DC Wall Supply?

Putz · Post by **Putz** » Mon Jul 05, 2004 2:31 pm

I've been wondering for a while whether it would be feasible to incorporate a 12V DC supply system in a house.

(BTW, I tried searching the fora to see if this topic had been discussed before, but the keywords are far too common.)

What I'm envisioning is a large (probably not silent, unfortunately) "power supply" in the basement of one's house, with wiring throughout the house, and some kind of connector in every room to hook up electronics of all types. This infrastructure would supply DC voltage of some magnitude (possibly 12V), converted from the 1-, 2-, or 3-phase mains, wind/solar power, a hamster on a wheel, etc. One could extend this idea to include UPS functionality as well.

Then, devices that currently use internal or external (brick-type) AC-DC converters could simply run straight into the wall, thus eliminating the heat, noise, and space consumption that has become rampant in homes today due to the sheer number of transformers required. In my 8'x10' student apartment room, I have at least 15 of these things, for my computers, network gear, audio components, phone, alarm clock, battery chargers, etc.

Are there safety issues? Practicality "reality-checks"? Why hasn't this already been done? I understand the choice of ~120V AC way back when all it was used for was heavy-duty appliances and lightbulbs, but nowadays...

Imagine having a computer without a PSU at all (other than maybe something to regulate/adjust voltages for the various required rails)...

fancontrol · Post by **fancontrol** » Mon Jul 05, 2004 7:28 pm

Neat idea. We use 'built in' transformers for home low-voltage lighting installations all the time (halogen bulbs are typically 12V).
http://electronictransformer.com/cgi-bi ... i?pg=store for example.

Here's the rub: the lower the voltage the higher the losses in the line. That's why power is distributed at very high voltage. Or, at the risk of talking too much, power is the product of voltage and current. If something (a pc, for example) requires a given amount of power it will draw more current as the voltage decreases. Loss in the wire is the product of current squared and resistance. So, roughly, when you cut the voltage in half you end up with 4 x the line losses.

So, what do you do with the lighting stuff I linked to above? Use big wire (to reduce resistance, $$) and keep the runs short. Both of which kind of go against your idea.

I wonder what sort of wizardry off-the-grid types have come up with, since energy is stored DC (batteries). I wonder if, given the types of loads in a home today, a lower voltage on the mains (e.g. 50VAC) would be more efficient. Higher current on the line, but more efficient power supplies?

Also, I've see brick-eliminator things that take the place of many bricks. I've also read articles about super-fast inexpensive switching power supplies that are more efficient and less bulky than traditional bricks (but cost more to produce, no doubt, probably because bricks have been made by the millions for so long).

Just like USB brought power to so many things that don't have much to do with a computer (fans, lights, coffee warmers, cigarette lighters), perhaps the new power-over-ethernet standard will allow you to run your alarm clock, battery charger, etc. from the jack right next to your PC. No more unsightly bricks.

...dream on, eh?

Inexplicable · Post by **Inexplicable** » Mon Jul 05, 2004 11:44 pm

fancontrol wrote:Here's the rub: the lower the voltage the higher the losses in the line. That's why power is distributed at very high voltage. Or, at the risk of talking too much, power is the product of voltage and current. If something (a pc, for example) requires a given amount of power it will draw more current as the voltage decreases. Loss in the wire is the product of current squared and resistance. So, roughly, when you cut the voltage in half you end up with 4 x the line losses.

Yeah, power loss is one thing but the voltage drop alone could get difficult to manage if the draw is anything more than a few of amps. Maybe the middle ground would be to have one of those brick eliminators in each room. A new use for old discarded PC PSUs? I'm sure most of us have a few lying around.

EDIT: It just dawned on me that, in my case, the majority of the bricks are clustered around my computers, both of which have plenty of power to spare. Most of the bricks seem to produce either 5V or 12V, or both. The devices are there for the computer, so they don't need to run if the computer is off. Might as well take their power from the computer. Sacrifice a molex and a couple of PCI slots for power connectors, hmm...

sthayashi · Post by **sthayashi** » Tue Jul 06, 2004 5:24 am

Take care when trying to make DC mods. Be sure to make sure the adapter actually puts out 12 or 5V. Confirm it with a Voltmeter.

