Hacker News Reader

Comment by tesseract | Hacker News Reader

Long distance (including room-to-room) distribution of low voltage DC power is inefficient due to I^2*R losses in wiring unless impractically large wire sizes are used. There may be some marginal efficiency gains to be had with high voltage DC supply and DC-DC converters in electronic devices (this architecture is used in some datacenters) but house-wide distribution of say 5V is impractical.

Even within a system the size of a desktop PC, the architecture that has been determined to maximize efficiency and cost savings is an AC-to-12VDC power supply box, feeding additional DC-DC converters that produce the necessary logic supply voltages (~1.2V) as physically close to the actual load as possible.

View on HN

to maximize efficiency and cost savings is an AC-to-12VDC power supply box

Actually, no. That power supply box contains an AC-to-DC rectifier, followed by a DC-to-DC power convertor: high-frequency convertors are much more efficient than 50Hz transformers and can be built with much smaller components than a toroidal transformer. Yes, you don't want 5V power lines in your walls, but 150V DC would still be more efficient for most devices than 120V AC.

The reason (other than inertia) why we probably won't do that is because of polarity: AC plugs can be inserted either way; DC plugs might destroy the equipment if inserted the wrong way, and most existing plugs allow both orientations. Not sure many people would like to "re-learn" how to insert a power plug.

But, truth be told, I have been thinking about getting myself certified as an electrical engineer so I can install DC voltage lines in my home: I already have DC solar panels (48V), and DC line batteries. It's awfully wasteful to have AC convertors on them all, just because that's the standard we've settled on.

Yes, that is the architecture of a modern offline SMPS. But the rectification losses are minimal compared to other losses in a PSU and there are options, like active rectification, to improve efficiency in that block if needed.

I will concede there may be advantages to centralizing power factor correction. But on the flip side, components like circuit breakers, connectors, and switches for high voltage DC face arc quench challenges that their AC equivalents basically don't at domestic line voltages and currents due to the zero crossing.

In-home batteries are a case where the benefits of DC wiring may outweigh the drawbacks though. That is also the basic situation in those datacenters I mentioned, where power to the racks all comes from a UPS system.

People would also be shocked (haha) to discover that their 48V DC system can actually kill them. Most solar systems are stacked 50V panels up to 250-500V DC which is easily as dangerous as our 120AC (350Vpp) systems. We have something that works pretty well and running a lot of specialized LowVoltage around doesn't make much sense. I'd rather have PoE!

> Yes, you don't want 5V power lines in your walls, but 150V DC would still be more efficient for most devices than 120V AC.

Arcing can become a hazard with large-scale DC power distribution. Old American wall switches often had ratings for both AC and DC service, and the DC current rating was much lower due to arcing. An AC arc self-extinguishes 60 (or 50) times a second, but DC doesn't offer that luxury.

Even 48V can pack a wallop; I've read accounts of wrenches being vaporized when dropped onto bus bars in telco installations.

I have been thinking about getting myself certified as an electrical engineer so I can install DC voltage lines in my home

FYI in most countries low voltage DC wiring is not regulated in the same way AC is.

Hence the development of things like power-over-ethernet LED lighting: it can be installed by non-electricians.

You could design some sort of concentric DC plug that simply cannot have the polarity reversed.

And modern AC plugs are usually three prong or bigger prong anyway.

A standard DC plug that could be installed on an AC line with efficient converters and no zero-load power draw would be nice. Unfortunately the closest thing we have to a standard DC power plug is USB-A.

Just use a wall socket with usb?

Reply on news.ycombinator.com