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Was Thomas Edison Right about DC Power? 545

Posted by CowboyNeal
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Declan McCullagh writes "Everyone knows the alternating vs. direct current wars ended with Thomas Edison and Nikola Tesla. But now DC power is being seriously considered for data centers. DC advocates say that plugging servers into AC power is inefficient, and switching to DC cuts down on waste heat and component failure. The University of Florida has even bought 200 DC servers."
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Was Thomas Edison Right about DC Power?

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  • Was Thomas Edison Right about DC Power?

    Oh, well, nothing sensationalist about that headline. (*rolls eyes*)

    DC advocates say that plugging servers into AC power is inefficient, and switching to DC cuts down on waste heat and component failure.

    In this case they're right. With that much hardware that close together, it's easier to treat the entire room as a single device. As the article suggests, this cuts down on waste heat produced by inefficiencies in AC->DC conversion. In fact, it significantly cuts down on the amount of equipment needed in the entire room. The concept can be taken as far as to cutting down to a single power supply per rack.

    The amusing part about this is that the resulting racks might look a lot like Big Iron servers with pluggable motherboards. :-)
    • by Dukeofshadows (607689) on Thursday March 02, 2006 @10:51PM (#14840125) Journal
      Wrap the casket in copper, replace the headstone with a magnet, and expose corpse to this article. As Tesla turns in grave, free power.
    • by oddbudman (599695) on Thursday March 02, 2006 @11:03PM (#14840188) Journal
      The concept can be taken as far as to cutting down to a single power supply per rack.

      The article mentions the distribution will be done using 48V for distribution - you will still need DC:DC conversion at the boxes. These DC:DC converters will need to be run at higher current than an AC:DC converter. Higher current can cause more series loss in the system as well leakage losses in the switching supply.

      AC:DC converters, as mentioned in the article aren't really that ineffiecient (article itself quotes 90%). AC:DC converters are infact really DC:DC convertors, they just have a rectifier circuit to convert the AC to high volatge DC for DC:DC conversion.
      • by Anonymous Coward on Friday March 03, 2006 @12:35AM (#14840550)
        Meanwhile back in the real world ...

        In a standard PC power supply the incoming AC is rectified and stored in a capacitor. Energy only flows into the capacitor when the voltage after the rectifier exceeds that stored in the capacitor. This results in a waveform which departs considerably from a sine wave - no current flows for most of the time while much higher currents than expected flow at the peaks of the half cycles. Electricians interpret this as a bad "power factor" from their experience driving inductive loads where the current lags the voltage by as much as 90 degrees.

        Standard PC power supplies are nothing like 90% efficient largely because of this crude rectification of the mains. Compare the rating of your supply in watts with the input voltage multiplied by the input current. These values should all be marked on the case.

        Power Factor Corrected (PFC) supplies are available. The better ones use a switch mode circuit to charge the reservoir capacitor through most of the main power cycle, while the less good ones incorporate a capacitor across the mains to buffer the large peaks of current when the input voltage exceeds that stored in the reservoir capacitor.

        One advantage of AC is the ease of transforming it to other voltages using transformers and the ease of using it to drive motors especially with multiple phases. In the modern age where switch mode power supplies are cheaper than those using transformers operating at mains frequency this advantage no longer exists. One disadvantage of using DC is the difficulty in switching the stuff off - inductance in the load drives the current straight through an opening switch or fuse creating a nice sustaining arc which is not quenched by the current dropping to zero twice each cycle.
      • Is that quite so? Wouldn't there be taps on the transformer for approximately 12V and approximately 5V, and then the potentials finely adjusted using DC-DC regulators? Wouldn't that have less loss?
        Taking this a bit further, why not have an entire rack power supply that can deliver a rail of 3.3V, 5V, and 12V to each server, thus eliminating the need for a high-current DC-DC converter on the target board? I am excluding things like the exotic voltages for CPU and RAM, but still it is the 12V and 5V rails t
        • Because there's oodles of 48VDC power supply gear out there now, since telcos buy it in trainload lots to run their equipment. Battery for telephony has been 48V pretty much forever. Gear that can give you reliable 5V@1000A is probably rather scarce (pronounced "expensive").

          It's a good idea but it won't fly until DC becomes common in datacenters. And then it won't fly because the datacenters will have all been rigged for 48V. :-(
      • Ac to dc converters are more tricky because it is necessary to isolate the input from the output. That means using a transformer with either special insulated wire on the primary or the secondary or an insulating layer between the primary and secondary. This means the transformer is less efficient than if the copper was more closely coupled. Also you have to design for a wide variation of input voltages and account for minor brown-outs. Most switchers these days are designed to run on anything from 100V to
      • As the parent said, using DC to feed the racks still requires point-of-load DC to DC converters.

        In fact, the biggest problems with today's AC supplies is that the frequency is TOO LOW... this results in the transformers and other AC-AC converters being oversized. Pretty much every switching supply today, including the ones in PCs, chop up either AC or DC into much higher frequencies and this allows smaller components. Avionics have used this for quite a long time, as weight and size savings are crucial!


