5 ms·

If we could get in-house/building DC power (not to replace but to complement AC power) and get industries to make digital products work with this we could get a

If we could get in-house/building DC power (not to replace but to complement AC power) and get industries to make digital products work with this we could get another substantial savings in energy efficiency. Plus digital products would cost less b/c they wouldn't all need AC to DC adapters.

brandmeyer 11y ago
Unfortunately, unplugging DC power plugs under load tends to damage them much more than the equivalent AC plugs. Similarly, DC rated circuit breakers and fuses are much larger than their AC counterparts. The reason is that the zero-crossing of the AC voltage waveform naturally allows the developed arc an opportunity to be extinguished. DC current/voltage has no such zero crossing.
- badsock 11y ago
  I believe transfire is talking about low voltage DC.
  - simcop2387 11y ago
    The problem with low voltage DC wiring is that it requires much more metal to prevent serious losses in the lines. You could switch to higher in the lines and then a converter in the wall but then you're back to the same circuit breaker problem. AC deals with this by the fact that zero crossing gives a great way to break arcs and you can convert it to DC really efficiently.
    - caf 11y ago
      I believe the idea is probably to have one large switchmode converter running off the mains at the POE that supplies, say, dual rail 20VDC / 5VDC throughout the building, replacing the umpteen smaller converters in the average house. Not sure it would end up being a net win.
      - simcop2387 11y ago
        That's the point, if you don't do the conversion to low voltage at the source you lose a lot in losses from the resistance in the lines. Let's say you're using the normal 14 gauge solid core wire commonly found in houses right now for a 5V supply. A 200ft run of that in the house (both ways) isn't out of the question if there's a central supply in a 2 story building. That's about 1 ohm [1], which doesn't sound like much at first until we look at what it ends up meaning. A phone might take an 1 Amp to charge at it's fast rate (tablets 2 or more), so we've got 1 Amp, with a 1 Ohm resistance. That means we're going to have a 1 Volt drop to that outlet, which means the phone is likely not going to charge at the correct current since it'll only see 4 Volts at the outlet. It also means you've just had a 20% loss in the power line in order to have a larger central power supply. Combine that with the fact that most Switch-Mode Power Supplies aren't actually that efficient when not under load (part of the problem) then it might not be unreasonable to have it running at 80% efficiency to begin with. At this point you're down to 64% total efficiency instead of the 80-90% that the small wall warts end up getting you. You could use a higher voltage, 48V isn't uncommon because of the telephone industry, but then you need conversion at the outlet again and end up with the same problem you've got with wall warts. The other option is to use more copper to carry the current, say 10 gauge, which brings the losses to 0.4V and about 10% losses, but you need 150% more copper than 14 gauge. [1] http://www.cirris.com/learning-center/calculators/133-wire-resistance-calculator-table http://www.cirris.com/learning-center/calculators/133-wire-r...
        upofadown 11y ago
        Trying to regulate 5V across 14Ga wiring isn't anything anyone would do. If 48V became some sort of DC standard then devices would work off 48V. That might not make sense of course and how much sense it might make could depend on line voltage (120V vs 240V). There is a cultural thing here too. If you live in a place where all heating is done with combustion then there is no particular reason so not do everything with low voltage DC. Now that lighting is LED based there wouldn't be anything that would need the higher voltage and power.
        simcop2387 11y ago
- upofadown 11y ago
  I doubt that anyone would be running space heaters or ovens off the DC part of the system. The idea is more to prevent having to do a zillion high voltage AC to low voltage DC conversions.

brandmeyer 11y ago

Unfortunately, unplugging DC power plugs under load tends to damage them much more than the equivalent AC plugs. Similarly, DC rated circuit breakers and fuses are much larger than their AC counterparts. The reason is that the zero-crossing of the AC voltage waveform naturally allows the developed arc an opportunity to be extinguished. DC current/voltage has no such zero crossing.

badsock 11y ago
I believe transfire is talking about low voltage DC.
- simcop2387 11y ago
  The problem with low voltage DC wiring is that it requires much more metal to prevent serious losses in the lines. You could switch to higher in the lines and then a converter in the wall but then you're back to the same circuit breaker problem. AC deals with this by the fact that zero crossing gives a great way to break arcs and you can convert it to DC really efficiently.
  - caf 11y ago
    I believe the idea is probably to have one large switchmode converter running off the mains at the POE that supplies, say, dual rail 20VDC / 5VDC throughout the building, replacing the umpteen smaller converters in the average house. Not sure it would end up being a net win.
    - simcop2387 11y ago
      That's the point, if you don't do the conversion to low voltage at the source you lose a lot in losses from the resistance in the lines. Let's say you're using the normal 14 gauge solid core wire commonly found in houses right now for a 5V supply. A 200ft run of that in the house (both ways) isn't out of the question if there's a central supply in a 2 story building. That's about 1 ohm [1], which doesn't sound like much at first until we look at what it ends up meaning. A phone might take an 1 Amp to charge at it's fast rate (tablets 2 or more), so we've got 1 Amp, with a 1 Ohm resistance. That means we're going to have a 1 Volt drop to that outlet, which means the phone is likely not going to charge at the correct current since it'll only see 4 Volts at the outlet. It also means you've just had a 20% loss in the power line in order to have a larger central power supply. Combine that with the fact that most Switch-Mode Power Supplies aren't actually that efficient when not under load (part of the problem) then it might not be unreasonable to have it running at 80% efficiency to begin with. At this point you're down to 64% total efficiency instead of the 80-90% that the small wall warts end up getting you. You could use a higher voltage, 48V isn't uncommon because of the telephone industry, but then you need conversion at the outlet again and end up with the same problem you've got with wall warts. The other option is to use more copper to carry the current, say 10 gauge, which brings the losses to 0.4V and about 10% losses, but you need 150% more copper than 14 gauge. [1] http://www.cirris.com/learning-center/calculators/133-wire-resistance-calculator-table http://www.cirris.com/learning-center/calculators/133-wire-r...
      - upofadown 11y ago
        Trying to regulate 5V across 14Ga wiring isn't anything anyone would do. If 48V became some sort of DC standard then devices would work off 48V. That might not make sense of course and how much sense it might make could depend on line voltage (120V vs 240V). There is a cultural thing here too. If you live in a place where all heating is done with combustion then there is no particular reason so not do everything with low voltage DC. Now that lighting is LED based there wouldn't be anything that would need the higher voltage and power.
        simcop2387 11y ago
upofadown 11y ago
I doubt that anyone would be running space heaters or ovens off the DC part of the system. The idea is more to prevent having to do a zillion high voltage AC to low voltage DC conversions.