4 ms·

It was really mind blowing to me the first time I did an inventory in my house of appliances and gadgets that had to convert AC back to DC after being collected

It was really mind blowing to me the first time I did an inventory in my house of appliances and gadgets that had to convert AC back to DC after being collected as DC and converted to AC. Not just the electricity being used but also just the wasteful resources needed for wall warts and built in transformers.

junon 3y ago
Yes though AC is still safer and easier to transport. It's just the conversion that is wasteful. DC grids might be a good option but not without risks, especially at higher voltages.
- pipo234 3y ago
  I might misremember, but aren't some of the submerged sea power cables (Norway, Germany, UK) DC? If so, why? And how does that rhyme with AC being more efficient?
  - junon 3y ago
    Disclaimer that I'm not an expert. As far as I understand, AC runs on the outside of the cable rather than through the conductor. DC does not. That's one of several reasons why.
    - deleted 3y ago
      [deleted]
  - cfn 3y ago
    I don't know if it is the same issue but AC lines longer than a certain distance will approach the wave length of an antena and start to radiate energy (like a radio broadcast antena). In that case you use DC. When I studied this back in school I was told there was such a line in South Africa (if I recall correctly).
  - joneholland 3y ago
    There’s a dedicated DC trunk line running down the entire west coast. It’s how hydropower generated in Washington state is sold to California. High voltage DC to bridge grids is really common. https://en.m.wikipedia.org/wiki/Pacific_DC_Intertie https://en.m.wikipedia.org/wiki/Pacific_DC_Intertie
  - wesleyd 3y ago
    My understanding here is that the saving comes from needing fewer cables, although I’m sure there’s a lot more to it than that.
  - ben_w 3y ago
    A whole bunch of different effects. AC/DC have different costs for voltage converters and per-distance efficiency, so there are some distances where AC makes more sense and others where DC makes more sense; the distance changes as tech improves. AC mostly conducts on the outer surface of the wire, while DC conducts with the whole cross section, giving you different scaling issues as the current changes. Under water, AC suffers from significant capacitive loss — the wire acts as one side of a capacitor and the entire ocean as the other. At certain frequencies and wire lengths you also get inductive losses, though IIRC that affects only RF cables in practice and and the design of the trans-Siberian railway in theory as no other place even seriously considered having a sufficiently long conductor for the frequency used.
  - tuetuopay 3y ago
    As others mentioned, undersea cables suffer from capacitive losses. But another big factor is grid synchronisation: two AC grids cannot be joined if they are not perfectly synced. Their phase must match. DC having no such phase, it’s very useful to connect independent grids (e.g. UK to the rest of the EU, or the three main grids in the US)
- fsh 3y ago
  AC is quite a lot more dangerous than DC for humans at ~hundreds of Volts. ElectroBOOM has made a nice demonstration of this: https://www.youtube.com/watch?v=snk3C4m44SY https://www.youtube.com/watch?v=snk3C4m44SY
  - 10u152 3y ago
    DC arcs are much harder to extinguish. Switching 10A of 240v AC is trivial. Switching 10A of 240V DC is much more complicated in terms of switchgear. Sustained arcs are dangerous.
- briffle 3y ago
  There is a DC long distance line from the Dalles, Oregon to LA that has been around since the early 70's. Its been upgraded a few times, and now runs 3GW of power over it. its very distinctive from the other lines nearby, since it only has 2 wires: The grounding loops are very impressive. 6 mile loop of buried cable at either end. https://en.wikipedia.org/wiki/Pacific_DC_Intertie https://en.wikipedia.org/wiki/Pacific_DC_Intertie
  - londons_explore 3y ago
    It's generally understood now that using the earth for transmitting electrical current is a bad idea... It causes more corrosion in building foundations even hundreds of kilometers away, and soil microbes that navigate with electric fields die. Therefore, most new DC transmission systems have a balanced pair of cables, and only use earth return for emergencies
    - crmd 3y ago
      The article says that the intertie has one +500Kv line and one -500Kv line. So why does current need to travel via earth? I would think the DC current flows in a loop via the two conductor lines.
rini17 3y ago
But this is about 380V DC which is not going into your living room anytime soon. Sadly, it is less safe than AC. Automakers tried to increase DC voltage beyond 12V but it causes sparks that cause mechanical switches to fail. AC sparks are extinguished whenever the voltage wave crosses zero. Ubiquitous power DC needs high power/high voltage solid state switches to become cheaper and more reliable than mechanical ones. Perhaps SiC or GaN transistors will do eventually.
- deleted 3y ago
  [deleted]
- aramachandran7 3y ago
  re: 12V automotive LV systems: it’s a little more nuanced than that. teslas cybertruck will be running a 48V LV system[1] and while relays fuses efuses and DCDC converters across the LV system have to be rated for higher voltages - you gain efficiency back with I^2R losses across the entire harness, and can drop your required wire gauge since the necessary current carrying capacity is reduced by 4. So it’s a nuanced trade off, and if the industry shifts (which tesla is banking on since they’re the ‘leader’) then economies of scale can be reached with higher voltage fuses switches relays etc. [1] - https://auto.hindustantimes.com/auto/electric-vehicles/tesla-cybertruck-to-mark-automaker-s-switch-to-a-48-volt-system-41678600605740.html https://auto.hindustantimes.com/auto/electric-vehicles/tesla...
- londons_explore 3y ago
  It is possible for high voltage DC to be made safe. Currently, techniques to do so are neither cheap nor off-the-shelf. For example, imagine I want a 3000 volt DC wire to power a portable air conditioner. The air conditioner will be 10 kilowatts, so 3.3 amps. The wire can be thinner than headphone cables (two 0.3mm conductors, +-1500 volts, 150um PTFE coating) if desired. Obviously, with such a thin insulation, the system needs to be human safe when chewed through by a baby. To ensure that, the current flow through the baby must be under 1 milliamp, or 10 milliJoules through the baby's heart. That can be ensured by tracking the current through each conductor, accurate to 1 milliamp, and shutting off the supply if there is ever more than 1 milliamp unaccounted for (either to earth, or to the other conductor). When the shutoff occurs, it must therefore happen within 1 microsecond (assuming the worst case fault, that is all three amps direct to the baby's heart). That in turn puts capacitance and therefore length limits on the cable - it wouldn't be possible for this cable to be safe longer than ~1000 feet. TL;DR: It is very possible, with today's technology, to design very high voltage DC systems safe enough for use within a home. However, no hardware available off-the-shelf yet can do this, due to no demand.
eternityforest 3y ago
They would have to do conversion regardless. It might be a little more efficient DC DC, but it's not like you could just have 5v lines everywhere, you might wind up using as much copper in the thicker lines as you do in the power adapters. Especially now with USB C. I suspect in 20 years a lot of today's PD supplies will be perfectly good. It's less wasteful when they're that reusable.
RetroTechie 3y ago
The #1 purpose of such wall warts is to provide safety by means of galvanic isolation (in use, any part you can touch on wall wart or device it powers, has no direct electrical connection to HV side). That's where a transformer comes in. When using a transformer, the voltage conversion comes 'free'. Modern electronics makes this smaller, lighter & using less metal (not more reliable, btw ;-) On the generation side (for example rooftop solar), it isn't a big deal to have 1, powerful, high-efficiency converter.

