USB Current Draw - Page 1
Forum section: electronics / beginners

author: skinny
author_rank: Contributor
timestamp: April 14, 2016, 12:00:18 am
content: Hello Everyone,Lately I've noticed several products that get power from USB Y-adapter cables. At one end is 2 usb plugs and at the other is 1 plug. The two plugs at one end are placed into two usb ports. One of these two plugs do not have the data pins connected. The other end of the cable plugs into the device being powered. The thought is that if you have 2 USB 2.0 ports supplying 500mA each, this cable will allow you to pull 1A of current to power the device.I intuitively kinda get this. My question is what happens when one port, for instance a USB 3.0 port, can supply 900mA and the other port being a USB 2.0 port can only supply 500mA. Will the result be additive in nature and you end up with 1.4A to use or will the over demand on the USB 2.0 port somehow limit the overall current to something lower. When the ports are unequal, I can't quite wrap my head around it yet.Thanks for the help!Skinny

author: Kilrah
author_rank: Unknown
timestamp: April 14, 2016, 04:29:31 am
content: These were actually more common 5-10 years ago than now, when most USB 2 ports were not able to supply more than the 500mA they were supposed to. Nowadays given pretty much everybody misused ports that way nearly all manufacturers exceed the specs and support 1 or even 2A by design so the need for these kludges has reduced/disappeared.And if you have an USB3.0 port then it by spec can supply enough so you don't need it in the first place.You can't wrap your head around it. Is's something that according to specs shouldn't even work, it only does in various ways due to partial implementations and spec abuse so by definition YMMV depending on the actual machine you plug it to.

author: skinny
author_rank: Contributor
timestamp: April 14, 2016, 11:27:52 am
content: Thanks for the reply. I see how for a laptop things may not be implemented to spec so here's a more concrete example. I have a usb charger battery pack that provides 1A and 2.1A USB jacks. I have a USB device that requires greater than 12W of power. Because of that fact, the manufacturer included a Y cable. Will this USB charger be enough if I combine the 1A and 2.1A ports? Will they combine for 3.1A or is the limiting fact the 1A port? Thanks.

author: cowana
author_rank: Frequent Contributor
timestamp: April 14, 2016, 11:43:47 am
content: Most of the time, if you look inside the powerbank you will see that the '1A' and '2.1A' power jacks are connected in parallel.In fact, the only difference is the voltage on the USB's D+/D- pins which signals to the device how much current to draw. So if you used a 'Y' cable, it would be no different at all to using just one of the ports.

author: exe
author_rank: Unknown
timestamp: April 14, 2016, 12:27:27 pm
content: JFYI: the usb ports I've seen (including chargers) didn't want to provide more than 60-70mA without negotiation.

author: jeroen79
author_rank: Frequent Contributor
timestamp: April 14, 2016, 12:29:25 pm
content: A simple Y cable wouldn't know how to balance the current draw.

author: Ian.M
author_rank: Super Contributor
timestamp: April 14, 2016, 02:08:10 pm
content: Worst case: A standards compliant USB host port may be designed not to supply more than 100mA before enumeration. Unless the cable contains a chip to enumerate the auxillary plug as a HID device requiring 5 unit loads, + a MOSFET ideal diode circuit to combine the two Vbus feeds, it may have problems supplying 1A from a fully standards compliant host device. If you have that situation, you'll need to interpose a USB power injector or a powered hub that isn't strictly standards compliant.N.B. Do *NOT* connect the auxiliary plug to a separate power source!Charging ports are a whole different ballgame.