        • by olman (127310) on Friday March 03, 2006 @09:04AM (#14841869)
          The only limit on using higher frequencies comes when you start to get magnetic losses in transformers and chokes, so in practise a few hundred kHz is the useful limit in switched mode PSUs.

          Thus, if you were starting again with an electric grid system, 500 or 1000Hz would be a much better solution.

          IAAPD (I am a PSU designer)

          1MHz SMPS is nothing fancy these days. In fact they're available even as integrated chips which combine FET switches and the controller into one IC. If you count in point-of-load charge pumps and such, you see up to 3MHz.

          Generally speaking, worst offender is high-current FET gate charge which eats up more power than all the other losses combined for synchronous buck transformer (higher DC to lower DC topology, most common type in use probably) Small (1-10uH) inductors are much better behaved in comparison. One reason to use such high frequencies is indeed that you can get smaller inductors and you have less ripple current.. But you're limited by the fet gate charge. Of course, if you're driving some 200W load, you can just say "h*ll with it" and build high power driver circuit to drive your switches, 5% waste heat on switching losses doesn't bring down the house when you can use dirt cheap inductors.
    • by LoRdTAW (99712) on Thursday March 02, 2006 @11:25PM (#14840273)
      Easy UPS too, if the servers use DC then batteries can easily be hooked right to the power bus that feeds them. No ac-dc dc-ac ups systems. If you have a 48V server you get 4 12v lead acid batteries hook em in series and hook them in parallel with a charging resistor and discharge diode. I know its a little too simple but at least it could be easily rigged up in case of emergency.

      Thing is how much more efficent is it to have one large ac-dc converter and then smaller dc-dc converters( ary.html [])? You are still converting ac-dc and then dc-dc again just like a normal power supply in a computer. If they oversize the main ac-dc then over size the dc-dc then I can see how it might be better.

      LoL It would be fun to get the DC from old rotary converters for a data center. Big mountain of spinning cast iron with slip rings, commutator, brushes and plenty of copper windings. Put in an old marble switch board with carbon arc breakers , synchro scope, volt/amp meters and knife switches. You then have yourself a turn of the century power system running new millenium computers :). Not efficent and high maintenence but how cool would that be!
      • by njh (24312) on Friday March 03, 2006 @12:10AM (#14840456) Homepage
        LoL It would be fun to get the DC from old rotary converters for a data center. Big mountain of spinning cast iron with slip rings, commutator, brushes and plenty of copper windings. Put in an old marble switch board with carbon arc breakers , synchro scope, volt/amp meters and knife switches. You then have yourself a turn of the century power system running new millenium computers :). Not efficent and high maintenence but how cool would that be!

        I believe this is SOP in EMP hardened bunkers.

        And 4 12V lead acid batteries would be 57.6V :) 3 is 43V, which is by complete coincidence the same voltage proposed for new cars.
        • by Almost-Retired (637760) on Friday March 03, 2006 @01:23AM (#14840726)
          57.6 volts? Under heavy charge rate and fully charged maybe. Thats so high the electrolyte in the batteries will be history in 3-5 days regardless of the formulation of the individual battery.

          I at one time had an older NCR ups, a huge old 150 pound honk rated at 1.2 kva, but it could output that 1.2kva for quite a length of time, running these two machines and one of the monitors for about 2 hours one day before I go nervous and did a gracefull shutdown till Allegheny Power managed to roust out a crew into our neighborhood 3 damned days later.

          It originally came with a 4 pack of 12 volt gelcell batteries in it of about 12ah each, but when it came into my posession they were toasted.

          On checking the float voltage I found it about 2 volts above what I would have called a good float voltage when divided down to a per cell rating, so I knew they'd been overcharged and dried out. I put in 4 18ah motorcycle batteries after setting it down to 52 volts, and boiled them dry in 9 months. I dropped it another volt and replaced them again, this time they lasted about a year before they were bone dry. As I'd rigged the overflow tubes to dump into a small jar of soda, I checked to see if the soda was affected, but it was still as pristine and white as the day I set it up. As that was about $120 a year for batteries, I said to hell with it, stuck a 2 wheeler under it and parked it on the back porch, replaceing it with the same size Belkin, which turned out not to be anywhere near big enough, shutting itself off rather unceremoniously at about 60% of its rated load. I yelled at Belkin and they sent me a much larger unit thats worked for about 4 years now with one battery replacement about 8 months ago.

          Idealy I should have been able to run the wet batteries in a stationary environment for 5 to 7 years, and possibly could have if I'd figured out the right float voltage for those batteries.

          Perhaps even a fixed trickle of about a milliamp once charged would have worked, but thanks to NCR's habit of burning old docs, I had none on that unit.

          I once ran a set of 225ah big truck batteries for 8 years on a standby generator after reducing the trickle charge till there was no more gassing, which was a current of about 5 ma. At the end of 8 years, they would still turn that Cummins 335 hard enough the first cylinder comeing up fired. And the next, second cylinder firing spun it on up enough to kick out the starter, a total elapsed time of maybe 1/4 second from hitting the button and it was only another second to make 1500 rpm and energize the alternator, for a total power outage to on generator elapsed time of about 3.5 seconds. Those 2 batteries would check at about 27.1 volts anytime.

