I just yesterday wrote about commercially make USB soldering iron product. But how building a DIY USB soldering iron?
USB Soldering Iron article shows how to convert an inexpensive battery powered Soldering Iron into a slick USB powered soldering iron. The conversion idea is to cut USB cable and wire it to the battery contacts inside the iron (that is normally operated with 4.5v battery power).
USB Soldering Iron – Powering Stuff w/USB Part 1 article shows another similar conversion project from battery power to USB power.
Converting the HobbyKing Battery Soldering Iron to use LIPO or USB video shows clearly how you can convert a battery powered soldering iron to use USB power or LIPO battery.
This looks interesting project because I have years old Biltema 20-388 battery powered soldering iron, that looks pretty much same as the soldering iron on the video (my soldering iron is just colored red). The specifications for this soldering iron I have is 6W power and 4.5V operating voltage. Looks like a good candidate for modification.
Still one issue to think about: The soldering iron tip on this kind of soldering irons is in direct contact with negative power terminal. This means that if your USB power source has any considerable leakage current (like on some laptops and some mobile phone chargers), that can potentially damage sensitive electronics that you try to solder. So you have been warned.