Just got a cheap Archos MP3 player and it connects and charges with the USB on computer etc. I have a Samsung Digi cam that came with similar cable , but also a mains charger. Am I right in thinking I am able to use this mains USB charger to charge the MP3 rather that the computer all the time?
Many Thanks for any advice.

If the plug that goes into the MP3 player and the digicam are identical size, then you should have no problems, as USB charging is based around a standard voltage and a standardised maximum current. Before actually attempting to charge anything with the other cable, be sure that both plugs go into both devices to ensure they are identical, as there has been a number of slightly different sized mini and micro USB plugs used in the last few years.

Edit: the above info is given in good faith but I cannot guarantee it is correct, and accept no responsibility for any damage which may result if you follow it.

Voltage and current!

Similar/same voltage and same current are important.
The current certainly has to be the minimum required and only more if the adapter is 'regulated'.
Unregulated charger/adapters means the voltage dangerously varies according to current usage

Look on the charger, it should say 5V DC 500mA. That's the USB standard, so if it says that and has a USB plug on it, you should be fine.

I don't know if it's covered by the above, but I read that some cheap and/or badly made mains-USB chargers don't convert the current from AC to DC.

Thank you all. Just checking the Samsung plug and output is 4.5v 400mA/ However, upon checking have noticed the camera connector is slightly bigger. So will err on the cautious and just use the computer .
I presume all the car cigarette chargers would pose no problems with the mP3 player.
I really should have stuck to my CDs !

Definitely stick to using the computer to charge it then, as this is a classic example of two small plugs which at first sight look the same, but are in fact slightly different sizes (I've got lots of them, thus my suggestion you try plugging both of them into each other device without the other end connected to anything to ensure they are actually identical size).

Thankfully this plug silliness should be at an end now as a standard micro USB charging plug has been agreed by pretty much everyone for future devices.

If you really want to charge away from the computer, you can get mains chargers with USB sockets on them that provide the same power and voltage as a standard USB socket.

I sometimes charge my MP3 player from the USB port on the Sky+ box too.

I have a plug in mains USB charger plug but my Creative ZenV+ MP3 player will not charge from it. Not from another charger plug a friend has. Is this a Creative issue? Would a firmware upgrade help? I am loath to do that as my old Creative MP3 player was bricked (or should that be pebbled) when I did the upgrade.

The trouble is everyone is bending the usb standard to suit their own needs. Even if the transformer brick has a usb socket, that does not mean that it can be used for any usb-charged device.

I have a few at home which range from 4.5v (like the OP) to 5.2v, with massive variations in the current, and it just makes a mockery of so-called standardisation.

For instance my iPad charger will not charge my Blackberry, something I found out at the beginning of a 10 day business trip. I presume the ratings are fixed voltage and maximum current, so the high current capability of the iPad charger shouldn't be the problem, and 5.2v compared to the 5v usb spec shouldn't cause any problems either.

The 4.5V to 5.2V range should be a non-issue as that is all within the rated spec of USB charging. It is preferable if the charger sticks as close as possible to a regulated 5V though (4.75-5.25V in practice) over the current range it is designed for. The maximum current capability of the charger shouldn't matter either provided the voltage remains within spec at much lower loads.

Provided the charger is also able to let the device know it is connected to a USB charger (over the data-lines), then I see no reason why the iPad charger wouldn't charge a Blackberry (unless the Blackberry is specifically designed to only work with its own proprietary chargers that send specific responses over the data-lines, which is the sort of thing that helps nobody in the long run).

I look forward to the day when one charger does them all, unlike today where I have something like three chargers for various larger devices serving quite different purposes, seven for various other gadgets all computer related, and nine for mobile phones most long dead. One charger to do them all, one charger to simplify them, one charger to combine them all would be an awful lot more convenient.