          When you've lived where car batteries can freeze and split overnight if not fully charged, one tends to finetune the voltage regulators in the vehicles that must just start, for each battery. I've had batteries that were happy at 15.8 volts without gassing excessively, but in that home made regulator I had strong negative temperature comp too, slopeing down to about 13.8 at 70F, and the next one boiled like crazy at 14 sloping down to 12.4. Each battery has its own 'personality' I guess. Go figure. Yeah, its the old fart again, pontificating a bit about that which he's played with.

          Cheers, Gene
    • by MillionthMonkey (240664) on Thursday March 02, 2006 @11:44PM (#14840358)
      I don't understand this whole issue with AC and DC. Both require massive investments in overhead wiring, which despoils the beauty of our suburbia, causes copper shortages, introduces losses from line transmission, requires odd things to be done to trees, and gives birds a comfortable place to sit directly above your car.

      Why not just deliver the electricity off a truck like everything else in life? This country has gotten so "addicted" to current electricity that we've forgotten that static electricity even exists. A single charged capacitor can supply enough power to run a modern datacenter; the only limitation is capacitance. Say your datacenter runs on 10 kilowatts (that's just a guess) so you need 240 kWh of power a day, or 864,000,000 Joules of energy. Can a capacitor deliver that amount of power?

      Sure it can, if it has enough capacitance. Energy storage is 0.5*C*V^2. Say the cap is 1 Farad, and we choose a reasonable charge voltage of 500 kV. [] How much energy is that? 125,000,000,000 Joules! WOW! That will keep you all set for 144.675926 days of continuous uptime! Every couple months, the electricity truck arrives and delivers your charged cap, and you give your spent cap back to the electricity man to be recharged at some high-sulfur coal plant in another state. (That means recycling which will help get the "greens" on board.)

      Of course then, you have the nitpickers. "But what about the gasoline for the truck? Isn't that a wasteful means of electricity transmission?" Just use the energy in the caps to run the truck! It's like hydrogen! Hydrogen has already been shown to be politically viable.
      • by Waffle Iron (339739) on Friday March 03, 2006 @12:15AM (#14840476)
        Considering that it would be storing the energy equivalent of 30 tons of TNT, if you were to notice the lid starting to bulge on that cap, it would be wise to run like hell.
        • by MillionthMonkey (240664) on Friday March 03, 2006 @01:53AM (#14840851)
          Considering that it would be storing the energy equivalent of 30 tons of TNT, if you were to notice the lid starting to bulge on that cap, it would be wise to run like hell.

          Actually I remember doing some physics problem where I had to calculate the energy density of a simple high voltage paper/oil capacitor charged to near its breakdown voltage. I got an energy density for the cap that was 3% that of gasoline. Chemical fuels are just amazing.

          You could run your datacenter off a huge current in a superconducting ring kept near its superconducting transition temperature. As the magnetic field slowly collapses, a circular electric field forms around it. You stick a coil in that field and connect it to your datacenter. Cold magnetic rings can be delivered by (refrigerated nonmagnetic) truck. This scheme is only limited by the current in the ring when it comes off the truck.

          You could use a spinning disk. I'm guessing a steel disk spinning almost fast enough to structurally fail and fly apart might have an energy density similar to that of a fully-charged cap. Maybe it's possible to create an ultra-spinnable disk using carbon nanotubes. Then you could spin the disk much faster, and keep your datacenter running longer. I'm too lazy to figure out how fast you can spin a disk like that. But the edge can't go faster than c, or weird relativistic things start happening to the disk. Carbon nanotubes can only get you so far. They could spin the disks up in China, and send them here. But they would have to be careful. If every person in China spun up such a disk at the same time, it might affect the position of the North Star or change the length of the day.

          You could just run your datacenter off the 30 tons of TNT.
    • by Doc Ruby (173196) on Friday March 03, 2006 @01:03AM (#14840656) Homepage Journal
      Edison wanted to pump DC across longdistance lines, which would have consumed much more than the 10-20% losses in Tesla's AC. A hundred years ago, electronics complexity was so low that a single electric motor's mechanical power was often distributed around an entire factory by pulleys, rather than use multiple motors. Now we've got more complexity on a single square CPU inch than existed in the entire world when Tesla and Edison battled.

      I'd love to see how Tesla would have applied his high frequency/voltage engineering to photonics.
  • by 9mm Censor (705379) * on Thursday March 02, 2006 @10:19PM (#14839946) Homepage
    I heard of this new power system. Seems like a mix of AC and DC, to create the ultimate power form. AC *lightningbolt* DC was the name, and with a lightning bolt in the name, it has to strike you like thunder.
  • No, he was having a pissing match with Tesla.