Several years ago, I had lost the AC adapter to my cordless phone. Me being the bright EE that I was, decided to build a new one to replace it. The phone claimed it took 12V in. So I went about my way to build an AC adapter that put out 12V. Well, when I plug it in, much to my disappointment, it doesn't work.

6 months later, I find the adapter that went to it and I measure the output. 18V. Even though it was clearly labeled 12V.

bomba · Post by **bomba** » Tue Jul 06, 2004 12:30 pm

No safety issues that I can think of, up to 50Vdc is safe to touch.

Regarding line losses, Fancontrol is right on. I've worked with monorail type lighting w/ remote 12v transformers and was amazed at the huge wire guage required to limit voltage drop, check out the guidance given in this document, scroll to the bottom of the page. BTW, the document recommends 4ga wire for a 40' run from the transformer to the lighting rail for a 300w load @12Vdc!

Putz · Post by **Putz** » Tue Jul 06, 2004 2:57 pm

Then what about 48V DC? The addition of a DC supply is the more-significant part of my idea, since it seems to me that a "simple" 48V-12V step-down would be much more efficient heat-, noise-, and space-wise than the typical AC-DC converters. Would voltage drop/current leakage still be a problem at 48V DC?

bomba · Post by **bomba** » Wed Jul 07, 2004 6:26 am

Putz wrote:Then what about 48V DC? The addition of a DC supply is the more-significant part of my idea, since it seems to me that a "simple" 48V-12V step-down would be much more efficient heat-, noise-, and space-wise than the typical AC-DC converters. Would voltage drop/current leakage still be a problem at 48V DC?

I think this will be workable. FYI, tlecom equipment is commonly powered by -48Vdc. Found a voltage drop calculator, here. Good luck

Edit, found a company that sells ATX style PSU's for telecom use w/ DC input.

Inexplicable · Post by **Inexplicable** » Wed Jul 07, 2004 8:24 am

bomba wrote:Edit, found a company that sells ATX style PSU's for telecom use w/ DC input.

Efficiency 70%. Ouch.

mpteach · Post by **mpteach** » Wed Jul 07, 2004 10:20 am

This is not a bad idea if it was only use for small brick fed devices. it would reduce clutter and Ups funtionality would be great for brrick fed devices like my kitchen cordless phone.!!!

Unfortunatly this idea is not eonomically feasble to replace high power devices like computer power supplies. Some CPU's use over 45 Amps by themselves. To make things worse at those low volages their not much room for any volage drop at all. 3.3V - 2.2V is disatorous while 120v - 110V is not.

Putz · Post by **Putz** » Wed Jul 07, 2004 1:44 pm

Assuming the devices to be powered require less than 48V, if the stepdown mechanism at the device end is of the self-regulating variety (think Zalman FanMate), a voltage drop from say 48 to say 45V in the line wouldn't matter.

However, I was under the impression that a DC-DC voltage-fixer-upper* would be more efficient than an AC-DC type PSU... if the one linked above (at 70%) is any indication, I was mistaken, hence eliminating any heat-related benefit I was anticipating.

(* non-technical term used intentionally to display my lack of actual knowledge in this subject)

My dream house would have such a system, but unfortunately, it's probably either not feasible, or, more likely, not marketable. And it's a little bit too big of an undertaking for me to DIY-it-up. However, if anybody else is inspired enough to actually attempt this feat, please be sure to let me know how it goes!

msde · Post by **msde** » Thu Jul 08, 2004 5:17 pm

Solar, wind, and hydro powered systems generally have to deal with this, unless they feed their power directly into the grid. When you decide to build one of these to power your home, you end up storing your energy in a bank of batteries in the basement. They usually run at 12V, 24V, and 48V, and have to deal with the problem you have.

mpteach · Post by **mpteach** » Thu Jul 08, 2004 5:24 pm

Putz wrote: However, I was under the impression that a DC-DC voltage-fixer-upper* would be more efficient than an AC-DC type PSU... if the one linked above (at 70%) is any indication, I was mistaken, hence eliminating any heat-related benefit I was anticipating.

