One of the bigger problems of eco-friendly bulbs is that they have to be build with AC/DC converter which raises their cost, makes them bulkier and more susceptible for failure…
So my question is why new houses aren’t built with separate DC wiring for lighting with one big power supply unit?
Off-grid houses ofttimes are.
There’s essentially zero demand for this with conventional construction, though.
The converter in CFL bulbs is absolutely necessary for their operation (needs high voltage AC, DC quickly ruins fluorescent bulbs, plus they have a weird voltage-current characteristic) and LED bulbs should have a DC/DC converter for constant current control, especially if you want them to be dimmable (an AC/DC converter is the same thing with a rectifier added). Although having a separate DC power line has the advantage of being isolated from the AC wiring, reducing the chances of surges, plus there isn’t any concern about power factor.
It wouldn’t be much of an improvement, if at all. Most electronic devices nowadays use switching circuits for their internal power supplies whether the input is AC or DC, and DC input is only modestly more efficient than AC input. Furthermore, such a system would need a rather large, centralized, DC power supply (with its own switching circuit), as it must be capable of supplying full power during peak use. Most of the time it won’t be operating at peak output power, which further adds to the inefficiency of the system.
Also keep in mind that arc suppression (for breakers, switches, and contacts) and circuit protection is more difficult and expensive with a DC system vs. an AC system, all else being equal. And unlike AC, the accidental reversal of a load’s polarity might prove to be catastrophic, depending on how it is designed.
Over time dedicated low voltage DC wiring will prevail. Among other things we’ll eventually buy light fixtures that don’t have a removeable bulb anyway since it would only need replacing once every 25 years. But in the mean time we need bulbs that fit into the same old sockets and use the same old electric supply that everyone already has.
And what about floor lamps, have a seperate set of receptacles too? Even if you want to restrict people to eco-friendly bulbs that could run on 12 volts, with the losses in a low voltage system you’d need two sets of heavy wires, one for lighting and one standard AC for every thing else. Although I don’t like to mix lighting and receptacles on the same circuit, you can do it everywhere but a kitchen and it’s commonly done; cheaper to tie the light into the receptacle circuit and put a cheap Chinese switching power supply on the bulb.
Because not everything needs the same DC voltage. And DC can’t easily be stepped up or down like AC, so if your devices still need DC-DC converters inside anyway, why not just stick with AC supply and switch-mode power supplies?
You’ll still need standard AC outlets for other devices. You probably wouldn’t wire outlets for the lighting, just the DC supply for the built in fixtures. The floor and table lamps wouldn’t need the miniature power supply in the bulb, it would be built into the lamp, away from the bulb so heat won’t degrade the LEDs. Or they’ll just use wall transformers like all the other modern appliances.
Switching power supplies convert AC to DC before stepping up/down, so using DC as an input is more efficient.
I don’t think so, Tim…
The problem with low voltage is it needs high amperage. 23W 12V is about 2A. 100W 110V is only 1A and 23W (good CFL bulb) is 0.2A; since wire thickness and heat from resistance is an issue with number of amps, (Power=RI^2) the higher the voltage, the lower the amps, the better. That’s why the Europeans like 220V.
I think the earlier posts have it right. First its chicken-and-egg. Nobody will make bulbs for homes unless there’s DC, and nobody will install DC unless there are bulbs. Installing DC means the converter, a overly large device just so you can do what AC already does. The cost appeal is not there to retrofit old houses. and so on…
It will start with the elimination of bulbs. If you’re building a new house you might as well have much smaller LED lights installed that never need changing in the life of the product, so there will be no need for sockets. For a chain of fixtures there will be one AC-DC converter. Floor lamps won’t have bulbs either, and probably won’t even look like lamps since that shape is based on bulbs anyway. I’d expect most lighting to take the form of panels or decorative objects. Bulbs are an artifact of existing lighting that will disappear along with the candle snifters, and high voltage AC is an artifact of old energy greedy heaters we call light bulbs.
Now I can’t prove what’s going to happen in the future, but I’m willing to bet. So I’ll wager that in 25 years bulbs will be an anachronism and fixed lighting will be powered by DC. I’ll have to have my executor take care of collecting from you since I don’t expect to make it that far.
I might note that SMPSs don’t actually convert AC to DC; a rectifier is used for that purpose, which provides a DC output that is then converted. It is also true that some power is lost during rectification, although it is only around 1% at 120v (a bigger concern is that if power factor correction isn’t used, which means a complex electronic circuit in this case, current is drawn only at the AC voltage peaks; while not a direct cause of power loss, this does increase the apparent power drawn).
