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Comment by dmfdmf | Hacker News Reader

Does anyone know if there are any plans or way to add universal DC to the standard electrical system? It seems with all the electronic devices it is wasteful to generate AC and then convert to DC in switching power supplies and in wall-warts, etc. This plug device looks like it just puts the transform in the wall so does nothing to solve the conversion losses. Would it be practical to add a DC offset, say +5v to the AC sine wave to directly drive electronics in the home? I'm not an EE so maybe the idea is crazy but I hope someone can explain.

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A DC offset would have the negative side effect of causing AC wall transformers to saturate, redering them useless. Also, as far as I know it would cause problems for many kinds of AC motors. True, you could bypass the DC offset with a series capacitor, but then you would suffer efficiency losses in larger appliances, in addition to added cost and complexity. The problem with having +5V centrally regulated and then distributed throughout the house is wiring resistance losses. Let's say you decide to use 16AWG wire, and have a run of 100' to a hypothetical outlet (not uncommon in even a mid sized house). that means your round trip length is 200'. Given the resistivity of copper, the resistance of such a wire is 1.26Ω, and with .5A of current your voltage drop would be .63V. Therefore, 5V would leave the distribution box, and only 4.37V would be available at the outlet. Worse, Pin=5V.5A=2.5W, Pout=4.37V.5A=2.19W. Efficiency of just the transmission system is 87%. You can get a good power factor corrected switching converter that can do better than this just plugging into the standard wall outlet.

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Or to put it another way: There is a reason that our power stations transmit AC, not DC. It is the electromagnetic equivalent of twisted pair versus straight copper: AC signals propagate over long distances. DC signals have a high loss.

Look up the Edison-Tesla rivalry for an interesting historical debate on the subject (Edison was a real jackass).

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