If you are like me you will have accumulated surplus power adaptors for all kinds of things, modems, external hard disk drives, and so forth. Here's how to put them to use and avoid causing more 5V power adaptors to be made.
5V has become a common standard for chargers and digital circuits. On eBay you can buy buck voltage converters that take 6-24V input and output 5V at 2A or more for about a dollar. As a bonus they have a USB A socket on the board ready to receive a USB cable. They are small, only a little longer than the USB socket. You just need to solder the input wires onto the board (double checking polarity first) and it's ready to use. Here's one powering a Raspberry Pi 1B.
The primary current drawn depends on the input voltage and the output current. Assuming 90% conversion efficiency, the formula is:
So for a current draw of 2A and an input voltage of 12V, the input current is just under 1A.
One thing to be aware of: If you are using the input voltage for other purposes at the same time, the negative lines of the input and output are common, there is no isolation.