How many of those little Power Cube thingies do you have around your house? Here's how many I found around mine:
2. The cordless phone
3. The cell phone
4. The electronic anti-flea ultrasonic noisemaker (we have 4 dogs)
5. The digital camera
7. The video camera battery replacer/recharger
8. The digital clock in the bedroom
9. The electric toothbrush
10. The electric razor
11. The electric screwdriver
12. The electric drill
13. Both sides of the baby monitor
14. The laptop computer
15. The office phone
16. The ink jet printer
17. The speaker system on the computer
18. The tape recorder that records notes in the office
19. The calculator recharger
20. The 9-volt battery replacer for the radio
21. The radio shack univeral power cube
22 of them! (remember, the baby monitor has 2) And they are all different - they have different plugs, different voltage ratings and different amperages.

So let's take one of these apart and see what's inside. Here's the transformer we will be exploring today:

Under the cover

This transformer came with a rechargeable electric screwdriver. This particular transformer is rated at 3 volts and 240 milliamps. Once you convince the cover to come off here is what you find inside:

What you can see here are two windings. The purpose of a transformer is to convert one AC voltage to another AC voltage. In this case the transformer converts the normal 120 volt AC current in your house down to three volts. The 120 volts comes in on the primary winding on the left. Running down the middle of that winding (as well as around the outside) is an iron core. The AC current in the primary winding creates an alternating magnetic field in the iron just as it would in an electromagnet. The other winding, known as the secondary winding wraps around the same iron core. In the secondary winding the magnetic field in the core creates current. The voltage in the secondary is controlled by the ratio of the number of turns in the two windings. So if the primary and secondary windings have the same number of turns, the primary and secondary voltage will be the same. If the secondary winding has half as many turns as the primary then the voltage in the secondary will be half that of the voltage in the primary. You can see in the following figure that the primary in this particular transformer uses very fine wire while the secondary uses much thicker wire. To drop down to 3 volts, there needs to be 40 times more turns in the primary than in the secondary.

Turning the AC current into DC current

On the other side of the transformer you find two diodes wrapped in rubber insulation. The diodes act as a rectifier, turning the AC current into DC current.

Most transformer cubes that you find around the house produce a low-voltage DC current (3 to 12 volts, and less than an amp of current). DC current is necessary because rechargeable batteries store DC current, because most electronics require low-voltage DC current and because small DC motors run directly from batteries and are the least expensive motors available.

The big question remains, however - why can't somebody standardize all of these little cubes?!