Can I run Computers/monitors off high voltage DC instead of AC ?

[Electrons]

Hi

I have multiple PCs in the office that I would like to run off a Scavenged battery bank of an 10kva UPS . the battery bank is 240v DC (20 x 12v7Ah SLA batteries) . I have a charger that will be connected at all times to the battery bank , and when Electricity goes off (which happens a lot) the batteries will keep suppling the load for <30minutes.

I know that all PCs and monitors have SMPS so in theory it should accept DC since it is converting AC to DC inside anyway. My concern is that all new models with active PFC circuits and whatnots that might go crazy trying to correct the DC input ? Can anyone know for sure if it is safe (technically) to do so ?

cheers

[KaneTW]

Active PFC might go insane, the fuses *must* be DC rated or they won't be able to actually interrupt the arc, etc. Definitely not something I'd be considering unless the efficiency gains are on the order of tens of grand a year.

[jonpaul]

 

Batts>>IInverter (sine) >>PCs, loads

See PriUPS.com for a fine but old solution!
http://www.priups.com/

Jon

[BeBuLamar]

I think they would work.

[wraper]

DANGEROUS and will void warranties and violate electrical codes

Claims about voiding warranty make me laugh in the vast majority of cases. Simply because unless you tell what you've done yourself, there is no way to know for sure why the device failed, especially if it failed for some other reason. Even say flashing custom or wrong firmware, usually there is no (reasonable) way to tell if the device is completely dead.

[Cerebus]

DANGEROUS and will void warranties and violate electrical codes

Utter rubbish. There's a awful lot of the whole world's telecom infrastructure run off -52V DC hooked up to huge racks of lead acid batteries. As long as the distribution system is properly designed there's no reason for it not to be (a) safe, (b) compliant with regulations (which in the UK where the OP is, despite what every self interested party will tell you, boils down to "you have a duty to ensure that the installation is safe"), (c) compliant with the manufacturer's warranty as long as they rate their supplies for DC input, which many do.

[RoGeorge]

Try.  Not guaranteed, but most of the times it will work with a DC mains, too, instead of AC mains.

Switching power supplies usually rectifies then filter the mains AC voltage before using it internally, so it shouldn't be much of a difference for the power supply.  I've tried a couple of times with laptop power bricks, and they worked just fine with DC instead of AC.

[Capernicus]

That would be interesting to see.   dc is so much simpler, I was on Edisons side. (until he started electrocuting innocent animals, then Tesla was the way regardless.)

[penfold]

I wouldn't encourage it necessarily but exercise great caution if you proceed. They certainly should function well, and as long as the DC voltage is in the ballpark of the AC RMS rating, then losses will be similar. I've done some tests historically for "second-life" recycling of ATX PSUs and they all functioned correctly.

As pointed out, fuses would need to be DC rated AND rated for fault current of the battery bank, I don't suppose it would be out of the question to install an additional DC fuse externally to the PSU?

Earthing would need some thinking, I think you would need some specific filtering, less so to remove the interference/noise but to provide some defined HF impedance between +ve, -ve and earth, otherwise, I'm reasonably sure that the loop of batteries would radiate pretty chronically. I would avoid a directly earthed -ve terminal for safety reasons...

With regard to "safety"... with DC, you lose the nice protective features of an RCD, so typically a DC system would have something like an Insulation Monitoring Device (see Bender GmbH for examples of DC protection).

I want to avoid giving too much of my personal opinion in a safety matter, but, if I was forced to use DC and I had a limited budget... I would be seriously contemplating whether an 'over-seas' inverter was genuinely safer than a DC system without earth fault or insulation monitoring.

[TimFox]

I have used other devices (specifically, switch-mode high-power drivers for AC servo motors) that started with a full-wave bridge into a DC-bus capacitor followed by the switching circuitry, where the manufacturer rated the input for AC or DC, since the DC merely flowed through two diodes in the bridge.  In face, we would sometimes use an external 3-phase diode bridge to provide DC into such a device (designed for single-phase AC or DC).  However, I never recommend that anyone use power not specified by the manufacturer as allowed for the equipment.

[golden_labels]

Since no one mentioned that yet: you will need to either run at power much lower than the rated one or use the rectifier rated for at least twice the current. Under AC the diodes conduct only half of the time. With DC one half will always be completely off and the other half will conduct all the time. I put stress on “at least twice”, as those are still rated for operation where heat is dissipated across the entire package, not localized in two spots.

[Cerebus]

As pointed out, fuses would need to be DC rated AND rated for fault current of the battery bank, I don't suppose it would be out of the question to install an additional DC fuse externally to the PSU?

And the prospective fault currents from battery banks can be terrifying.

The 13A fuses we use everyday for AC in the UK in plugtops are rated for a 6kA breaking capacity, a single 60 Ah, 12V lead acid battery has a fault current of about 1.4kA - so 5 paralleled 60Ah 12V lead acid batteries would exceed the 6kA (AC) breaking rating of a fuse that is safe for something designed with the prospective fault current of the mains electricity supply in mind. This is just for illustration of scale purposes, the DC breaking capacity of that fuse would in fact be a fraction of its AC breaking capacity and they are not rated for DC. The smallest fuse I have seen in a Telecoms DC distribution system had a breaking capacity of 80 kA, and that's one at the equipment end of the distribution system, not at the supply end where fuses with breaking capacities of 1000kA and higher are quite normal..