junon 3y ago

Yes though AC is still safer and easier to transport. It's just the conversion that is wasteful. DC grids might be a good option but not without risks, especially at higher voltages.

pipo234 3y ago
I might misremember, but aren't some of the submerged sea power cables (Norway, Germany, UK) DC? If so, why? And how does that rhyme with AC being more efficient?
- junon 3y ago
  Disclaimer that I'm not an expert. As far as I understand, AC runs on the outside of the cable rather than through the conductor. DC does not. That's one of several reasons why.
  - deleted 3y ago
    [deleted]
- cfn 3y ago
  I don't know if it is the same issue but AC lines longer than a certain distance will approach the wave length of an antena and start to radiate energy (like a radio broadcast antena). In that case you use DC. When I studied this back in school I was told there was such a line in South Africa (if I recall correctly).
- joneholland 3y ago
  There’s a dedicated DC trunk line running down the entire west coast. It’s how hydropower generated in Washington state is sold to California. High voltage DC to bridge grids is really common. https://en.m.wikipedia.org/wiki/Pacific_DC_Intertie https://en.m.wikipedia.org/wiki/Pacific_DC_Intertie
- wesleyd 3y ago
  My understanding here is that the saving comes from needing fewer cables, although I’m sure there’s a lot more to it than that.
- ben_w 3y ago
  A whole bunch of different effects. AC/DC have different costs for voltage converters and per-distance efficiency, so there are some distances where AC makes more sense and others where DC makes more sense; the distance changes as tech improves. AC mostly conducts on the outer surface of the wire, while DC conducts with the whole cross section, giving you different scaling issues as the current changes. Under water, AC suffers from significant capacitive loss — the wire acts as one side of a capacitor and the entire ocean as the other. At certain frequencies and wire lengths you also get inductive losses, though IIRC that affects only RF cables in practice and and the design of the trans-Siberian railway in theory as no other place even seriously considered having a sufficiently long conductor for the frequency used.
- tuetuopay 3y ago
  As others mentioned, undersea cables suffer from capacitive losses. But another big factor is grid synchronisation: two AC grids cannot be joined if they are not perfectly synced. Their phase must match. DC having no such phase, it’s very useful to connect independent grids (e.g. UK to the rest of the EU, or the three main grids in the US)
fsh 3y ago
AC is quite a lot more dangerous than DC for humans at ~hundreds of Volts. ElectroBOOM has made a nice demonstration of this: https://www.youtube.com/watch?v=snk3C4m44SY https://www.youtube.com/watch?v=snk3C4m44SY
- 10u152 3y ago
  DC arcs are much harder to extinguish. Switching 10A of 240v AC is trivial. Switching 10A of 240V DC is much more complicated in terms of switchgear. Sustained arcs are dangerous.
briffle 3y ago
There is a DC long distance line from the Dalles, Oregon to LA that has been around since the early 70's. Its been upgraded a few times, and now runs 3GW of power over it. its very distinctive from the other lines nearby, since it only has 2 wires: The grounding loops are very impressive. 6 mile loop of buried cable at either end. https://en.wikipedia.org/wiki/Pacific_DC_Intertie https://en.wikipedia.org/wiki/Pacific_DC_Intertie
- londons_explore 3y ago
  It's generally understood now that using the earth for transmitting electrical current is a bad idea... It causes more corrosion in building foundations even hundreds of kilometers away, and soil microbes that navigate with electric fields die. Therefore, most new DC transmission systems have a balanced pair of cables, and only use earth return for emergencies
  - crmd 3y ago
    The article says that the intertie has one +500Kv line and one -500Kv line. So why does current need to travel via earth? I would think the DC current flows in a loop via the two conductor lines.