  • by Anonymous Coward on Thursday March 02, 2006 @10:20PM (#14839954)
    In Washington State, Verizon (Was once GTE) runs almost all DC powered servers and Telco equipment in their Data Centers. Many of the IBM server my company buys support DC power.
    • by isdnip (49656) on Thursday March 02, 2006 @10:49PM (#14840118)
      Actually, that's the norm across the phone industry. Everything, and I mean everything, runs on -48V DC. Okay, not the fluorescent lights....

      This goes back to the telephone talk battery, which is -48 V DC. That powered the phones via old cord switchboards, and was the voltage of electromechanical (stepper, and later crossbar) switches, which basically used relays. Electronic gear was then designed to run on the same power plant. A telephone building has a big bank of batteries, powered by multiple "rectifiers" (DC supplies) which, btw, are normally engineered to not run over 40% of load. (That way they can still run the systems and recharge the batteries when one of them is kaput.)

      If you then put anything else into one of their buildings, the Network Equipment Building Standards (NEBS), which are Telcordia documents that practically carry the force of law, dictate that equipment be DC powered. Among other things -- NEBS gear has to meet the brick schytthaus test. (Sun Netras and many Cisco routers meet NEBS. Your basic rack server doesn't. And aluminum racks are STRICTLY forbidden; it has to be steel.)

      So because of the talk voltage on analog phones, lots of computing equipment is engineered for -48 V DC power. Sort of like the legend (I know, that one is not really true) about the railroad track gauge being based on Roman chariots. But in this case it's surprisingly effective.
      • by Myself (57572) on Thursday March 02, 2006 @11:30PM (#14840299) Journal
        The origin of the 48 volt number is that it was convenient, and now it just sneaks under the 50-volt "low voltage" cutoff in the NEC, which I think was written with telcos in mind. The glorious thing about this is that you don't need licensed electricians to do power wiring in a central office.

        And the reason it's negative with respect to ground goes all the way back to the telegraph system: Western Union initially ran bipolar lines and noticed that the positive ones corroded much faster. Sodium ions (from dissolved salt) are negative, and thus repelled from lines that're also negative. The whole phone system was built with positive ground because of this, and it's saved incalculable maintenance costs. It does tend to mess with people's heads the first time, if they're used to negative ground systems, but you get over it quickly. (A number of traditions use blue for "hot" and black for ground/return, to help escape your "red equals positive" association.)

        DC power as used by telcos is also always redundant. There's an A-side and a B-side for everything, and the cables are sized so that the entire load can run from just one side. This leads to some very fat copper, which is cheap compared to downtime. You don't achieve five-nines reliability with a system that contains single points of failure!

        Now, about rack-mounting: This was also invented by the telcos, originally in a very wide (40-inch?) format, for the panelboards and Strowger switches. Some of the old crossbar equipment is still in those huge racks, but the 23-inch width is infinitely more common now. All telco equipment is mid-mounted, with the ears approximately in the center of gravity on the shelf, so the force on the screws is shear. There's no torsion on the mounting flange unless you step on the front or back of the shelf. Cooling is always convective bottom-to-top, or occasionally front-to-back with fans. This leads to a "cool" front aisle and a "warm" back aisle between alternating rows of equipment.

        Now, the pro audio industry borrowed the rackmount idea fairly early on, but they were mostly mounting control panels and mixers, which are very shallow, so flush-mounting made sense. They also changed the every-inch Western Electric mounting holes to an alternating-spaces "EIA" standard, and narrowed the rack from 23 to 19 inches.

        Somewhere along the line, an absolute idiot decided that computers should be rackmounted, but they should be 19 inches wide, flush-mounted, and use EIA hole patterns. I'm sure this has something to do with mainframe legacy getting perverted by peecee people. The current mishmosh of mounting standards (19" vs 23", two-post versus four-post, flush versus mid, inch versus RU, front-cable versus rear-cable) is what every datacenter tech deals with on a daily basis. Throw overhead racks versus raised-floor cabling into the mix, and you've got a recipe for frustration!

        If you're familiar with the concept of "blade servers", where common components are separate from processor resources in the shelf, congratulations. Telco hardware has been built like this since the invention of the circuit board. Actually, the concept of replacable plug-in units goes back before that, but it got vastly easier with printed wiring boards and card-edge connectors in the sixties. Most of the "good ideas" in serious computing circles are actually century-old ideas in the telco industry. Spend a week shadowing a central office tech before you design a datacenter, please!

        Also consider: If your datacenter is already built for DC, throw some solar photovoltaic panels on the roof. Inverters are a large part of most PV systems' expense, and you can skip that part. Why not start offsetting your grid demand now?

        Also also: Edison was flat-out wrong about DC. The modern switching power supplies that make DC transmission lines practical didn't exist in his day. Besides, long-distance power transmission is an entirely other discussion.
  • by davmoo (63521) on Thursday March 02, 2006 @10:22PM (#14839966)
    Tesla and Edison were both right...and wrong. Like many Slashdotters do when debating which operating system is best for any given job, Tesla and Edison wanted to apply one power system to every job. Its like having a toolbox with only a screwdriver in it. Ever try to drive a nail with a screwdriver?

