I picked up a store-brand DVD writer at the electronics store in the mall over the weekend - it’s one of the ones that plugs into two different USB ports on your computer to pull power. Burned nearly a dozen DVD+R disks, seems to work fine.
But a friend who I chat with sometimes told me that I was courting damage to the USB ports or even the motherboard, that these disks try to pull more current than the bus is designed for or something.
What say you, dopers? Is this something that I really need to be seriously concerned about?
For older (several years older, as in prior to USB 2.0), that was a possibility, where the USB power was direct connected to the system +5VDC supply. With USB 2.0, a properly designed USB port should power manage itself, and only allow a maximum of 500mA to be drawn (each ‘unit load’ for USB is defined to be 100mA max, and a USB 2.0 port is supposed to directly support 5 unit loads).
USB 3.0 supports 150mA per unit load, and up to 6 unit loads per port, for a total of 900mA total per port.
Some PC’s also have dedicated charger ports, which support no data, but can provide 1.8 A max output.
The USB connector itself is rated at 1.5 A continuous power over it’s +V and 0V terminals.
If the DVD writer has two plugs, that means that it probably draws more than 500mA at full power, and could potentially overload the driver circuitry. You would have to check the spec of the actual DVD unit. With proper power management, it won’t use more then is available when connected to USB 2.0 or 3.0 ports. Increased demand is normally only during writes, and disc spinups and spindowns, and is averaged out.
For a branded device, I wouldn’t worry. For a no-name DVD burner, I’d be a bit more concerned.
The USB spec very clearly defines how much power you can draw from a USB port. Properly designed USB ports also should have some sort of overcurrent protection built into them. All of this has been built into the USB standard from day 1.
The problem is that a lot of manufacturers don’t bother to follow the standard. You will find some motherboards (even brand new modern ones) just hook the 5 volts from the power supply up to the USB ports with no current protection at all.
Other motherboards will allow a certain amount of current (which may or may not conform to the USB spec for max current) and have protection of some sort on them, but are also put out a certain amount of total current per root hub, and not per port on the computer. You may have several USB ports all connected to the same root hub. In these types of motherboards, using two separate USB ports to draw more power doesn’t accomplish anything because it ends up coming from the same source on the motherboard either way. This is the type that can get you into trouble with the type of DVD drive that you have.
As USB devices are getting more and more popular, and especially with higher current USB devices becoming more and more common, these cheaper USB implementations seem to be less and less common, so your DVD drive is probably going to work just fine. There is no way to know for certain though without knowing the design of your motherboard.
Note that your DVD drive is not the problem here. The problem is cheap and not quite to spec USB implementations on some motherboards. Go into the device manager and count the number of USB root hubs that you see. If this matches the total number of USB ports on the outside of the computer then it is much more likely that each port can supply the full spec USB current and you won’t have a problem. However, even if several of the external USB ports connect to the same USB root hub on the motherboard, that doesn’t necessarily indicate a problem.