4 ms·

I wondered for a long time why we don't have standard built-in DC in building codes that could power our lights, and most electronic devices. Really the few thi

I wondered for a long time why we don't have standard built-in DC in building codes that could power our lights, and most electronic devices. Really the few things in my house that require full line voltage are all in the kitchen. Everything else has a transformer attached.

Smoosh 4y ago
Me too - standard 12V and 5V rails run throughout the house would be great. I even thought about a wallpaper with conductive strips so the power could be invisibly delivered to any part of any room and "tapped" with a push-into-the-wall socket.
- xpe 4y ago
  The usual counterargument is: voltage drop can become a problem. Trying to use one big power supply and use DC as your only distribution mechanism probably isn't a good idea. But choosing a DC system for part of the house can make a lot of sense. For one residential new construction room, it can be practical to have one shared power supply rather than one per LED. Say you have a 12 V, 5 A DC power supply. Using a star wiring topology, this can serve 10 lights (at 500 mA) fine with 16 AWG.
  - djbusby 4y ago
    But how far can the power go before wire resistance causes too much Vdrop? Maybe one good transformer+rectifier per room? AC to the room and DC in the room. Those DC runs would be <5m each.
- thrashh 4y ago
  The wires would be thick asf Not practical
- lazide 4y ago
  Even 12V over 50ft+ will have noticeable voltage drop unless you have huge wires. 5v? Not even worth considering. And switch mode power supplies are relatively inexpensive and quite efficient.
midoridensha 4y ago
It's because of Ohm's Law. Any sophomore-level electrical engineering student could answer this question immediately.
- xpe 4y ago
  Until I see someone defining a representative example and running the numbers, I'm skeptical of their DC vs AC commentary. I say this, because I was guilty of this exact shortcut thinking (in another comment). But I paused and thought to myself "I should run the numbers before just repeating the usual voltage drop criticism". So I compared scenarios and it depends a lot on the topology, lengths, costs, and situation (new vs renovation). Sure, a whole house system doesn't typically make sense, but I don't think that's what people are really talking about. I think people are interested in hybrid systems; e.g. DC power supply for each room. I don't know if you meant it, but the sentence about "any sophomore level electrical engineering student can solve this" can easily come across as dismissive. I also think it gives too much credit to sophomore students. :) I would have more confidence in an electrician apprentice on this one. I think they'd have more practical experience when it comes to figuring out what are the right questions to ask. I did EE in college and do a fair bit of hands on residential electrical work. P.S. How many sophomore level engineering students learn to do a sensitivity analysis?
  - midoridensha 4y ago
    >Sure, a whole house system doesn't typically make sense, but I don't think that's what people are really talking about. I think people are interested in hybrid systems; e.g. DC power supply for each room. I completely disagree. Where exactly are you going to put a power supply in a room? Make a special electrical box for it? Won't it be unsightly in many rooms, or need some huge special panel that looks like a breaker panel? The comments I see seem to be advocating a whole-house solution, where a power supply is mounted in the breaker panel to supply LVDC to the whole unit. But this makes no sense for several reasons, especially the voltage drop. >I don't know if you meant it, but the sentence about "any sophomore level electrical engineering student can solve this" can easily come across as dismissive. I also think it gives too much credit to sophomore students. :) It's supposed to be dismissive, because this whole discussion is a bunch of software people trying to make up solutions for a perceived problem when they obviously don't know one of the most basic things about electrical theory, which makes all of their solutions unworkable. It's like a bunch of people trying to make a new kind of personal vehicle to replace cars when they don't even understand Newton's Laws. It's really annoying, because I see this kind of discussion pop up every so often, over many many years. I have another comment here I don't feel like copy-and-pasting, but basically this whole discussion is silly because people are trying to make a solution using a very expensive power supply to fix a problem they see because they're buying cheap $2 light bulbs that burn out quickly, instead of just buying light fixtures that were properly engineered in the first place. With modern SMPSs, you're not going to get any kind of benefit by centralizing the power supply to drive individual LEDs, you're only going to get problems. LEDs need a driver circuit to provide constant current, and that means the power supply needs to be matched to the emitters and kept very close to it.