    For moving power over long distances, AC is king. But for short distances with most modern electronics, DC would win. The first thing a desktop system or server does with AC is converts it to DC. So if you have a number of machines all in the same room, why not do the conversion in one spot, and eliminate the redundancy in every machine.

    Would it benefit the average user with one or two machines? Not at all. But for a major center with many machines in the same room, I can see quite a bit of benefit with going with DC.
    • by daniel_mcl (77919) on Thursday March 02, 2006 @10:32PM (#14840021) Homepage
      "Ever try to drive a nail with a screwdriver?"

      Nope, but I've put in screws with a hammer, even when I had a screwdriver on hand.
    • Yes, but I would still have more then ONE invertor. Redudancy in a data center is something you WANT. Redundant PDU's attached to two different substations as well as a generator backup.
    • by Phanatic1a (413374) on Thursday March 02, 2006 @10:35PM (#14840035)
      For moving power over long distances, AC is king.

      Nope. For the longest-distance transmission lines, you see DC being used. There comes a point when the capacitive losses you get from using AC encourage you to switch to DC, and for lines of several hundred miles, you start seeing DC transmission lines.
      • by hpa (7948) on Thursday March 02, 2006 @10:42PM (#14840080) Homepage
        In addition to capacitive losses, there is also the fact that you have to dimension your transmission lines to handle up to Vp (peak), not just Vrms which is what controls the amount of power that actually travels through your system. In effect, by going to DC, you can run the whole system at 1.4 times the voltage, and run more power through the same wires with no additional losses (other than conversion.)
    • The big problem is also the length of the transmission cable. A few feet of wire carrying 12 and 5 volts in your PC won't be too wastefull. But that wire is suddenly say 100 feet long than it can start to waste electricity as heat. Say at 12 volts your powering a component that needs 2 amps. And the long wire now acts say as a 1 ohm resistor. So using V=iR, the voltage drop becomes significant and your power supply now needs to account for it. But yes, you are right in that a few machines next to each
    • Actually the article gets it quite wrong, with a bogus explanation when it says "For Physics reasons"...

      HVDC is actually FAR better for long line power distribution due to ACs inductive line losses... IIRC DC _is_ used some places. (California)

      The downside is that AC requires only a series of transformers to step it down to various levels for local power distribution--- Makes for a relatively straightfoward infrastructure.

      DC for all practicle purposed MUST be converted to AC for this purpose, via big honkin
    • by Cordath (581672) on Thursday March 02, 2006 @11:41PM (#14840350)
      Tesla originally worked for Edison, but they had a bit of a falling out, which is possibly why the AC/DC competition was so heated. Edison embarked on a pretty ruthless and gruesome campaign to discredit AC power, at least by modern standards. He electrocuted stray dogs and cats with AC current in public demonstrations intended to show how dangerous AC power was.

      In one instance, he even electrocuted an elephant...

      During the construction of Luna Park on Coney island, an elephant used as a beast of burden went out of control and killed a couple of people. Topsy, as she was called, was condemned to death. However, there was a wee bit of a problem. Elephants aren't the easiest critters to kill. What happens if you walk up and fire a shotgun at it's head, only just to piss if off? They do have rather thick hides, and we are talking about a homicidal elephant the size of a couple SUV's here. There weren't any cliffs handy to stampede poor Topsy off of, and I doubt dynamite was ever seriously considered. Edison, being the generous person he was, gladly volunteered to execute the elephant with AC current and filmed the whole thing. He showed the resulting film, "Electrocuting an Elephant" (1903) publically on many occasions. It is quite probable that many a cat and dog escaped a crispy fate thanks to this film. If you decide to track down a copy of "Electrocuting an Elephant" today, please be warned that it's a rather gruesome little piece of history, and is not for the faint of heart, or SPCA members.
  • by hpa (7948) on Thursday March 02, 2006 @10:23PM (#14839971) Homepage
    It's true that DC-DC power converters are more efficient than AC-DC converters, only if you consider than the typical DC-DC converter has a much lower voltage ratio than the typical AC-DC converter. DC power distribution is usually done in the 12-48 V range, depending on application, whereas AC is 100-240 V. It's also only a win in if you don't end up losing that power in the wiring.

    How come there is no real difference? Because both modern AC and modern DC supplies start out by converting the power to high frequency AC (on the order of several kHz), and operate on that. That's what you actually want as input, if anything.

    The article states:

    By distributing redundant direct current power to each server--and replacing the standard AC power supply with a far more reliable and efficient DC power supply...server reliability is increased by as much as 27 percent, and monthly power costs are reduced by up to 30 percent.

    In other words, the DC supplies they use are more efficient than standard AC supplies, which are the cheap crap and notoriously inefficient.

  • So Edison wants to rise from the grave and defeat his nemisis, Tesla, eh? Now all Tesla has to do is rise up and strike back with his wireless power transmission system at GHz frequencies. Not only would this eliminate the per system power supplies but also the wiring and the master clock!

    I'm pretty sure I'm just joking about that idea...

  • by GoMMiX (748510)
    Computers already use DC power?