Also, even if lighting runs on DC in the future, I’m sure they will use a highish voltage, say 120 volts DC rather than 12 volts or so, for the reasons already mentioned; as an example, say you use 18 AWG wire to supply 1 amp for lighting (120 watts could very well be enough for an entire house in the future); at 120 volts, and a 50 foot run (100 both ways), you will lose 0.55 percent of the voltage. On the other hand, at 12 volts you would need 10 amps and for the same percentage voltage loss, you would need 000 AWG wire (which is almost half an inch thick and can carry over 300 amps according to the table here; note that the actual current handling capacity of wire isn’t ever really an issue; for 1 amp, you could use wire as thin as 29 AWG, 0.28 mm thick, and that is still conservative, although it would drop too much voltage (about 8 volts for 100 feet; 8 watts of power dissipated over such a length of wire is very low) to be practical for power transmission).
Are we expecting LEDs to get vastly more efficient than they are (I’d rather they improve the dimming performance and color rendering first, but I digress)? Even if I converted every light in the house to LED, I’d still be using several hundred watts, and I don’t have an especially big house.
I’m certainly not an expert on this subject, but I have read arc tracking (and arcing in movable contacts) is much more of a problem with DC verses AC, especially at voltages above 30 VDC. Assuming this is the case, I’m curious if the NEC standards that are currently used with 120 VAC residential power distribution systems can be used for 120 VDC systems, or if they have to be more stringent.
Or the mechanical switches could be eliminated for the DC lines, at least for fixed lighting. It might be a problem simply with outlets, though I don’t recall any 120VDC sparks, everything I’ve seen was lower voltage or much higher in the thousands of volts. It’s also possible that lower voltage AC could be run. But I think it more likely that 120AC (or 220 other places) would be the main supply lines and only reduced to low voltage DC for short runs between fixtures.
How these changes affect other appliances could be interesting. A lot of appliances could be run with much less supply. Alarm clocks, computers, TVs, etc. But homes will still have their high consumption appliances also, fridges (as the compressor kicks in), dishwashers, washing machines, electric dryers, electric heat, hair dryers, and many kitchen appliances.
I’ve already eliminated mechanical switches, except for a couple of closet lights, so I can see that happening. Electronic dimmers like I have should be able to run on low voltage DC easily.
Triac-based dimmers won’t work on DC; they will be stuck “on” because a triac conducts until current goes to zero, as it does during every AC half-cycle. It is possible to make a DC dimmer that works the same way, using a transistor to switch the current, but more efficient is to have a separate line (or transmit a signal over the power line) that is used by the driver circuit to adjust its output.
Mechanical switches would be much more of a problem on DC; in my experience, DC can very easily make sizable arcs at even moderate levels of voltage and current (e.g. I test power supplies I build with power resistors which I connect while it is running, at 100 volts or more and less than 1 amp, arcs commonly occur when they are disconnected, with sustained arcs over a quarter inch possible, even half an inch at 300 volts), although with derating you can use a switch on DC (for example, this relay is rated at 10 amps and 250 volts AC but only 5 amps and 100 volts DC); the size of household switches should allow for a large gap between open contacts and LED bulbs shouldn’t have much of a load.
AIUI, this is one big reason that the transition of automotive to 42 volt electrical systems has yet to happen. “12 volts” can’t really form or sustain an arc, but 42 VDC can which means in a car, more expensive and complicated circuit protection would be necessary.
Man, did you take an extra does of your pedant pills today?
What exactly, does a rectifier do, if it doesn’t convert AC to DC?
Yea, I was a little confused by that to.
To elaborate on what all of us already know: most AC-to-DC SMPSs converts the AC at your wall outlet to a lower and regulated DC voltage. In the past we used linear power supplies that contained a big, heavy, expensive transformer to do this. But SMPSs are much more efficient, so they’re more common now.
The SMPS does this using the following steps:
- Convert AC at wall outlet directly to DC using a rectifier circuit (diodes). Note that the DC voltage will be pretty high. For a 120 VAC input, it may be as high as 170 VDC.
- Store DC voltage on capacitors. The capacitors need to be rated for the high DC voltage, obviously.
- Connect capacitors to a switch (transistor) and the primary of a small transformer. Use an oscillator to supply a pulse train to the gate of the transistor at some frequency (e.g. 40 kHz). In essence, you are “pulsing” energy from the caps into the transformer, or (put another way) you are “pulsing” current through the transformer.
- The secondary of the transformer will be AC, and at a lower voltage.
- Rectify the AC to DC. The DC is the output.
- Lastly, you need a regulator circuit to regulate the output voltage. The regulator circuit should sense the output voltage and then adjust the duty cycle of the oscillator described in #3 to maintain a fixed voltage.
I’m glossing over a lot of details here such as isolation, PFC, EMI control, etc. But this is essentially how they work.