6 ms·

There is some validity to using DC distribution within a building or domicile, it can make it cheaper and more reliable to have a central power supply and batte

There is some validity to using DC distribution within a building or domicile, it can make it cheaper and more reliable to have a central power supply and battery system. That is not a new idea obviously, that exact approach has been in use in the telecom industry for decades. There is a AC feed to a system of rectifiers that converts the AC from the grid to -48VDC and continuously charge a string of batteries. There is a Generator that will go on automatically if the power from the grid fails and takes over. The -48VDC is distributed throughout the central office to the equipment bays. There are a number of benefits to doing it this way, the batteries help maintain a constant voltage level and provide back up until the generator can fire up or if the generator should fail to start it buys you time to get it going. For a home, I could see having a DC distribution system using USB 48V standard or maybe a 12VDC system with a wall battery and perhaps Solar system. Assuming that you could power all your devices off DC, it would eliminate the need for an Inverter. Most devices in homes today can be powered with 12VDC versions, with some exceptions. It's an interesting idea.

dhdc 5y ago
Since we are doing DC-DC, I'd say we go with the highest DCV we can get away with. 12V is just simply too low and the associated I^2R losses will be unacceptable.
- zw123456 5y ago
  Right, agree. But then there is the tradeoff of the popularity and availability of 12VDC devices due to RV/Boating. But strictly from a technical point of view you are spot on.
  - dhdc 5y ago
    > But then there is the tradeoff of the popularity and availability of 12VDC devices due to RV/Boating. Which is exactly why we need to get away from those 12V old garbages asap. 12V is so bloody wasteful, in terms of both wasted power and wasted conductor material. The only reason why its still around is people got used to it and don't wanna change.
    - zw123456 5y ago
      what is the optimal voltage based on empirical data ?
      - picture 5y ago
        > highest we can get away with Assuming this means highest voltage that's still mostly safe and does not require a whole bunch of insulation, we might be looking at something from 80 to 100 VDC? No idea where I read it from but apparently it takes about that much DC voltage for people to "feel something" when touching conductors with dry skin.
        sokoloff 5y ago
        The EU safety regs have a cutoff of 50VAC and 75VDC. (Those are only ~5% different in peak voltage.) Below those levels, you have only general product safety regs to comply with. At/above those and up through all “reasonably household” voltages, you (probably) have to comply with the EU low voltage directive. “Probably” because the LVD itself isn’t law but member states have generally implemented it in their laws.
        R0b0t1 5y ago
        You break skin at about 50VDC. We could keep ~120V for distribution. There are problems tuning control loops of buck converters dropping more than about 30V (though you can just have multiple buck converters in a row). Correcting the power factor from 120VAC gives you a boost circuit that gives you 360-400VDC. Some motor control and battery technology standardizes around this voltage. Cars are a big one, but also PFC direct to inverter motor control, which is becoming popular in white goods.
        picture 5y ago
        > You break skin at about 50VDC. What does breaking skin mean in this context? My understanding was that humans largely act like a resistor with some parasitic capacitance and inductance. Wouldn't more voltage equal more current in a mostly linear relation?
  - coding123 5y ago
    There are a few people that use 24 - 48V DC batteries/solar charge controllers in RVs and then step down for the motors, lights, etc.. for the rest of the rig. Pretty simple conversion for an RV.
arcticbull 5y ago
Correct me if I'm wrong but isn't the whole point of using AC that it's easy to convert voltages super-efficiently with transformers? Having DC power at home seems like the worst of both worlds. Low voltage DC at home causes large losses in the wiring, and high voltage DC causes large losses in the step-down regulator (leaving you with a buck-type regulator operating at the low extreme of its duty cycle). After all, DC-DC conversion relies broadly on turning the DC into something vaguely sinusoidal and using an inductor - so its basically DC-AC-DC anyways. Am I missing something?
- analog31 5y ago
  Switchmode power supplies already tend to rectify and filter the high voltage AC before stepping it down. At high frequency, the transformer can be a lot smaller.
- dreamcompiler 5y ago
  Not sinusoidal: Boost and buck converters use PWM square waves. Because the power transistors are almost always fully on or fully off the I2R losses in the transistors are quite low.
dreamcompiler 5y ago
I think 24vdc is pretty much ideal. It's low enough not to be dangerous (48 volts is right at the threshold of danger) while it could supply enough power via the 12 or 14 gauge copper that's already in most US homes to handle all lighting needs plus probably a decent fraction of other loads. When you decrease the voltage to 12 you start having to think about fatter wire--especially in larger homes--and that retrofit would be expensive.
- coding123 5y ago
  That's going to be some thick ass wires for the oven, water heater and a few other high wattage items.
  - dreamcompiler 5y ago
    The big loads would have to remain at 120 or 240vac.

