For novel ideas about building embedded systems (both hardware and firmware), join the 40,000+ engineers who subscribe to The Embedded Muse, a free biweekly newsletter. The Muse has no hype and no vendor PR. Click here to subscribe.
By Jack Ganssle
A couple of weeks ago I was pounding away at the computer when a shrill whistle sounded for a few seconds. The frequency rapidly shifted higher till it was inaudible. Seeing no obvious causes I resumed work. Then that ineffable electrical smell, signaling something seriously smoked, wafted through the room.
Alarmed, I searched for causes but found none in the jumble of computers and test equipment. The odor increased and for a moment I deliberated: act like a hero and try to isolate the problem, or call the fire department? Discretion beats seeing my place turn to toast, so 911 quickly dispatched a truck.
The firemen were courteous but admonished me about the proliferation of outlet strips along the floor. We don't pull many amps here, and the number of plugs isn't all that huge. But those annoying wall wart power supplies cover more outlets than they use. Outlet strips plug into outlet strips ad infinitum.
Afterwards I bought a bunch of APC power strips optimized for wall transformers. But those are Band-Aids. Wall warts are simply a stupid idea.
Sure, making the power supply an external brick shrinks electronics; a 2 pound Vaio notebook doesn't have much room for a transformer. And yes, putting a hot component out in the air does make a certain amount of sense.
But most are dumb linear supplies that waste enormous amounts of energy. Even when the equipment isn't in use these vampires secretly suck power, creating plenty of useless waste heat. The air conditioning works harder, burning even more power. I googled somewhat unsuccessfully for estimates of the amount of power wasted by these supplies; there's little data but some sites suggested they uselessly suck 5% of the nation's grid. Expensive - and ever scarcer - energy shouldn't be dissipated so uselessly.
So here's my idea: design a wall wart that fits entirely within the expanded grip on a power cord that goes into the wall - that molded bit of plastic from which the metal prongs sprout. Use switching technology to get 90% efficiency or more. Since most electronic devices take only an amp or two (sometimes far less) at 5 volts or so, practically no heat will be generated.
In Britain the plugs are larger than here in the USA so there's more room for parts. or for a higher capacity power supply.
The unit will be an off-line switcher, one that connects directly to the AC mains, since the small form factor won't allow a transformer. I have no idea what the regulatory requirements are for such a device, but off-line switchers are hardly rare. Somehow people manage to get approval for the technology today.
Check out http://www.supertex.com/pdf/datasheets/GN2470.pdf for one possible design. Power Integrations (http://www.powerint.com/) also sells some interesting off-line switcher parts.
In volume the costs would be low and customer satisfaction high. Everyone hates wall warts. Sell a product with the supply integrated invisibly into the cord and you'll have a competitive advantage over the dinosaurs pushing antique power bricks. And the reduced energy waste is a huge benefit in itself.
The firemen never did find the cause of the problem, as the smell stopped as soon as the engine arrived. But seeing the expression on my wife's face as she drove up to find the fire truck parked outside and 3 burly firefighters through the window was priceless.
What do you think? Why do we persist in building annoying and wasteful wall warts?