Getting rid of AC/DC adapters

Question

Many of us have lots of devices that uses AC mains (110V/220V) to DC (typically 12VDC, 9VDC and sometimes 15VDC) adapters. There's my Wi-Fi router, my powered speakers, printer, scanner, etc.

So I would like to combine all these into a single device; do the AC/DC conversion once and have everyone tap the DC line, instead of each doing the conversion themselves.

Regarding to this, I have two questions:

• is there any commercial solution out there to do this? (bench power supply for example)?
• suppose I have one of these beauties powering my stuff and this power supply can deliver 100W of power. Will the device always be consuming 100W of power or will the consume increase as I hang more devices?

thanks.

Answer 1

Your last question first: no it won't always consume 100 W, that seems to be a common misunderstanding. A 100 W power supply will supply 1 W if that's what the load needs. It's what you attach to the power supply which determines the actual power consumption. The 100 W is just the limit.

Bench power supplies are a possibility, but they are too sophisticated for this: they are designed to deliver 1 or more variable voltages, while you can do with fixed voltage levels. Also, a bench power supply usually has only a couple of voltage outputs, so it may depend on your needs if that's sufficient. You already mention three different voltages, but you might also add 5V DC, maybe others.

The lack of a standard makes it hard to design a universal supply; there's not even agreement on a standard polarization for the DC connector. So each power supply is designed for its own purpose: a given voltage at a given current. Today many adapters use switch-mode regulators for more efficiency, but also these are optimized for a certain output voltage. You could go from 15 V to 12 V to 9 V, but each level has its efficiency, and for the last link in the chain you would have to multiply all previous efficiencies, and the 15 V will need to be higher power than when it just has to power that router.

What you could do is remove the electronics from the adapters and put them all in a single enclosure, that would at least save you a couple of wall sockets. But you'll lose the possibility to wire every device up where you want; do you want to place your speakers in the same closet as your router?

Final note: a PC power supply delivers different DC voltages, but most power is available at the lower 5 V, maybe not enough at 12 V, and at tens of amperes at 5 V it's way overkill.

Answer 2

I too think that this superficially would be a great idea. When you look into the detail though, it's virtually impossible. What voltage would you distribute? A DIY magnetic in-defibrillator controlled by an Arduino might take 3.3v. But your door bell's buzzer takes 12v. And a LED night light might run on 2.4v. The industrial PLC watering those plants in the attic runs on 24v.

If you distribute 2.4v, you're going to lose a lot of power just through cable resistance.

If you have multiple voltages, you'll need multiple (somewhat redundant) circuits reaching all parts of your house.

If you have multiple parallel cable runs, that will significantly raise the material /installation costs of the building. This would be a difficult and unwanted proposition for starter home builders.

And how would you future proof it? When 0.5v CPUs become available for PCs, where would you get 0.5v from? So you're left with the lowest common denominator - 240VAC.

Answer 3

While this is old, I want to achieve the same thing. Forever, I’ve had many adapters near my “network”, which has grown into a network cabinet. Just now, I looked again and saw three adapters for three different thingies using some DC voltage. Each adapter is an odd sized plug or a brick that needs to be somewhere. The plugs prevent me from using some of the scarce 220V sockets. It’s a pain.

I looked for DC hubs and found nothing of interest, but I did find rack mountable dc power supplies. Some are expensive, but this last one is intriguing.

Apparently, each output has its own fuse limiting power to one of three amperages, they’re all 12V (I had hoped for different voltages as well, but maybe more searching down this path.) The benefit is that you can wire up the connectors yourself. It’s also an expired product, something they don’t carry anymore.

So the drawbacks in other answers are good. Because of them I wouldn’t opt for central 12V for a whole house, but for a rack enclosure like mine, this is awesome.

The product I found.

Answer 4

The disparate devices are not usually designed to have their negative voltage inputs connected at the power input. That could cause ground loops and EMC issues both in terms of susceptibility and radiation. To be general, we need DC outputs isolated from each other.

What would work is factoring-out of common components to create multiple DC outputs at a lower cost and better efficiency than duplicating entire power supplies. Namely:

1. A PFC front-end to rectify wide-range mains AC into the DC-link capacitor - typically 350-400VDC.

2. An EMI filter on the PFC input and output.

3. DC-to-AC inverter for a fixed frequency forward converter. This can be running at a frequency tailored to minimize the switching losses - say 30kHz. The transformer could be 3-phase to improve efficiency and lower EMI.

4. A transformer with multiple secondary windings - one for each output. The only high-strength insulation is between the primary and secondary. The insulation between secondaries can be of a functional nature and rated for 100V peak, thus using much less material than what the primary-secondary insulation would need.

5. A switching regulator on each secondary output. Since the secondary outputs are already AC, these are low-voltage low-loss switches.

6. A low-dropout wideband ("RF") linear regulator after each secondary switcher. It would provide low-ripple, low-noise output in the bandwidth up to about 1MHz or so. The remaining ripple and noise would be filtered with passive filter elements.

