I have an ipod touch and since then I've been charging it on the PC via USB. I just want to know if it is safe to use those wall socket chargers for USB devices? Is it different on every device, or is it safe to use it on any USB device?
What you do plug into the wall is a power supply of some kind -- a switched-mode power supply (for desktop and server computers) or a wall-wart (for laptops and rechargeable devices). This device has the job of converting the outlet's AC into some flavor of DC.
So you need to use a power supply that's appropriate for the 220v AC supplied by your site's wall outlets.
Charging over USB supplies 5v, 500mA DC. Any USB-charger wall-wart should output to that standard, and all devices that charge via USB should work, so all you need to worry about is whether the wall-wart can handle the input AC.
Normally, electrical characteristics such as acceptable input voltages (and amperages) and provided output voltages (and amperages) are printed on the power supply, so you need to literally read the fine print to make sure. If in doubt, it's probably safer to buy another one.
Apple's chargers are "universal," meaning they'll work with any country's AC power. All you need is an adapter to allow you to physically insert the prongs of the plug into the wall socket.
To determine if any given device is universal, look on the bottom or side of the unit. If something on the label reads "100-240v" or "110-220v" or something like that, it'll work. If, on the other hand, it reads something like "110v only", then it is not safe to plug into a 220v socket.
I just want to know if it is safe to use those wall socket chargers for USB devices?
Short answer: Yes
The usb spec tells us Vbus = 5VDC, and then the device can start with 0.1A (but the device "can ask" for as much as 0.5A).
The wall socked should transform 220VAC into 5VDC.
Please note that a lot of those wall sockets that short-circuit D+ and D- and then you don't have to negotiate how much current you can draw (and as much as 1A should be ok).
If you are unsure, just use a multimeter and check that the wall socket provides 5VDC.
As others have said, you need to make sure that the DC output of the charger matches the input requirements of the iPod, but even then it might not work. If possible get the charger from the manufacturer of the device.
To illustrate I'll relate my experience. The kids have got iPod Touches and were using the PC charger. We were due to go on holiday without the computer but with the iPods so they wanted a way of charging them. I investigated the options and went for a third party charger whose specs matched.
It arrived and we plugged it in. The iPod started charging then there was a little "pop" and faint electrical smell. The charger had fused. I contacted the supplier and they sent a 2nd unit out. We tried this one - there was a spark, a loud "bang" and a definite electrical smell. This one had nearly exploded. I contacted the supplier again and they refunded the costs.
I bought an offical Apple charger - which works quite well.
works for me. Probably you should check your device for 110-240 (or 220) written on it. Those tiny USB adapters that came with my iPhone in the US, work fine in Europe... I just need an adapter plug to be able to plug it into the socket.
Also I brought over from the states a Belkin "thing". dunno how to describe it, it is kind of like a brick with a bunch of power sockets. So like a mini powerbar, but with 2 usb ports also. That also works in Europe to power my iPhone dock and iPhone/iPod universal docks. After I plug it into the wall with a plug adapter.