    Is this not the entire point of the PS? To convert AC to DC?

    So basically all these new DC computers would be is a computer that relies on yet another source to convert AC to DC then still requires some sort of internal component to convert THAT DC to the correct voltage for the various devices within the computer itself?
    • Yes.

      It will make the servers smaller, save space by consolidating power supplies into one unit, save lots of power by having one large power supply that's more efficient than small ones, and make it way easier to remove waste heat by concentrating most of it into one big unit.
  • What a stupid headline. Sure he was right about DC power, but he was wrong about AC power and that still has nothing to do with the article.
  • Perfect (Score:5, Funny)

    by HangingChad (677530) on Thursday March 02, 2006 @10:31PM (#14840014) Homepage
    Let's see, how do we get a functioning data center to not just replace their computers, but their whole infrastructure? Replace AC with DC!


  • this is news? (Score:5, Informative)

    by iggymanz (596061) on Thursday March 02, 2006 @10:37PM (#14840048)
    for crap's sake, dc powered servers are nothing new, many have config option of "-48VDC standard telco" supply.
  • Is it worth pointing out that the phone company, whose switches and local distribution network all required DC to drive the (first) electromechanical components and (then) electronic ones, never made the switch? Commercial power was (and still is, AFAIK, although I've been out of that business for years) used to charge massive banks of 48V batteries that actually power the central office equipment. Once they made the decision to have UPS on that scale, AC/DC/AC conversions were expensive and hence minimiz
  • DC power is not very good for distributing power over anything other than short distances, in particular given how trivial AC-to-DC conversion is using modern solid state power supplies. Once you reach the end user, then DC starts making more sense.
  • No (Score:3, Informative)

    by dbIII (701233) on Thursday March 02, 2006 @10:41PM (#14840070)
    Short answer no. Long answer - sometimes. DC is somethimes useful right in front of you, but it's hard to get it there.

    I've seen houses wired with 12V DC from mini hydro and solar - but in those cases it was a long way to the nearest transmission wire and would cost a fortune to get mains power onto the site.

  • Telco switches normally ran on 48V DC back in the electromechanical days, and standard telco offices have rooms full of big honkin' batteries to act as a UPS for the building. And yes, power gets distributed on fat copper busses that you don't want to drop wrenches on. As electronic switching systems replaced the old mechanical ones, the capacity increased rapidly while the floor space for electronics decreased, but there's been enough opportunity to fill it back up again.
  • To Westinghouse (Score:2, Informative)

    by jheath314 (916607)
    Perhaps we'll see the AC group hitting back with demonstrations of how dangerous these DC powersupplies are to the hamsters and other wildlife native to big server rooms.

    Incidentally, that's how the electric chair came about:

    [Edison]AC is dangerous! Just watch what happens to these various animals when I close this circuit!
    Edison electrocutes some horses
    [US_Gov]Ooooo... I'll bet that works on people too!
    US_Gov introduces new grisly method of executions, while disregarding the main point of Edison's demonst
  • The long distance power distribution network relies on AC power, for reasons that I assume many/most slashdotters are aware of (high voltage to minimize voltage drop across lines, with transformers at the ends). That said, I've long thought it would be really nice to have a big DC power supply with a DC power distribution system in buildings. Just think about getting rid of all the wall warts and power supplies that we currently have to deal with and instead just having regular straight cables to plug DC-
  • Not that there's a reason the telco world has been running it's stuff on -48v power for, what, forever? Try to make that work well across town, though.
  • by gvc (167165) on Thursday March 02, 2006 @10:45PM (#14840102)
    For physics reasons, it's easier to transmit AC over long distances; DC requires thick copper cables or bars, instead of comparatively lightweight wires. But DC becomes a more serious possibility for power once AC reaches a building.
    What a load of crap. Low voltage (high current) requires thick wires - it has nothing to do with AC/DC. AC is horrible for long-distance transmission; up north megavolt DC is popular. AC is useful because it is easy to transform - you can step the voltage up or down with turn-of-the-previous-century technology and hence transmit at a higher voltage than you'd like to use.

    That said, if space and cooling are an issue it might well make engineering sense to get the transformers, capacitors, and rectifiers out of the computer boxes. Big 5v/12v power busses wouldn't even need to be insulated. So while the reporter badly mangled the story, the engineering sounds reasonable to me.

  • Therefore, if you live farther away than, what was it? 20 miles? TURN OFF ALL YOUR POWER! DC current can't go that far.