(* non-technical term used intentionally to display my lack of actual knowledge in this subject)

I believe the term "converter" and "-fixer-upper*" are used interchangeably

orionlion82 · Post by **orionlion82** » Sun Jul 11, 2004 8:58 pm

aww heck, i put in a power failure lighting system in my kitchen on top of the cabinets,...basically 12 volt outdoor (malibu) type lights stripped to bare bulb-and harness, battery clip from old dead screwgun#1, 9.6v battery and charger, allso from #1, and 12 volt battery and charger from scrwegun#2 wired it up ran about 20 feet of #18 speaker wire to a convenient place where the battery clip could live without getting lost or getting in the way, found the two chargers a home (keep the batteries there till needed and viola! a manual, rechargable emergency lights... 45 min runtime per batt.i think i got about 8 - 4 watt lights...dont know why the 9.6 has the same runtime (+or- 5 minutes), but of course 12 volt yeilds whiter light

fancontrol · Post by **fancontrol** » Mon Jul 12, 2004 8:43 am

I was waiting for the part where you shove an allen wrench in the chuck of the drill and crank it around as a generator.

I'd like to see someone show up at the airport with an Antec ARIA under one hand and a car battery under the other.

mpteach · Post by **mpteach** » Mon Jul 12, 2004 10:04 am

why didnt i think of that?

equisol · Post by **equisol** » Mon Jul 12, 2004 1:51 pm

12V DC and AC, depending on the application works great in houses for quite a few items.
Most electronics run on either 12V or 5V, including TVs, stereo and our computers. The first think you notice after the 110Volt wire is a power supply to reducre the voltage to 12 or 5 V.
The issue as identified by another user is the power loss, if the current does not exeed 10A, it would be safe to go with no 14 wire for runs up to 35 - 40 feet. Build the master power supply somewhere in thhe middle of the housein the basement and work your way up. There are quite a few power supplies that will give out 50+ Amps.

I personally use a set of golf cart batteries as ballast and charge these (they need to be in a ventilated area).

One year ago I convertedd all my computer to Mini-ITX models and they all run on 12V. Never had a problem since, and who cares about power outages or power surges that are quite common. The batteries will last for a full day with a computers running including monitors (flat panel all running 12V).

Here is a link to simulate the power consumption of a computer:
http://www.mini-box.com/powersimulator.html

For extra safety I use one fuse per outlet. Connectors are all the same, the same kind used for the VIA computers.

I also use 12V AC for lighting (12V halogen).

12 volt is much safer that 110.

Putz · Post by **Putz** » Mon Jul 12, 2004 1:54 pm

equisol wrote:12V DC and AC, depending on the application works great in houses for quite a few items.
Most electronics run on either 12V or 5V, including TVs, stereo and our computers. The first think you notice after the 110Volt wire is a power supply to reducre the voltage to 12 or 5 V.
The issue as identified by another user is the power loss, if the current does not exeed 10A, it would be safe to go with no 14 wire for runs up to 35 - 40 feet. Build the master power supply somewhere in thhe middle of the housein the basement and work your way up. There are quite a few power supplies that will give out 50+ Amps.

This is exactly what I had in mind. Maybe it's not that unreasonable after all!

orionlion82 · Post by **orionlion82** » Mon Jul 12, 2004 4:39 pm

fancontrol: was waiting for the part where you shove an allen wrench in the chuck of the drill and crank it around as a generator.

well, before i batteried it, i did just that, except with screwgun#3 (the only one that still works - decently) i just direct-drove #2 with some sort of shaft and ducttaped the switch on both... it worked, but it all had to be strapped down with plumbers tape, and made the most terrible noise! im glad i decided to simplyfy it, arent you?

orionlion82 · Post by **orionlion82** » Mon Jul 12, 2004 4:51 pm

on a more serious note, i am considering using a 12V power supply from a CB radio (vintage 70's) to ease the load on my POS 500w Psu that keeps crashing me. i see it as a candidate because it was made for electronics, (maybe not computer grade, but still) and i can take all the load from my POS thermattake Xaser iii fans out of the equation. thoughts?
a quick word about TT:the sheet metal and aluminum bezel are very nice, sturdy, and solid but the plastic drive mounts, fans, and fan controler are garbage-as is the cracked and crazing lexan window. i should have listened to yall when i first proposed the westminster project(search for it) /now called Knossos anyone know where i can get a new temp sensor?

