Why is there an extra USB connector on USB connection cable?

By Gaurav003 ·
I have a transcend optical drive that i use for reading and writing DVDs with my laptop. The connection cable is a USB type A-B cable. However, at the Type A end there is an extra provision for another Type A connection with a very short length of cable (about 20 cm). what is this third connector for?

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USB Connector

by avlwiz In reply to Why is there an extra USB ...

According to the manual the second USB connector provides additional power for the
CD/DVD Writer. Please make sure to connect both USB connectors to your computer???s USB ports.

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The second cable is unnecessary here...

by dldorrance In reply to Why is there an extra USB ...

Mine works just fine with the longer cable only plugged in. A second (extra power) cable might be necessary with a 3.5 inch HDD, whose motor requires more current.

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its for additional power

by markp24 In reply to Why is there an extra USB ...

when running/powering (with out an external ac adapter) the CD drive (or any device) from the usb (usually 1.1 will need the second usb connection)
the usb will provide power. The usb y cable is to use one port for a data/power connection and one port solely for power with a bus powered USB drive. Many laptop USB ports are not "powered", they may provide insufficient voltage for the USB bus powered drive. This allows you to draw power from two ports and generally this will be enough for both.

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plug both in, and use it anyway . . .

by Who Am I Really In reply to Why is there an extra USB ...

when reading a disc the power draw isn't that much and it may function just fine on the single plug,

but if you go ahead and try to write a full CD or DVD
you'll find that the extra power connector is required
writing requires anywhere from 1/3 to 2x more power than reading does
and the burn will likely fail to complete if the secondary power plug is not used


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Not more power, more current :)

by robo_dev In reply to Why is there an extra USB ...

It's all 5Volts,just double the current. The standard limit is 500 mA per USB port, while some old hard drives need more juice than that.

Note that hardware damage can result if you plug a high-current device into a standard port. While USB is supposed to detect over-current, and not supply power, I've seen some USB adapters make an acrid burning smell followed by them no longer working.

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Reponse To Answer

by markp24 In reply to Not more power, more curr ...

yes you right (mr technical... LOL) details details...LOL

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Reponse To Answer

by Who Am I Really In reply to Not more power, more curr ...

who cares

from a users perspective it's "More Power"
it even says this in user manuals

the second plug is to supply more power

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It never hurts to provide a teachable moment

by robo_dev In reply to Why is there an extra USB ...

In my opinion, more people need to understand the basics of electricity, such that connecting two 5V 500 mA USB connectors in parallel does not give you 10 volts, it gives you 5 volts at 1 Amp (5 watts).

In theory a 12 volt power supply could supply the same amount of power (5Watts), but the higher voltage would most likely burn out the interface and destroy any attached peripheral.

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Also something that has not been mentioned here

by OH Smeg Moderator In reply to Why is there an extra USB ...

The second USB Plug is often only a Power Plug it has no Data Connections inside it.

if you look at the actual Cable you'll see that the Second USB Plug has a thiner cable it only has the leads for the extra Power.

So if you where to plug in that plug the drive would spin up but not actually appear on the Monitor.

Also if you look at the specs of your computer you may very well find that it doesn't actually provide 500 Milliamperes at the USB Sockets. Modern NB's have reduced the Available Current at the USB Ports to prolong Battery Life. So while it may not apply to your current computer it could very well apply to any other computer that you plug the drive into. Of course if you where to use a Powered USB Hub things would be different but even then I wouldn't be trusting what should be happening as what actually happens is quite often different.


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Go to the source - the USB specification

by Realvdude In reply to Why is there an extra USB ...


Many responses have partial right answers. One thing to keep in mind about USB power, is that there is a process between the requesting (device) and supply (host) side of things. Therefore and high current device will not draw more than can be supplied and conversly the device will know if not enough power can be supplied. That is supposed to keep both devices safe.

Now for that second connector, and other devices that may cause problems:

If you want something more confusing, check out Power Over Ethernet.

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