    However, if you enjoy distributed power, no, no he certainly was not.
  • by fm6 (162816)
    Everyone knows the alternating vs. direct current wars ended with Thomas Edison and Nikola Tesla.
    Thanks to my fellow trivia dweebs at Wikipedia, I found out that Consolidated Edison still sells DC power. Not a big profit center, though.
    • Re:Not over! (Score:3, Interesting)

      by b0s0z0ku (752509)
      I found out that Consolidated Edison still sells DC power.
      Yep. My dad was the building superintendent of a church on 96th St. in NYC in the early 70s. The church building has a DC mains supply - mostly to run elevator and fan motors, but some of the outlets in the building were DC, and were identical enough to normal AC outlets that you could plug a regular plug into them. Well, before he knew better, my dad plugged an old TV into a DC outlet. Transformers don't take DC input very well - fireworks en
  • by sflory (2747) on Thursday March 02, 2006 @10:50PM (#14840120)
    The reason you want to use DC is that a computer's power supply converts AC into DC. The power supply of most computers isn't that efficient at it. This basically converts some of your electricity into heat. (Heat in a 1U server in a big rack of 1 U is really bad.) In theory the data center's big AC to DC converter is more efficient and better cooled. Thus you save money in power bills, air conditioning, and rack space (less heat, and power draw means more servers per rack). Plus in theory your servers should last longer as the power supply is one of the more likely points of failure.
  • "I'm sick to death of people saying we've made 11 albums that sounds exactly the same, Infact, we've made 12 albums that sound exactly the same"
    -Angus Young "AC/DC"

    If you don't get this post, I deserve -Troll, -Offtopic and -Flambait karma from each and every one of you.
  • by Animats (122034) on Thursday March 02, 2006 @11:02PM (#14840181) Homepage
    There are some advantages to operating on 48VDC, but unless you have the big battery room of a telephone central office, they're not that big.

    What Rackable is really pushing is a system where AC to 48VDC conversion takes place in a unit at the top of the rack, and 48VDC is local to the rack. That, at least, simplifies the cable management.

    One big advantage of 120/240VAC power distribution using US standards is that the connectors are standardized and reasonably idiot-proof. That is, if you can plug it in, you won't overload the power cord or the connector, and if you overload the branch circuit, a breaker will trip. Outlet strips have circuit breakers, so you can't overload the cord to the outlet strip without a breaker trip. There are NEMA standard power plugs [] for 15A, 20A, and 30A circuits, 120/240VAC, and single and three phase configurations. All this is standardized nationally and enforced by the National Electrical Code.

    In contrast, there are no simple standards for 48VDC. Most 48VDC gear has big screw terminals. There are no standard plugs and sockets. Somebody, preferably a licensed electrician, has to check all the data plates, add up the current loads, calculate voltage drops, size the wire and breakers, and torque the big screw terminals to the correct torque, using the correct lockwashers. Every time you add or change a load, somebody has to recheck the math. Errors can cause a fire. None of this is all that hard if you have basic power technician skills, but you can't just go casually plugging stuff in.

    Although, since the development of the low-cost clamp-around DC ammeter, things have become easier in the DC world.

  • by Omega Hacker (6676) <omega.omegacs@net> on Thursday March 02, 2006 @11:18PM (#14840243)
    Last I checked, pretty much every data center worth its name has a bank of UPSs. That means that power is coming in AC, and being converted to DC to charge the batteries. AFAIK any decent UPS in use in a server these days is "on-line", which means that instead of attempting a fast switchout between mains and battery, all outgoing AC power is re-generated from the DC battery bus. If you assume a 10% loss in both steps, you're at 81% right away. Add a bazillion AC power supplies at 10% loss and you're down to less than 73% efficiency.

    Contrast this with a properly designed DC system a la old-school telco: The same front-end of the UPS is used, with a 10% loss converting AC to battery voltage. Then you run that into DC supplies that, with modern electronics, are going to be doing a lot better than an AC supply, so let's say 5% loss. That puts you at better than 85% efficiency.

    The critics cited in the article are actually probably not far off in calling the Rackable solution over-hyped, if you only take into account the isolated-rack design. Rackable puts 2-3U of beefy redundant supplies at the top of the rack and does DC to the servers. Efficiency-wise this is only fractionally better than a bazillion AC supplies, and quite possibly dead even because of the DC->DC losses in each server on top of the AC->DC->AC->DC setup implicit with AC-based UPS systems. However, AFAICT from a glance at their site, Rackable's systems are designed to drop right into existing DC datacenters, which eliminates the AC supplies at the top and the DC->AC->DC stages.

    The issue is what kind of infrastructure is needed to feed the selected DC voltage (which is going to be -48VDC) into the racks with the lowest bus losses, but this is someone I would expect is either a) already solved by the decades-old telco industry, or b) going to be solved in at the appropriate 384-cores-and-100TB-per-7ft-rack scale RSN, by "the market".

    I know that if I were in the position of designing a big datacenter right now, I would be looking very hard at DC systems.

  • by Ungrounded Lightning (62228) on Thursday March 02, 2006 @11:39PM (#14840342) Journal
    The article conflates several things.

    First off: Digital electronics generally requires several voltages. And they're all low, requiring high currents, massive conductors, and local filtering and regulation. So even if you're providing DC power from outside the room, you'll have a switching power supply (or several) in each piece of equipment to convert whatever the rough DC power is to whatever you need, smooth it, and regulate it.

    But while some electronic devices use a common switcher to generate all the voltages with one conversion step, others use a "roughing" supply and a bunch of local supplies. Part of that is to get better regulation - part is because the roughing supply must run from 60 (or 50 or whatever) Hz and thus requires big caps to tide you over the low part of the cycles - caps you don't want taking up space near the components.