rini17 3y ago

But this is about 380V DC which is not going into your living room anytime soon. Sadly, it is less safe than AC. Automakers tried to increase DC voltage beyond 12V but it causes sparks that cause mechanical switches to fail. AC sparks are extinguished whenever the voltage wave crosses zero. Ubiquitous power DC needs high power/high voltage solid state switches to become cheaper and more reliable than mechanical ones. Perhaps SiC or GaN transistors will do eventually.

deleted 3y ago
[deleted]
aramachandran7 3y ago
re: 12V automotive LV systems: it’s a little more nuanced than that. teslas cybertruck will be running a 48V LV system[1] and while relays fuses efuses and DCDC converters across the LV system have to be rated for higher voltages - you gain efficiency back with I^2R losses across the entire harness, and can drop your required wire gauge since the necessary current carrying capacity is reduced by 4. So it’s a nuanced trade off, and if the industry shifts (which tesla is banking on since they’re the ‘leader’) then economies of scale can be reached with higher voltage fuses switches relays etc. [1] - https://auto.hindustantimes.com/auto/electric-vehicles/tesla-cybertruck-to-mark-automaker-s-switch-to-a-48-volt-system-41678600605740.html https://auto.hindustantimes.com/auto/electric-vehicles/tesla...
londons_explore 3y ago
It is possible for high voltage DC to be made safe. Currently, techniques to do so are neither cheap nor off-the-shelf. For example, imagine I want a 3000 volt DC wire to power a portable air conditioner. The air conditioner will be 10 kilowatts, so 3.3 amps. The wire can be thinner than headphone cables (two 0.3mm conductors, +-1500 volts, 150um PTFE coating) if desired. Obviously, with such a thin insulation, the system needs to be human safe when chewed through by a baby. To ensure that, the current flow through the baby must be under 1 milliamp, or 10 milliJoules through the baby's heart. That can be ensured by tracking the current through each conductor, accurate to 1 milliamp, and shutting off the supply if there is ever more than 1 milliamp unaccounted for (either to earth, or to the other conductor). When the shutoff occurs, it must therefore happen within 1 microsecond (assuming the worst case fault, that is all three amps direct to the baby's heart). That in turn puts capacitance and therefore length limits on the cable - it wouldn't be possible for this cable to be safe longer than ~1000 feet. TL;DR: It is very possible, with today's technology, to design very high voltage DC systems safe enough for use within a home. However, no hardware available off-the-shelf yet can do this, due to no demand.

eternityforest 3y ago

They would have to do conversion regardless. It might be a little more efficient DC DC, but it's not like you could just have 5v lines everywhere, you might wind up using as much copper in the thicker lines as you do in the power adapters. Especially now with USB C. I suspect in 20 years a lot of today's PD supplies will be perfectly good. It's less wasteful when they're that reusable.

RetroTechie 3y ago

The #1 purpose of such wall warts is to provide safety by means of galvanic isolation (in use, any part you can touch on wall wart or device it powers, has no direct electrical connection to HV side). That's where a transformer comes in. When using a transformer, the voltage conversion comes 'free'. Modern electronics makes this smaller, lighter & using less metal (not more reliable, btw ;-) On the generation side (for example rooftop solar), it isn't a big deal to have 1, powerful, high-efficiency converter.