Smoosh 4y ago

Me too - standard 12V and 5V rails run throughout the house would be great. I even thought about a wallpaper with conductive strips so the power could be invisibly delivered to any part of any room and "tapped" with a push-into-the-wall socket.

xpe 4y ago
The usual counterargument is: voltage drop can become a problem. Trying to use one big power supply and use DC as your only distribution mechanism probably isn't a good idea. But choosing a DC system for part of the house can make a lot of sense. For one residential new construction room, it can be practical to have one shared power supply rather than one per LED. Say you have a 12 V, 5 A DC power supply. Using a star wiring topology, this can serve 10 lights (at 500 mA) fine with 16 AWG.
- djbusby 4y ago
  But how far can the power go before wire resistance causes too much Vdrop? Maybe one good transformer+rectifier per room? AC to the room and DC in the room. Those DC runs would be <5m each.
thrashh 4y ago
The wires would be thick asf Not practical
lazide 4y ago
Even 12V over 50ft+ will have noticeable voltage drop unless you have huge wires. 5v? Not even worth considering. And switch mode power supplies are relatively inexpensive and quite efficient.

midoridensha 4y ago

It's because of Ohm's Law. Any sophomore-level electrical engineering student could answer this question immediately.

xpe 4y ago
Until I see someone defining a representative example and running the numbers, I'm skeptical of their DC vs AC commentary. I say this, because I was guilty of this exact shortcut thinking (in another comment). But I paused and thought to myself "I should run the numbers before just repeating the usual voltage drop criticism". So I compared scenarios and it depends a lot on the topology, lengths, costs, and situation (new vs renovation). Sure, a whole house system doesn't typically make sense, but I don't think that's what people are really talking about. I think people are interested in hybrid systems; e.g. DC power supply for each room. I don't know if you meant it, but the sentence about "any sophomore level electrical engineering student can solve this" can easily come across as dismissive. I also think it gives too much credit to sophomore students. :) I would have more confidence in an electrician apprentice on this one. I think they'd have more practical experience when it comes to figuring out what are the right questions to ask. I did EE in college and do a fair bit of hands on residential electrical work. P.S. How many sophomore level engineering students learn to do a sensitivity analysis?
- midoridensha 4y ago
  >Sure, a whole house system doesn't typically make sense, but I don't think that's what people are really talking about. I think people are interested in hybrid systems; e.g. DC power supply for each room. I completely disagree. Where exactly are you going to put a power supply in a room? Make a special electrical box for it? Won't it be unsightly in many rooms, or need some huge special panel that looks like a breaker panel? The comments I see seem to be advocating a whole-house solution, where a power supply is mounted in the breaker panel to supply LVDC to the whole unit. But this makes no sense for several reasons, especially the voltage drop. >I don't know if you meant it, but the sentence about "any sophomore level electrical engineering student can solve this" can easily come across as dismissive. I also think it gives too much credit to sophomore students. :) It's supposed to be dismissive, because this whole discussion is a bunch of software people trying to make up solutions for a perceived problem when they obviously don't know one of the most basic things about electrical theory, which makes all of their solutions unworkable. It's like a bunch of people trying to make a new kind of personal vehicle to replace cars when they don't even understand Newton's Laws. It's really annoying, because I see this kind of discussion pop up every so often, over many many years. I have another comment here I don't feel like copy-and-pasting, but basically this whole discussion is silly because people are trying to make a solution using a very expensive power supply to fix a problem they see because they're buying cheap $2 light bulbs that burn out quickly, instead of just buying light fixtures that were properly engineered in the first place. With modern SMPSs, you're not going to get any kind of benefit by centralizing the power supply to drive individual LEDs, you're only going to get problems. LEDs need a driver circuit to provide constant current, and that means the power supply needs to be matched to the emitters and kept very close to it.