    If you're going to do it in two stages anyhow, you can put your roughing supply OUTSIDE the room and only have the final supplies inside. The roughing supply has a lot of heat dissipation so you save a bunch on your cooling.

    Second: There are two standards for power distribution in electronics rooms:
      - Your local power line stuff. (120/240/480/208-3-phase in the US)
      - The telco standard: x2-redunant 48V DC.
    A lot of equipment - especially networking equipment - is manufactured for sale to tellcos and other operations that use the standard. They might have initially used it because some of their equipment was co-located in tellco sites, where only 2x48VDC is available - and they got a quantity discount for buying a bunch of the same stuff and went to 48V for their own sites. Or they might use it because it's MUCH simpler to do backup power with floating batteries and century-old technology than with a building-sized UPS. (Note that a UPS CAUSES at least one outage when first installed and on the averate at least one more within the first year of operation from some malfunction. And a UPS dissipates more power than a roughing power supply or a battery charger.)

    But the standard for 48VDC is REDUNDANT 48VDC supplies, with the equipment only requiring one (and typically doing "cutover" with diodes B-) ). With the equipment already set up for redundant supplies it's not a lot of cost or work to wire both sides and put in two 48V feeds to the equipment room. (Four diodes are a LOT cheaper than a pair of 120V roughing power supplies at each box, too.) So of course the users of such equipment normally give it dual supplies. (Even if it's a single rack and so they just put two roughing supplies in the rack fed from two different 120V feeds.)

    The result is that all the equipment has redundant power supply, and keeps operating glitch-free through a number of kinds of partial outages - AND power supply repair and replacement. This is what's responsible for much of the claimed increase in reliability.

    The whole Edison/Tesla DC/AC war had to do with the economics of CROSS-COUNTRY power transmission. AC beat DC there because a century or more ago it was virtually impossible to jack DC voltages up to levels suitable for long-distance transmission and back down to levels safe for distribution within houses, while AC could do that easily and efficiently. So Westinghouse/Tesla could ship cheap power from Niagra Falls to New York City while Edison had to build fuel-burning power plants IN the city. It has essentially nothing to do with shipping the power around within a single building.
  • right and wrong (Score:5, Informative)

    by IGnatius T Foobar (4328) on Thursday March 02, 2006 @11:45PM (#14840365) Homepage Journal
    Edison and Tesla were both right. Remember, the DC vs. AC wars were fought back when the load was mostly made up of lights, motors, very utilitarian things. AC is fantastic for transmission over long distances (and for running three phase motors, but that's another story). DC happens to be better at running precision equipment like computers -- heck, they all run on DC already. All we're really talking about here is taking advantage of an economy of scale by doing one big power supply (or a few, for redundancy) instead of one for each machine.

    Ever seen a telco rack? Everything runs on -48VDC. Everything. A telco rack always includes a couple of DC power supplies, and all the equipment just ties in to a common DC bus. The best part of all: the UPS simply consists of four "car batteries" (not exactly, but you get the idea) wired in series and tied directly into the bus! No pesky inverters to deal with.

    The telecom industry has been doing it this way for decades. It's about time the computer industry got on board.
  • by The Cisco Kid (31490) * on Friday March 03, 2006 @10:23AM (#14842240)
    The Edison/Tesla one was about long distance transmission of power, and AC is still the winner there.

    TTL logic has to run on DC, so you have to convert the supplied AC to DC. This is just recognizing that instead of converting it individually in each of dozens or hundreds (or more) machines, that it is more efficient and reliable to have one (and perhaps a redundant standby) converter providing DC to the same machines.
  • by ecloud (3022) on Friday March 03, 2006 @02:22PM (#14843955) Homepage Journal
    I believe the article makes an oversimplification by stating that AC is better for long-distance power transmission. Rather, it's easier to generate AC power (no rectifiers are needed), easier to switch (because the arc when the switch opens is much easier to extinguish - current flow actually stops for a short period of time, and the arc goes out), easier to run a motor from AC (no commutator), and easier to do voltage conversions (you only need a transformer). For really high-power long-distance transmission lines (like between states) they use very high-voltage DC because it is in fact more efficient. But I'm not sure how they do the conversion from DC back to AC in that case (would guess it's just a rotary converter - a motor running a generator). The losses from doing the conversion on both ends are acceptable only when they are less than the losses that would occur in such a long transmission line.

    Losses are especially bad in AC transmission lines when the power factor is not correct, because while currents which are out-of-phase with the generated voltage waveform are expressed using imaginary numbers, in fact they are very real currents, and they cause increased heating losses in the transmission line. So the power companies switch large capacitors in and out of the circuit to try to keep the current and voltage in phase. (And they would appreciate if every device on the grid was power-factor-corrected, but this doesn't happen, mostly because motors are inherently inductive, and motors are the largest consumer of electricity. Sometimes they at least manage to persuade large industrial customers to manage their own capacitor bank, to correct for the inductance of their own motors, and give them a discount in exchange.)

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