I have some USB 3.0 ports on my motherboard which can deliver up to 900 mA of current I believe. If I plug in a USB 2.0 cable, I can still get up to 900 mA right? I have some devices that uses USB charging and outputs from 1A to 2A but I'm not sure if the cable provided is a factor or not since I'm just using a generic USB cable for this.
65 Answers
If the output is only 900 mA, your device will still charge, albeit, a little more slowly.
USB 3.0 is designed to be backwards compatible with USB 2.0, and for this, I believe it is. See everythingusb.com and the Power Requirement Comparison table (from Buzzle.com for details.
4I think the answer you are looking for is all about the cheap cables some companies supply, and if they can cope with carrying the full 900mA.
No, not all cables are capable of carrying the full 900mA. It will depend on what size the wires are in both diameter (gauge) and length. The length is seldom considered when using cables. If you try to draw 900mA down a short cable then you may be able to, but if you try to draw the same 900mA down a 3 meter cable then most likely you will fail and it could even cause a fire by either the connector or the cable getting hot.
Some of the makers of USB cables try saving money by using wire so small that it cannot cope with the extra 400mA difference between the 500mA of a USB2 and the 900mA of USB3.
If you do go ahead, then make sure you test to see if the cable or plug gets hot after using it for a while. 10 to 15 minutes at least, and do not just plug it in and then come back after the time is up - make sure you keep an eye on it for the full duration. I personally keep one of those laser thermometers next to me to check the temperature of the wire and plugs before starting the test, and the temperature as time passes.
0WHY is this so difficult? Because people keep talking about the SPEC and not the hardware! The spec refers to how much power a USB device CAN use and therefore how much power the USB connector must supply. USB2 is 500ma, so all USB2 connections supply 500ma. USB3 is 900ma, so all USB3 connections supply 900ma (some may supply more to comply with the charging spec). HARDWARE wise, the power pins on a USB3A or B connector are the SAME pins as on a USB2A or B connector. The USB3 connector simply adds 5 pins which are +Rx, -Rx, +Tx, -TX, and GRD/DRAIN. Notice, not more power pins. SO, if you plug a USB2 device into a USB3A or B connector you can draw up to the limit of that connection.
What this means in real life. A USB2 device will charge faster in a USB3 connector. A USB2 portable HDD will probably work without the need to plug in both A connectors or to add an external power source. If the USB3 port supports charging currents, you could probably use a USB2 4 port expander without an external power source. If you plug a USB3 device into a USB2 socket, it may or may not work depending on how much current the USB3 device needs. If it is labeled as backward compatible, then is must draw 500ma or less.
PLEASE note: Current or AMPS or ma is the amount of current AVAILABLE. How much current the device DRAWS is a function of the device. You could have 10,000 amps available, and no harm will come to your device because it only needs 500ma and that is all it will draw - unless of course your DEVICE is broken, then it might be a pretty fireworks show - or not so pretty. VOLTS on the other hand is the pushing force of electricity. Applying 120V to a 12V device will push the electricity clear thru the device and out the other side taking no hostages. This is where you research Wall Wart VOLTAGES. 12V on a 5V device is not good.
I hope that helps
Can a USB 2.0 cable support USB 3.0 charging?
USB 2.0 and USB 3.x (3.x includes 3.0, 3.1, and 3.2) have the same charging capability. What muddies the water is that some ports support USB-BC and USB-PD.
The current provided by USB 1.1, 2.0, and 3.x breaks down something like this...
100 mA - The maximum allowed by a USB 1.1 or USB 2.0 device before being "enumerated", that is being recognized by the host as connected. After this the device can request more.
150 mA - The maximum pre-enumeration current allowed by USB 3.x.
450 mA - Minimum allowed current supply by a host to meet USB 1.1 spec.
500 mA - Minimum allowed current supply by a hub to meet USB 3.x spec.
900 mA - Minimum allowed current supply by a host to meet USB 2.0 and USB 3.x spec, maximum current device can draw under USB 1.1 spec.
1.5 A - Maximum allowed current draw by a single device under USB 2.0 and USB 3.x.
But it doesn't end there. With the trend to people using USB-A ports as a means to charge batteries on portable electronic devices there's there's a means by which a USB-A port on a computer, hub, or charger to supply more than 1.5 A.
1.0 A - USB-BC minimum allowed current from a charger to meet spec.
1.5 A - USB-BC maximum current draw from a host.
2.4 A - USB-BC maximum allowed current for a charger, some computers will source this much current using USB-PD. There are some older USB-BC specs that had lower maximum allowed current than 2.4 amps, there are few chargers built to such spec in circulation in my experience.
3.0 A - USB-PD maximum allowed current for USB-A, but few ports on computers and chargers are built to supply this current, at least to my knowledge.
Because USB 2.0 and USB 3.0 use the same voltage and current for power supplied to any device no device will charge faster under either.
But...
When there's USB-PD involved then a computer will supply 2.4 amps to devices, but not all host and devices support this. As other people answering have pointed out the actual current draw depends on the device. A well behaved USB device will ask the host how much current it can draw and then draw no more than given permission to draw. Not so well behaved devices will simply take however much current it likes.
USB 2.0, USB 3.x, USB-PD, and USB-BC are independent standards. By that I mean a USB-A port can support any one, all, or some mix of them. The exception to that is USB 3.x must support USB 2.0, going forward though I would not be surprised if USB 2.0 becomes optional under USB 3.3 or something.
While USB 3.x and USB-PD are independent specs there is a trend for a host that supports USB 3.x to also support USB-PD. Newer computers will have USB 2.0 "charge and data" ports built to include the USB-BC and/or USB-PD and therefore supply more power than any USB 3.0 port that doesn't support USB-PD or USB-BC.
If I plug in a USB 2.0 cable, I can still get up to 900 mA right?
Yes.
I have some devices that uses USB charging and outputs from 1A to 2A but I'm not sure if the cable provided is a factor or not since I'm just using a generic USB cable for this.
All USB cables built to spec will handle the current. If you have some old USB 1.1 cables, or cables not built to spec (indicated by a lack of the USB trident icon on the cable ends) then they can be unreliable in making a connection.
Clear as mud now?
No, it can’t because a USB 2.0 cable is different than a USB 3.0 cable, and both the cables have different outputs. For example, USB 2.0 delivers 500 mA and USB 3.0 delivers 900 mA.
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