Linus · Post by **Linus** » Tue Jul 13, 2004 10:09 am

Putz wrote:However, I was under the impression that a DC-DC voltage-fixer-upper* would be more efficient than an AC-DC type PSU... if the one linked above (at 70%) is any indication, I was mistaken, hence eliminating any heat-related benefit I was anticipating.

Actually, Opus Solutions has 90W and 150W DC-DC PSU models with ">90% efficiency at optimum loads". Using one of these would be far preferable for battery setups, where you would otherwise need an overly complex and inefficient DC-AC-DC setup.

Once I get a solar panel and a battery, I'll be using the 150W Opus to power both my LCD TV and my computer, eliminating the need for an inverter and replacing a low-efficiency 185W AC-DC PSU and a 60W AC-DC 12V brick with an ATX-sized DC-DC PSU.

fractal · Post by **fractal** » Sat Jul 17, 2004 10:15 am

A couple of random thoughts.

1 - It is better to run a higher voltage than a lower voltage. Voltage drop is a function of current and a one volt drop at 120 volts is less significant than a one volt drop at 12 volts.

2 - all modern consumer power supplies are what are known as "switching power supplies". The mains come in, are rectified and filtered, then are "chopped" at very high frequencies. A high frequency transformer is much smaller and less expensive than a low frequency (50 or 60 hz) transformer for the same power rating. Hence, just because it plugs into the wall and gets AC, the AC is immediately turned into DC and back into AC. A DC to DC converter simply eliminates the first conversion. The efficiency of the supply should be independant of whether the supply started off as AC or came in as DC. I.E. you will see *NO* heat savings by running 12, 24 or 48 dc around the house and using a dc-dc converter at the point of use.

Combine those two and you can see why 12v is probably not the best voltage to run around the house if you intend to use it to run a dc-dc converter. 48v is a common dc voltage for the solar community. 24v may be even more common. Buying a combination charge controller / inverter and running ac through the house is likely to be your most efficient, both in terms of heat and money, way to utilize your solar cell.

mpteach · Post by **mpteach** » Sat Jul 17, 2004 3:39 pm

as i said before

1. dc-dc conversion is good if you are running of of a solar panel or batteries

2. Dc wall outlets would only be good if you wanted to run a bunch of low power devices and get rid of all those ugly power bricks, or want ups backup on your powerbrick devices

hvengel · Post by **hvengel** » Thu Aug 26, 2004 2:31 pm

DC to DC conversion where volts in < volts out requires an expensive DC to AC to DC conversion. In the case where volts in > volts out does not.

By the way with AC a voltage change up or down is not costly since it can be done with a transformer and is close to a nearly perfect lossless conversion. This is one of the key reasons why the electric grid is AC. Most of the power loss in a AC to DC power supply happens during rectification (changing the AC to DC) and in voltage regulation. Not in converting the AC to the new AC voltage or the base DC voltage to other lower voltages.

I had a place that used solar power. The best thing to do is to use as much of the power as DC as you can and only use AC when there is no other option. So I had both a DC distribution system and an AC distribution system. The solution for DC was huge copper cables. Some of those in my system were 00 gauge (about the size of your thumb) but I was also doing the following with DC:

All lighting
stereo
TV
washing machine
well water supply pump (60 feet of head)
communications
solar water heating controls and pumps

For things like the washing machine (Matag wringer machine that are still being made) and water pump I was using DC motors from Honeywell that were about 90% effecient. So a 1/2 hp motor was only using about 400 watts. Good inverters are close to 90% effecient but typical AC motors are only about 70% so the extra cost of setting up the DC motors reduced the cost of the system significantly for the same results. Typicaly I would only turn on the inverter for specific uses and then only for short periods of time. Most days it did not get turned on at all.

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