[penfold]

Since no one mentioned that yet: you will need to either run at power much lower than the rated one or use the rectifier rated for at least twice the current. Under AC the diodes conduct only half of the time. With DC one half will always be completely off and the other half will conduct all the time. I put stress on “at least twice”, as those are still rated for operation where heat is dissipated across the entire package, not localized in two spots.

Excellent point. But if the PSU is designed for a wide input range of 85 to 264Vac for example, then the current when operated at 264V would be close to 1/3 of that when operated at 85V - its a slightly extreme example, but even relatively between 100V and 240V the difference would be better than 1/2. I'll happily take your word on the two hotspots causing a problem (I think I've had a similar issue, just never needed to investigate), but any guess on what level of safety factor might be needed?

[jmelson]

Well, you have to make sure ANY piece of connected gear doesn't have a line-frequency AC transformer in it.  I'm especially thinking of old wall-wart supplies.
Most modern computer gear is all using switching power supplies.  Now, running these off 240 V DC runs them at a lower voltage than rectified mains power, which will be about 1.4 X the RMS AC rating.  Some power supplies may not like this.  There are some multi-range supplies with ratings like 90-145; 180-265 V that do not handle voltages in that gap between the two ranges.  MOST wide-range supplies, however, will handle anything anywhere in the entire range.
To check, turn the switch on and apply a voltmeter in Ohms range to the power prongs and see if there is a low reading (say, below 200 Ohms or so).  Wait a few seconds for the reading to stabilize.  If you see the low reading, it has a line transformer  and will not handle DC (probably destructively).
Jon

[golden_labels]

Excellent point. But if the PSU is designed for a wide input range of 85 to 264Vac for example, then the current when operated at 264V would be close to 1/3 of that when operated at 85V - its a slightly extreme example, but even relatively between 100V and 240V the difference would be better than 1/2.

And that’s an excellent observation too. I missed that. I have only seen people experimenting with equipment made for European market, so rated for 240V (± tolerance). Perhaps the PSUs were indeed made for wider range, but not sold as such, so assumptions must have been made to stay on the safe side.

I'll happily take your word on the two hotspots causing a problem (I think I've had a similar issue, just never needed to investigate), but any guess on what level of safety factor might be needed?

No idea, honestly, other than an educated guess. “Being above 2×” is clear for obvious reasons. The diodes are not thermally isolated from the rest of the case, so my feel for that is: 10–20% more. The few projects I encountered were simply adding 50% more, but that was based on a rule of thumb, not actual predictions.

[BrianHG]

Since no one mentioned that yet: you will need to either run at power much lower than the rated one or use the rectifier rated for at least twice the current. Under AC the diodes conduct only half of the time. With DC one half will always be completely off and the other half will conduct all the time. I put stress on “at least twice”, as those are still rated for operation where heat is dissipated across the entire package, not localized in two spots.

Careful, lower voltage on the AC mains means a higher current and higher duty cycle for the flyback converter.  This can shorten the life of a switching power supply.  And, with AC, the current through the diodes may be far shorter than half the time depending on the power supply's power-factor.

My rule of thumb: if you want to use DC on as your mains voltage, purchase hardware with power-supplies which contain listed DC capability.  Tricky to find, but, they do exist.

[CaptDon]

Some of the switched mode supplies that run from 120vac 60hz actually have a voltage doubler on the input and tend to run around 280-300vdc on the combined filter network. If your monitors have that sort of SMPS you could tap right in at the filter stack. The 3AG or F or T fuses probably are not suited for breaking D.C. faults at 280 volts. Fusing will be a problem for you.

[Cerebus]

My rule of thumb: if you want to use DC on as your mains voltage, purchase hardware with power-supplies which contain listed DC capability.  Tricky to find, but, they do exist.

They're pretty common in fact. The Meanwell OEM supply I've got sitting on my bench at the moment is DC rated (120 - 370V) and I trip over DC rated supplies all the time. Even if the end product isn't DC rated there's a good chance that the OEM SMPS inside is. That's fine for folks like us who would trust the rating on an OEM supply's datasheet but I doubt that the average paperpusher would be convinced unless it was on the overall product's datasheet or stamped on the back.

[Electrons]

very interesting points have been raised to summarize :

A) Add DC rated Fuses

B) Think about Earthing /Safety by looking into Insulation monitoring

C) Rectifier overheating might be an issue since only half the diodes are working which might create hotspots

D) Active PFC might go insane

E) buy DC rated Power supplies . ( not all equipment manufacturers have this option )

F) Inrush current is higher / or not controlled ? ( I have found this by simulating an active PFC circuit in LT spice )

regarding point C as pointed out by penfold if the PSU is designed for wide input 80-265 it can probably handle the extra current.

regarding point D it has not been discussed which is my main concern since all new power supplies certainly have active PFC circuit and searching on the subject there is multiple topologies (Interleaved - boost -buck -buckboost - Bridgeless - totem pole bridgeless ... ) most digitally controlled.
So I am scratching my head around it , if the PFC job is to try to make the input current follow the input voltage shape, what will happen if the input Voltage is flat (DC) now it should make the current flat also and can it do that?! 

[Terry Bites]

We've been here before https://www.eevblog.com/forum/renewable-energy/powering-ac-switch-mode-power-supplies-with-dc/