Ken Shirriff's blog: Tiny, cheap, and dangerous: Inside a (fake) iPhone charger
http://www.righto.com/2012/03/inside-cheap-phone-charger-and-why-you.html
Stay away from super-cheap AC adapters built by mystery manufacturers. Spend the extra few dollars to get a brand-name AC adapter. It will be safer, produce less interference, and your device's touchscreen will perform better.