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DC power has been an option for datacenter equipment since I was a young lad racking and stacking hardware. Cisco, Dell, HPE, IBM, and countless others all had
by otterley 7mo ago
DC power has been an option for datacenter equipment since I was a young lad racking and stacking hardware. Cisco, Dell, HPE, IBM, and countless others all had DC supply options. Same with PDUs. What’s old is new again.
See e.g. https://www.dell.com/support/kbdoc/en-us/000221234/wiring-instructions-for-a-dc-power-supply-unit https://www.dell.com/support/kbdoc/en-us/000221234/wiring-in...
- bluGill 7mo ago48vdc was common in phone exchanges. They filled the basement with lead-acid batteries and to could run without the grid for a couple weeks. In turn the phone was 99.999% reliable for decades.
- mjuarez 7mo agoNot to be _that_ guy, but it was technically -48V DC. Honestly, that was pretty surprising to me when I had to work with some telco equipment a couple of decades ago. To this day, I don't think I've encountered anything else that requires negative voltage relative to ground.
- SAI_Peregrinus 7mo agoLots of amplifier circuits need a bipolar supply: both positive and negative voltages with respect to ground.
- yostrovs 7mo agoCheck out older English cars.
- bluGill 7mo agopositive ground used to be in all cars. When they went from 6 volts to 12 the disadvantages became appearant fast and so everyone went negative ground then (mid 1950s). I am not clear why positive ground was bad (maybe corrosion?)
- servo_sausage 7mo agoIs that something other than a labelling convention? Is ground actually connected to a earth stake?
- CamperBob2 7mo agoCathodic protection against corrosion was the goal of using -48V, in the telcos' case.
- myself248 7mo agoAnd the telegraph lines before that.
- magicalhippo 7mo ago> Is ground actually connected to a earth stake? Yes, or something similar[1]: A few of the more efficient grounding electrodes for buildings and structures are: - Metal Underground Water Pipe - Metal In-ground Support Structures - Concrete-Encased Electrode (also known as “footer ground” or “Ufer ground”). - Ground Ring As mentioned this is particularly important for telecom and similar systems which have signal wires going literally through the ground. [1]: https://www.nfpa.org/news-blogs-and-articles/blogs/2021/09/27/grounding-understanding-the-essentials-for-building-the-foundation-of-a-structures-electrical-system https://www.nfpa.org/news-blogs-and-articles/blogs/2021/09/2...
- jacquesm 7mo agoYes, and that tiny little difference can cost you a lot of expensive gear if you run it off the battery and plug in a serial port or something like that. You'll also learn first hand what arc welding looks like without welding glass.
- aidenn0 7mo agoRTL and DTL both needed negative-voltage relative to ground, as do many analog circuits.
- em3rgent0rdr 7mo agoSome old guitar effects used -9V DC.[1] And the convention with guitar effects power adapter is the barrel is center negative (which is motivated with facilitating easy wiring of the socket's switch to connect to a 9V battery inside). [1] https://www.analogisnotdead.com/article26/what-is-going-on-with-positive-ground https://www.analogisnotdead.com/article26/what-is-going-on-w...
- HWR_14 7mo agoCan you explain why it's -48 VDC as opposed to 48 VDC with the + and - inputs mislabeled?
- jojobas 7mo agoIn short, ground.
- crote 7mo agoBecause the chassis is connected to ground (as in, a literal grounding rod hammered into the soil) and by definition your 0V reference point. The crucial difference is the direction in which the current is flowing: is it going "in to", or "out of" a hot wire? This becomes rather important when those wires are leaving the building and are buried underground for miles, where they will inevitably develop minor faults. With +48V corrosion will attack all those individual telephone wires, which will rapidly become a huge maintenance nightmare as you have to chase the precise location of each, dig it up, and patch it. With -48V corrosion will attack the grounding rod at your exchange. Still not ideal, but monitoring it isn't too bad and replacing a corroded grounding rod isn't that difficult. Telephone wires will still develop minor faults, but it'll just cause some additional load rather than inevitably corroding away.
