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I'm wondering why, in a standard house, instead of using many low power AC-DC rectifier, there isn't a big (like 12V 20~50A) AC-DC rectifier which supplies some wall socket with a DC voltage? In the long run this should be less expensive, given the fact that AC to DC in one big apparatus is done better than in many low-quality transformers, right?

Also, modern chip DC voltage regulators have efficiency of >90%, so smaller (or even larger) voltages are still efficient and possible.

So we could use a 12V socket to charge a phone, desktop computer, laptop, radio etc... and still have an AC socket to move washing machine engine and other things that work better with an AC supply.

Why isn't this done?

edit: real question is: is it convenient, given a standard family (2 laptop, 1 desktop, 4 smarthpone, 1 big tv and some minor electonic) to do so?

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  • \$\begingroup\$ Phones were powered by the telephone line. Other than that the desktop, laptop, chargers etc are all (relatively) recent innovations and none of them had defined DC voltages. USB 5V is mostly used now, so why not 5V? Or 19V for laptops? \$\endgroup\$ – carveone Aug 28 '14 at 12:22
  • \$\begingroup\$ don't look at that particular, we can go even with 25V, DC step up or down (regulator) are >90% efficiency, so you can use whatever you like. I can understand OLD houses does not have this trick, but what for new house? \$\endgroup\$ – Lesto Aug 28 '14 at 12:28
  • \$\begingroup\$ You've answered your own question - STANDARD HOUSE. \$\endgroup\$ – JIm Dearden Aug 28 '14 at 12:29
  • \$\begingroup\$ You would have to make custom cords for each of your devices that requires charging. \$\endgroup\$ – tcrosley Aug 28 '14 at 16:30
  • \$\begingroup\$ @tcrosley at the beginning, but once it become standard, device will have the old and/or new chord, based on witch plug is more efficient. \$\endgroup\$ – Lesto Aug 28 '14 at 16:39
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I think you are focusing on a small fact about power conversion efficiency, and ignoring all the more important factors:

  • extra cost of copper in house wiring to carry significant DC power
    (this is partly why Tesla's AC beat Edison's DC system),
  • the lack of a single standard DC voltage so there is still conversion required,
  • the requirement for higher-voltage AC anyway for existing and likely future home devices,
  • the reliability, availability and economy of low-power AC-DC converters,
  • the flexibility of AC-DC conversion techniques
  • the flexibility and scalability of converting from a single AC standard to the specific requirement of the device
  • the extra complexity of house wiring if two 'standards' have to be routed, e.g. the need for new plug standards, extra cables routed through building
  • there will still be a period of transition when both systems must be supported, and IMHO no sane democratic government would mandate the change-over,
  • I also believe as power increases the cost of components for DC-DC increases quite quickly above modest power conversion requirements

Edit:
Copper is expensive, London Metal Exchange - copper lists one tonne of copper (cash) is $7050, and that hasn't been processed into cable.
By comparison, London Metal Exchange - steel billet lists one tonne as $430, i.e. copper is 16x more expensive than steel.

So how thick must the 50A 12V cable be so as not to waste more energy as heat than AC? (Remember most of the benefit sited is efficiency, so it seems reasonable to ensure the losses due to heat in the DC cable are not worse than 230V AC.)

Lets compare AC mains cable rated at say 15A (Europe 230V), to 12V 50A DC cable.

Power loss = I^2 R

15^2 Rac = 50^2 Rdc

Rdc = Rac x 15^2/50^2 = Rac x 0.09,
i.e. Rdc must be ~11x lower resistance than Rac to achieve a similar power loss

DC cable would need to have over 11x greater cross sectional area than AC cable to reach the same power loss in heat. Put another way, DC house wiring would need 12x more copper so that the losses in the DC cables were no worse than AC mains.

Further, while the 230V 15A AC cable could carry 3.6kW, the 12V 50A DC cable carries 600W for the same losses.

Yes, for 11x more copper (to maintain comparable energy losses to AC) the DC cable carries 1/6th the power. Even at the scale of a house, the proposed low voltage DC cabling is not viable vs 230V AC.

The significant and important economic, practical, political and transition costs seem to dwarf any notional benefit.

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  • \$\begingroup\$ "(this is partly why Tesla's AC beat Edison's DC system)" FTFY. \$\endgroup\$ – Ignacio Vazquez-Abrams Aug 28 '14 at 15:57
  • \$\begingroup\$ please not i don't ask for FULL DC but for DC+AC system. It is true that there is more cable, and a new socket standard, and decide witch voltage to use.. But cable are not so expansive, given that they will carry 20/50A (they are used a lot in the RC world) As far as i know AC to DC supply also are never put directly into electronics as they has a lot of ripple, and so they pass anyway to a regulator. those will bring down point 1 2 3 6 7 8, 4 is a matther of adoption, and i can't understand point 5 \$\endgroup\$ – Lesto Aug 28 '14 at 16:47
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    \$\begingroup\$ @lesto There is nothing preventing householders from installing other systems such as low voltage DC, optical data systems, audio etc. for their particular needs.However,the basic AC power circuits fitted are the final parts of a very large and expensive power distribution grid, huge investment made in producing engineering standards across the globe,training and qualification bodies to ensure high standards of installation and safety. Gbulmer correctly points out no sane democratic government would mandate the change.Everything that you require can be achieved with the current (no pun) setup. \$\endgroup\$ – JIm Dearden Aug 28 '14 at 18:21
  • \$\begingroup\$ @JImDearden, ok so i'll edit the question to be more clear, point is, will the gain be more than exanse? \$\endgroup\$ – Lesto Aug 28 '14 at 20:16
  • \$\begingroup\$ @Ignacio Vazquez-Abrams - "... why Tesla's AC ..." fixed. We should take every opportunity to remind people about Nikola Tesla. I'm British, so it annoys me that people think Edison invented the light bulb. \$\endgroup\$ – gbulmer Aug 29 '14 at 0:33
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For a given power required by a device, the lower is the voltage the higher should be the current. That would imply much thicker wires, and a possible overheat where the section is not enough.

