power supply to household

why household supply is only a.c, why not d.c?

For mainly 2 reasons:

• Most electricity plants are based on generation typically by alternated devices ( dynamos ), therefore no further conversion is required.
• Easiest to scale to higher voltages - which leads to lower transmission losses - employing solely passive devices ( transformers ).

In an age before semiconductors how do you efficiently convert 100+V DC to a few volts for something like a valve filament?

Transformers are simple and anyone who can design an electric motor can design one almost by definition, but they ONLY work on ac.
Motors also do better on AC for many things (You cannot run an induction motor on DC).

Finally the biggies, circuit protection for HV DC is a bear (And still somewhat unsolved), arcs do not self quench when the supply is DC, which is why the big HV DC links tend to be point to point so the protection can be on the AC side.
Three phase alternators are more efficient then DC machines, and control of a network of ac machines is quite a bit simpler then the equivalent probelem for the DC case (Remember no transformers in a DC system also no line reactors) how do you move power from point a to point b without creating an excessive voltage at point a?

Now there actually were DC supplies to building back in the day, early lifts were powered by separately excited DC machines as the speed control was easier then it is in an AC machine (Remember, this is before semiconductors), and in fact some old office blocks in the US only had the last of the DC ripped out a few years ago.
Other uses were arc lamp supplies in Cinemas and plating plants, neither exactly a domestic situation.

On a technical level, given the technology of the time it was never even a debate, for all that edison tried.

Regards, Dan.

In short words, DC electricty is suitable to store energy, whereas AC electricity is suitable to distribute energy.

Low voltage DC is OK (<=50V) in your home, If you were on an island with Solar power as your only source.

But HVDC gets into bigger problems with galvanic corrosion and charge buildup and then random discharge ( just like lightning can cause weird problems) and is only reserved where the savings in long-haul distribution losses exceed the costs of much higher insulation protection from partial discharge and full arc breakdown using expensive high technology UHV power distribution equipment.


That being said, I think all new homes ought to distribute 24V or 48Vdc from the Breaker Panel throughout the home for LED lighting and accessories. It would allow standardization for residential LED lightning. (How many homes have an octopus centre with a half a dozen wall transformers feeding DC powered peripherals and chargers)

Personally I use 18~19V a Laptop charger in basement to feed DC power outside, to power all my yard LED lights which usually have 6 CREE LEDs on ALUM strip., which I can distribute using 18, 16 or 14 AWG speaker wire.

Phone jacks are becoming obsolete for use with wireless phones now, so those could be used for 48Vdc power for LED engines in homes, powered by your own central PSU.