dhdc 5y ago

Since we are doing DC-DC, I'd say we go with the highest DCV we can get away with. 12V is just simply too low and the associated I^2R losses will be unacceptable.

zw123456 5y ago
Right, agree. But then there is the tradeoff of the popularity and availability of 12VDC devices due to RV/Boating. But strictly from a technical point of view you are spot on.
- dhdc 5y ago
  > But then there is the tradeoff of the popularity and availability of 12VDC devices due to RV/Boating. Which is exactly why we need to get away from those 12V old garbages asap. 12V is so bloody wasteful, in terms of both wasted power and wasted conductor material. The only reason why its still around is people got used to it and don't wanna change.
  - zw123456 5y ago
    what is the optimal voltage based on empirical data ?
    - picture 5y ago
      > highest we can get away with Assuming this means highest voltage that's still mostly safe and does not require a whole bunch of insulation, we might be looking at something from 80 to 100 VDC? No idea where I read it from but apparently it takes about that much DC voltage for people to "feel something" when touching conductors with dry skin.
      - sokoloff 5y ago
        The EU safety regs have a cutoff of 50VAC and 75VDC. (Those are only ~5% different in peak voltage.) Below those levels, you have only general product safety regs to comply with. At/above those and up through all “reasonably household” voltages, you (probably) have to comply with the EU low voltage directive. “Probably” because the LVD itself isn’t law but member states have generally implemented it in their laws.
      - R0b0t1 5y ago
        You break skin at about 50VDC. We could keep ~120V for distribution. There are problems tuning control loops of buck converters dropping more than about 30V (though you can just have multiple buck converters in a row). Correcting the power factor from 120VAC gives you a boost circuit that gives you 360-400VDC. Some motor control and battery technology standardizes around this voltage. Cars are a big one, but also PFC direct to inverter motor control, which is becoming popular in white goods.
        picture 5y ago
        > You break skin at about 50VDC. What does breaking skin mean in this context? My understanding was that humans largely act like a resistor with some parasitic capacitance and inductance. Wouldn't more voltage equal more current in a mostly linear relation?
- coding123 5y ago
  There are a few people that use 24 - 48V DC batteries/solar charge controllers in RVs and then step down for the motors, lights, etc.. for the rest of the rig. Pretty simple conversion for an RV.

arcticbull 5y ago

Correct me if I'm wrong but isn't the whole point of using AC that it's easy to convert voltages super-efficiently with transformers? Having DC power at home seems like the worst of both worlds. Low voltage DC at home causes large losses in the wiring, and high voltage DC causes large losses in the step-down regulator (leaving you with a buck-type regulator operating at the low extreme of its duty cycle). After all, DC-DC conversion relies broadly on turning the DC into something vaguely sinusoidal and using an inductor - so its basically DC-AC-DC anyways. Am I missing something?

analog31 5y ago
Switchmode power supplies already tend to rectify and filter the high voltage AC before stepping it down. At high frequency, the transformer can be a lot smaller.
dreamcompiler 5y ago
Not sinusoidal: Boost and buck converters use PWM square waves. Because the power transistors are almost always fully on or fully off the I2R losses in the transistors are quite low.

dreamcompiler 5y ago

I think 24vdc is pretty much ideal. It's low enough not to be dangerous (48 volts is right at the threshold of danger) while it could supply enough power via the 12 or 14 gauge copper that's already in most US homes to handle all lighting needs plus probably a decent fraction of other loads. When you decrease the voltage to 12 you start having to think about fatter wire--especially in larger homes--and that retrofit would be expensive.

coding123 5y ago
That's going to be some thick ass wires for the oven, water heater and a few other high wattage items.
- dreamcompiler 5y ago
  The big loads would have to remain at 120 or 240vac.