- tinyhitman 7mo agoDo you also happen to why this is not more common? Must be useful for more than just telephone wires.
- bluGill 7mo agoMost large scale systems are AC because transformers are relatively cheap, low maintenance, and efficient. When the system is AC ground makes no difference. With DC systems you generally think about the issues - which is why modern cars are negative ground. However other than cars most people never encounter power systems of any size - inside a computer the voltages and distances are usually small enough that it doesn't matter what ground is. Not to mention most computers don't even have a chassis ground plane (there are circuit board ground planes but they conceptually different), and with non-conductive (plastic) cases ground doesn't even make sense.
- fecal_henge 7mo agoI am STILL designing hardware for -48v telco standard. The first thing we do is convert -48 -> 48v. That's 4 square inches of PCB space we waste.
- eqvinox 7mo agoWhat do you need +48V for?
- pluies 7mo agoWell if it's negative 48V the electricty flows out of your circuit and back to the grid, so you need to make it positive to have the electricity come in.
- djmips 7mo agoLikely as a basis for converting to other useful DC voltages.
- fecal_henge 7mo agoWe go -48 -> 48 -> 12 -> 3v3,1v8 etc etc. If you went 48 straight to POL voltages then you would have horrific converter performance.
- eqvinox 7mo ago> If you went 48 straight to POL voltages then you would have horrific converter performance. What's horrific converter performance in numbers? An isolated flyback (to 12V) should be able to hit >92% and doesn't care if it's fed -48V or +48V or ±24V. TI webench gives me 95% though I'd only believe that if I'd built and measured it. What's the performance of your -48V → +48V? [with the caveat that these frequently require custom transformers... not an issue with large runs, but finding something that can be done with an existing part for smaller runs is... meh]
- fecal_henge 7mo ago-48 to 48 claims something like 97% (load dependent of course). It also needs to arbitrate between two input supplies for glitchless redundancy, plus have PM bus and other spec mandated stuff. There is no technical reason why you cant go -48 -> 12 as you state with good efficiceny, but we cant get hold of a part that ticks all the boxes. Horrific performance by my definition would be 48v to say 1v. We only realistically use buck topologies for POL supplies. Such a ratio is really bad for current transients, not to mention issues like minimum on times for the controller.
- swed420 7mo ago> I don't think I've encountered anything else that requires negative voltage relative to ground Automotive collectors can probably still relate to cars from the 1920s-50s having a "positive ground."
- fuzzfactor 7mo agoWith positive ground the traditional more-sacrificial spark plugs lasted longer.
- divbzero 7mo agoInteresting, so this is why the phone line still worked when power was out across the whole town.
- eqvinox 7mo agoYeah, it used to be that you could still make calls (particularly to emergency services) even in complete power outages, for as long as your local exchange has batteries for. (AFAIR that tended to be on the order of hours, but probably differs quite a bit across locations and regulatory domains/countries.) Another thing we lost in the age of VoIP landlines, but then again mobile towers also have batteries. Just don't be unlucky and have a power outage with 3% battery on your phone...
- shiroiuma 7mo agoLuckily, phone batteries and phone power consumption seem to have improved a lot in the last half-decade.
- MathMonkeyMan 7mo agoYeah I always heard that the phone lines carried their own power, and in Florida the phones did keep working when the power went out, but I never knew why. So the grid was always charging up the lead acid batteries, and the phone lines were always draining them? Or was there some kind of power switching going on where when the grid was available the batteries would just get "topped off" occasionally and were only drained when the power went out?
- bluGill 7mo agoGrid charging batteries, phone draining them as I understand. Of course there were switches all over the us so I can't make blanket claims but from what I hear that was normal.