I've made one of those a few decades ago running at 1kHz, using three identical 60Hz transformers with about a dozen secondaries each, reclaimed from a "junk" pile. PFC used a junk mains frequency inductor/choke, running at 2kHz. To minimize audible whine from the cores, I've added a bit of spread spectrum. Easy to do since both switching frequencies were derived from sawtooths, so a noisy comparator threshold fed from amplified Zener diode noise gives spread spectrum. Making it electrically quiet was an interesting exercise, but in the end it was better than the cheap noisy switchers in some of today's equipment. I still use it. The voltages can be adjusted 4-24VDC, and each output is current-limited as well, with the current limit roughly inversely proportional to voltage, so that each output has a roughly fixed power envelope.

Answer 5

A house power system has to serve a wide range of loads - lighting, heating, appliances and so forth. As things have stood, this is more practical using a higher AC voltage, then converting as needed at the point of load. (Westinghouse won that 'war'.)

(There’s a point to all this. I promise.)

AC also suits motors better, especially large ones used for refrigeration and pumping. Incandescent lighting historically hasn't cared so much as it could work on either AC or DC. Historically then, it's most economical, if not also most efficient, to run everything on AC.

The introduction of LED lamps has changed the game. Their low power consumption means it’s now possible to illuminate interiors on smaller, low-power grids such as 12 volts. And, in certain situations (like RVs, campers, boats and fixed micro grids) they do exactly that. Speaking of RVs, even refrigerators can now run very efficiently on 12V DC using a Danfoss dual-voltage compressor.

While my camper works fine on 12V, and even keeps the beer cold, back in the house things are a bit different. As things stand now, in my and many other homes there’s ever more AC power demand. Yet my house is old, provisioned with only 100A for two structures. As I contemplate a shift to more efficient electric devices like heat-pump water heater, mini-split heat pump and heat pump dryer to replace gas fueled units, I have to up-provision my electric. There is also some pressure / incentive to eliminate the gas stove in favor of electric (I'm splitting the difference with a dual-fuel.)

Add to this possible at-home EV charging. This alone doubles what my (and many older) homes are provisioned for.

My point? None of these big-draw appliances are practical on low-voltage DC. In the big picture, the cost I am undertaking to upgrade my AC to 200A is 10-100x the cost of what I might lay out to make a 12V microgrid for lighting in my house and powering my low-voltage tchotchkes. And for what? To get rid of a few power bricks here and there?

Ok, so the bricks matter to you. Say we limit the scope of devices on the low-voltage grid to the ones you mentioned. Is there a possibility of a 'universal DC' solution like you propose? USB-C comes pretty close for that class of devices (5 ~ 100W). So could PoE for the router and wifi.

However, looking at your list of items to be powered by DC, there's two that stand out: powered speakers and printers. These can have peak draws of several hundred watts, beyond the reach of a 100W-class solution.

My second (!) point being, even within your limited example, there are enough high-power 'exceptions' in potentially low-voltage devices that using a single, low-voltage DC will force some tradeoffs somewhere.

Meanwhile, off-the-line AC-DC conversion gets ever more efficient and economical. Innovations in semiconductors and power supply design continue to net improvements in efficiency to make AC-DC less objectionable in terms of efficiency and ‘vampire power’.

You may have noticed how much smaller AC-DC converter sizes have become (just look at how much smaller laptop bricks have become) and how much cooler they run. And, with reduced size and heat brought by innovation, it’s increasingly economical to just incur the up-front costs of embedding the AC-DC supply in the appliance.

For household use then, the benefit of using a DC microgrid only comes into play when considering other factors, like integrating batteries and / or solar inputs to your mix. In limited cases for lighting and low-power appliances it also make sense where safety is improved (like under-cabinet lighting in kitchens) where AC voltage can pose a hazard.

Otherwise it's hard to entertain a case where having a low-power DC microgrid is helpful / more efficient / more elegant, despite USB-C attempting to do this.

Is there a place for high-power DC? Yes. Some large server farms have moved to a 48V DC grid. This enables UPS integration more efficiently than the usual method of AC-to-DC then DC-to-AC conversion. Instead, they use just one AC-to-48V DC conversion, then use DC-DC point-of-load regulation in the rack units. Result? About 5-10% improved efficiency with less heat being shed in the server racks. For a big install this justifies the extra upfront cost.

But it doesn’t solve all the problems. The chillers and air-handlers for the buildings still use AC, being as they use 3-phase motors (some of them with variable-speed drive, further improving efficiency.)

Answer 6

Gator GTR-PWR-12 Pedalboard Power Supply, 12 Outputs - 2300Ma. Maybe it won't replace every one of the adaptors in a house, but could possibly take care of 1 room at a time. At least unclutter a desk with a computer,printer, scanner, speakers, router etc....

Answer 7

One possible problem is the risk of starting a fire or zapping a connected device.

Something plugged into its personal 1W supply would normally never get more than 1W of power.

But if it is sharing a 20W supply with 20 devices, when anything goes wrong, 20W of power is available. And that's more than enough to melt a low-voltage wire and start a fire, or to destroy the innards of the device.

That's one reason that house wiring has many individual 15A breakers and individual supply lines rather than having a single 100A wire supplying all the outlets.