If your system doesn’t need to be portable, this is the most obvious choice. What comes down the pipe is AC and far too high a voltage to be of immediate use to a digital system. It must be converted to a DC voltage of significantly lower magnitude. There are plenty of solutions for doing this. You can use DC lab power supplies, standard PC supplies (probably overkill for your needs), or AC adapters. The last of these is probably the best choice for most applications.

AC adapters (also known as plug packs or sometimes power bricks) are the little black boxes that come with your cell phone and a host of other appliances. They are a cheap, easy, and reliable solution and can be purchased from any good electronics vendor. Typically, they will provide an output voltage somewhere in the range of +5VDC to +12VDC and can supply a current of up to 500mA, depending on the particular plug pack. Choose one that can supply an appropriate voltage and current for your system. One caveat with plug packs is the polarity of the connector. Some plug packs have the positive voltage on the center of the connector jack and ground on the outside. Other plug packs have the exact opposite arrangement! Not knowing which you have could lead to disastrous consequences for your embedded system. As always, check the technical data. A better way is to incorporate a bridge rectifier as part of your design (Figure 3-1). The input power is DC, but the polarity of the connection makes no difference. The embedded system uses the output of the rectifier as its power source and has internal voltage regulation. (We’ll discuss regulators shortly.)

Figure 3-1. A bridge rectifier makes an embedded system “polarity proof”