- pocksuppet 7mo agoThe phone grid predated the electrical grid. There was no other choice for power. Actually, there was one. Even earlier phones had their own power. A dry-cell battery in each phone, and every 6 months, the phone company would come around with a cart and replace everyone's battery. Central battery was found to be more convenient, since phone company employees didn't have to go around to everyone's site. Central offices could economize scale and have actual generators feeding rechargeable batteries.
- qingcharles 7mo agoIt's a pretty decent chunk of power down a POTS cable too, as it was designed to ring multiple big chunky metal bells in the days of yore. I was wiring in a phone extension for my grandma once as a boy and grabbed the live cable instead of the extension and stripped the wire with my teeth (as you do). I've been electrocuted a great number of times by the mains AC, but getting hit by that juicy DC was the best one yet. Jumped me 6ft across the room :D
- rdtsc 7mo agoThe teeth. Yikes! But yeah, I remember having the rotary phone disassembled and touching the wires adjusting something when a ring came. Gave me enough of a jolt to remember.
- idiotsecant 7mo agoI still have a bunch of 48vdc comms gear in my powerplant.
- beAbU 7mo agoWhy do you have a powerplant?
- bluGill 7mo agoPower plant is the convention for any large company that has backup power. A few UPSes for the server room - they are the power plant. A backup generator - power plant. Sometimes even just the room with all the break boxes from where the grid comes in is called the power plant (though normally power plant is reserved for backup power). It is extremely common for commercial buildings to have their own power plant. Most of their power comes from the grid in all cases, but they have a power plant. At commercial scale you can often save money by buying a backup generator powerful enough for your whole building so you disconnect from the grid when grid power is in highest demand (see your utility, then your accountant: for details if you can afford a generator this large)
- idiotsecant 7mo agoBecause I am an electrical engineer in a powerplant and I am responsible for those systems. I don't own it personally, I say it in the same sense someone says 'my building' when they mean 'place my apartment is at'
- tverbeure 7mo ago-48V! :-)
- boricj 7mo agoI'm working on stuff in that market, it's still largely is. DC Power System Design For Telecommunications is still a must read and it doesn't even cover the last 15 years or so of development, notably lithium batteries and high efficiency rectifiers. I will say that this is a surprisingly deep and complex domain. The amount of flexibility, variety and scalability you see in DC architectures is mind-boogling. They can span from a 3kW system that fits in 2U all the way to multiples of 100kWs that span entire buildings and be powered through any combination of grid, solar and/or gas.
- amelius 7mo agoYou need thick cables if you want to power a rack with 48V.
- superxpro12 7mo agoThis reminds me of the early google data centers that directly soldered those massive duracell lantern batteries directly to the motherboards as a primitive battery backup. I'm struggling to google examples of it, this would have been back around 2008, but i have a vivid memory of it. edit: found it https://www.cnet.com/tech/tech-industry/google-uncloaks-once-secret-server-10209580/ https://www.cnet.com/tech/tech-industry/google-uncloaks-once...
- AbanoubRodolf 7mo ago[flagged]
- _fizz_buzz_ 7mo agoObviously 48VDC has been around and internally they will probably still step down to 48V. But these 48V islands are nowadays inter connected by regular AC grid. They want to replace that interconnection with a 800VDc bus. I kind of assume they chose 800vdc because there are already bunch of stuff available from EVs which also have 800vdc battery packs now.
- 15155 7mo agoThey chose 800VDC because it's a convenient multiple that is the peak possible with a two-level 650V (probably GaN) FET arrangement.
- eternauta3k 7mo agoAnd why is 650V special?
- 15155 7mo agoHistorical, physical, engineering reasons. Much of the world's mains-voltage electronics run at 240V (historical) and have PFC circuits (which are essentially just boost converters) that run at ~400V DC link voltages. 650V gives you enough headroom to tolerate overshoots and still have an 80% safety margin with a single level topology. This voltage also coincidentally is a convenient crossover point where silicon MOSFETs start to become inefficient and GaN FETs have recently become feasible and mass-produced.
- _zoltan_ 7mo ago800V DC is definitely not "old".