familiar with the USB standard please tell me if the 500mA current limit for USB 2.0 is the absolute maximum, or does USB 2.0 support scenarios where say you can draw 750mA for a few ms?

author: uer166
author_rank: Super Contributor
timestamp: October 18, 2024, 06:17:07 am
content: It should be 100mA when initially connected, and 500mA after negotiation. I don't believe USB 2.0 supports more than that on its' own. For more, look up BC 1.2 standard for dedicated USB type A chargers (not really revant here probably).

author: wraper
author_rank: Unknown
timestamp: October 18, 2024, 06:21:48 am
content: 500 mA is a default stadard. However most computer ports these days actually support more. I suggest having enough bulk capacitance that can store required energy. Be aware that USB allows 10uF maximum directly on VBUS, so there should be a charging current limit.

author: Phil1977
author_rank: Frequent Contributor
timestamp: October 18, 2024, 06:48:31 am
content: If you pull 750mA only for a few ms and then return to something below 500mA then it´s out of spec, but will probably work anyhow. The overcurrent protection of USB-ports is not that sensitive and such a short current spike will easily be buffered by the caps.But why can't you use an own cap for this? As wraper said, there is this limit of 10uF for direct bus connection, but there are many ways to make it less direct.Another solution could be to use a USB3.0-port for the USB2.0 device. USB3 is specified to 950mA, so even if it´s still out of spec because it may not be correctly negotiated, the port should physically be able to deliver this. If you have a given USB2.0 port then you can connect a powered USB3.0 hub.Then there also are these Y-cables that combine the power output of two ports into one. Absolutely out of spec, definitely not recommended to be used, but if you need a quick and dirty solution, why not?

author: RoGeorge
author_rank: Super Contributor
timestamp: October 18, 2024, 08:34:45 am
content: Some USB ports have polymer fuses on the Vcc. These are considered re-triggerable, in the sense that they recover after an over current, however, they do not fully recover, after each overcurent their internal resistance increases. So, pulsing overcurrents from USB Vcc might apparently work at first, then fail with time.If you only need short pulses of more than 500mA, maybe charge slowly a local capacitor, or a supercapacitor/rechargeable battery, at less than 500mA, then use the stored energy for pulses above 500mA.

author: ANTALIFE
author_rank: Frequent Contributor
timestamp: October 24, 2024, 05:13:03 am
content: Thanks everyone, yea 500mA is the absolute limit according to the USB 2.0 standard:

author: coromonadalix
author_rank: Super Contributor
timestamp: October 25, 2024, 02:31:29 am
content: in some cases i've seen popups warning about usb overloads ... you had to remove "the problems"and after a while it resetted by itself ?

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 25, 2024, 03:57:10 am
content: If you want a quick way to check if the device is plugged into a USB 3 port, check if the USB 3 ground actually connects to ground, it will be left floating when connected to USB 2. (That trick won't work with USB-C, sadly.)