Small wall warts, anyone?
Brad Thompson, Contributing Technical Editor -- Test & Measurement World, 4/1/2001
About the time California suffered its first bout of rolling-power blackouts, I experienced my own power crisis in the form of a thrift-store Hewlett-Packard DeskWriter inkjet printer. Late on a Sunday afternoon, the DeskWriter's power supply stopped working. I used a pruning saw to open the power supply's plastic case and discovered that the innards consisted of an always-energized transformer and two secondary-side fuses-one of which had a fractured element. After replacing both fuses and patching the case's saw cuts with silicone sealant, I had a working power supply.
The DeskWriter transformer's core runs slightly warm to the touch. So do the several AC adapters (or "wall warts") on my test bench that recharge batteries and power a frequency counter, a PC add-on digital storage oscilloscope, and the lab PC's amplified loudspeakers. Around the house, another dozen wall warts run everything from a network hub to a security camera.
Available from multiple sources, wall warts are inexpensive and carry safety-agency approvals. For overseas markets, you can design a small instrument, sans power supply, and ship a wall wart to match each customer's domestic AC supply standard.
But conventional wall warts waste watts. My colleague Martin Rowe controls his collection with switched AC outlet strips. I unplug wall warts that don't need to provide continuous power, but sometimes I forget.
We'd collectively save untold kilowatts of AC power if wall warts could sense load demand. Imagine a "smart" wall wart that would draw only a few microamperes while asleep but awaken to provide full power.
Think of a smart wall wart as an instrument in its own right: on the AC side, a trickle of current powers circuitry that periodically measures load impedance across an isolation barrier. An open circuit indicates that the load is either unplugged or switched off.
Sensing low load impedance invokes a start-up mode during which the wall wart monitors current. If the initial load-current surge decays normally, the wall wart supplies nominal power and continues to sense load current. A persistent heavy current drain puts the wall wart into "safe" mode.
Design constraints include complexity and cost-a smart wall wart's lifetime power savings must clearly exceed its initial purchase price. It may take a few more California-style power crises, but the smart wall wart's day is coming. If you know of such a device, write me at brad@tmworld.com. T&MW