Even creating your DC rail, what voltage you would choose? If you choose 12VDC, for instance, you should add one or more DC-DC for lower voltages. At that point, where is the convenience?

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  • \$\begingroup\$ small and cheap DC to DC are in efficeincy range of 90%, while normal wall AC-DC are less than 70% \$\endgroup\$ – Lesto Aug 28 '14 at 12:31
  • \$\begingroup\$ I believe you're mixing two things together. If you pick a transformer (thus AC-AC) its efficiency is nearly 100%. What's taking down the efficiency is the way to rectify the AC (thus AC-DC). The old-yet-cheap way is "wasting" energy via some load, typically a transistor or so. The new-way is switching energy back and forth via some energy accumulator, typically a coil. So, the problem is not AC or DC, high or low voltage, but the way the builder made the power supply. \$\endgroup\$ – Mario Vernari Aug 28 '14 at 12:37
  • \$\begingroup\$ Supplying a 500 W appliance (like a PC) at 10 V DC would require 50 A. That would mean something like AWG 6 wire in the wall, with corresponding added costs. \$\endgroup\$ – The Photon Aug 29 '14 at 0:28
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I've thought that if things were done over, knowing what we know now, houses would better be supplied with 3-phase 60Hz or 50Hz power.

It's easily turned into DC with 6 diodes (no capacitor required for moderate ripple), easily transformed into other voltages with a mains-frequency transformer, and will directly run efficient quiet 3-phase motors that do not require 'run' capacitors. If you want a variable frequency 3-phase motor, you don't need a huge capacitor that is the first thing to die.

Presently, in North America 3-phase power is used in virtually all commercial and industrial installations, and many apartment buildings (though not brought to individual units). It is split out on a street-by-street basis in subdivisions.

The savings in billions of gadgets would probably vastly outweigh the added capital cost.

http://coefs.uncc.edu/mnoras/files/2013/03/Transformer-and-Inductor-Design-Handbook_Chapter_12.pdf

But, it's too late for that, and standards were set well before any thought of electronics on a mass scale. Most consumers were happy with a few primitive incandescent bulbs, a bulky weak motor in the electric ice box and washing machine etc., none of which was all that concerned with single phase vs. three phase.

If you made an outlet with (say) 300VDC out it would run many of the electronic gadgets we have now (the input filter capacitor life would no longer be an issue). They would not be much simpler, but they might be more compact. However, even today, many devices would go up in smoke with DC input (anything with a mains-frequency transformer or motor, and probably some others on top). So, you'd need a different plug.

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  • \$\begingroup\$ In principle I agree that might have been a better choice. However, I still own devices which need an "Earth" to be 'safe', and I assume I am not alone in that. So it would require an extra wire. \$\endgroup\$ – gbulmer Aug 28 '14 at 13:12
  • \$\begingroup\$ The infrastructure to deliver 3 phase to every house is more than you make it out to be. Usually every 1-3 houses has a step-down transformer. Those would be rather more bulky and expensive if they had to be 3 phase, and there would be a lot of them. There there would be the extra cabling to bring 3 phase everywhere. \$\endgroup\$ – Olin Lathrop Aug 28 '14 at 13:14
  • \$\begingroup\$ @OlinLathrop There would be an extra wire down the streets that don't currently have three phase (mine does but it would be a big deal in rural areas) and then to the houses, and to at least some of the outlets. A 3-phase transformer could use less core material than a single phase for the same kVA, but of course the shape may be less convenient. I think the mass of copper used in wiring would also be less, but I have not worked it out. In any case, it's a moot point, as outside of Europe it's not happening. \$\endgroup\$ – Spehro Pefhany Aug 28 '14 at 16:08
  • \$\begingroup\$ With a transformer that has both delta and wye secondary, 12-diode rectifier runs even smoother. \$\endgroup\$ – venny Aug 28 '14 at 17:10
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Because a "Standard House" came into existance a Century (or so) ago, and slowly modified as needed (public safety and standard electrical code), at the speed of buerocracy. People are very resistant to change. Once the IEC was formed, and consolidation of standards happened, anything too different must have wide support to spread. Which is a catch 22. Unless there is demand, there is no supply.

Since everything already works on the current system, with all the negatives invisible to the common user, negatives that are very neutral. Effeciency doesn't matter to someone who just cares on if it works or not.

That said, low voltage DC is a standard used in many homes. Or I should say alongside homes. It is used in landscaping lighting. And new products pushing DC exist. In wall 2+ Amp usb chargers. AC outlets with builtin usb charger ports. I give it a decade before they have become so common that a house without them will be shunned by young first time buyers.

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