Little Things Mean a Lot

Published in IEEE Spectrum Magazine, July 2015

The most complex electronics that has a presence in households is undoubtedly the smartphone, which epitomizes the brilliance and wonder of current electrical engineering technology.  The depth of complexity in its circuitry, software, and algorithms is such that I doubt that any single engineer understands its entirety. However, today I write in praise of the simplest piece of electronics in households -- a little gadget that is both ubiquitous and unnoticed. It is something that is unappreciated until you forget it take it when you're traveling -- it's the little AC adapter that converts household AC to the five volts that keeps your other electronic gadgets happy.

There are at least a half dozen of these adapters with their connecting cords tangled at my feet right now.  More are scattered around the house, and upstairs there is a box of older adapters that I'm afraid to throw out.  I have no idea what they connect to, and maybe I'll find that I need one and that it is irreplaceable.  All different kinds of connecting plugs are represented, and few of them are labeled with any information that indicates what device they are supposed to support. Still other AC adapters are embedded in electronic equipment such as televisions and audio.  Moreover, there are a lot of other AC adapters around me that are hidden in the bases of the LED light bulbs.

The older adapters are mostly heavy wall plugs -- the kind that use up your sockets while blocking adjacent ones. They use sizeable transformers and are probably linear power converters.  The newer adapters have gotten smaller and smaller, and the newest ones -- those that are associated with the present generation of cell phones -- are no bigger than standard AC plugs.  Obviously, they use switching supply technology.

The appeal of the AC adapter to me as an electrical engineer is that it is a return to the roots of engineering.  So many of us today work at much higher levels than is represented by the simple circuitry of AC power conversion.  No one could design a cell phone from scratch.  There is a hierarchy of complexity where an engineer works to interconnect existing modules of software and hardware to effect some new functionality without ever having to understand the detail at the lower levels.

In contrast, the circuitry of an AC adapter looks like a diagram from an EE101 textbook.  There is a full-wave rectifier whose output is chopped by an oscillator to produce a higher-frequency wave, which is then regulated and converted through a split transformer that serves the purpose of both energy storage and isolation.  A feedback circuit adjusts the chopper to produce the correct output voltage irrespective of the input AC voltage.  It's actually a little complicated and a detailed analysis involves many issues familiar to an engineering education, but perhaps seldom needed in modern design. There are obvious trade-offs between size, cost, and capacity, but also design alternatives -- chopper frequency, transformer size, output ripple, efficiency with and without load, risk of core saturation, electrical noise, and so forth.  Lots to chew on here.

I'm really tempted to cut open one of my new adapters, but I'm afraid of what I'd find.  Maybe there wouldn't be anything inside at all.