DC current to computers' PSUs?

Hi all, would the PSU of a computer work if I'd provide 220V DC current to it?
If yes: why everybody wants an expensive sinusoidal UPS, with a big
expensive inverters in it, instead of using some 20 car batteries in a series (+ a rectifier to keep the charge), in parallel to the computers?
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Most of them will work on DC, but it needs to be closer to 330V. Better ones with active power factor correction will not though, and smaller power supplies designed for 120V only often need more like 170V.
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| |
| |> Hi all, |> would the PSU of a computer work if I'd provide 220V DC current to it? |> |> If yes: why everybody wants an expensive sinusoidal UPS, with a big |> expensive inverters in it, instead of using some 20 car batteries in a |> series (+ a rectifier to keep the charge), in parallel to the computers? | | | Most of them will work on DC, but it needs to be closer to 330V. Better ones | with active power factor correction will not though, and smaller power | supplies designed for 120V only often need more like 170V.
What about the continuous voltage range PSUs that handle 100-240 volts?
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abu wrote:

Computer power supplies do usually rectify the incoming supply and then apply that dc voltage to the rest of the supply. So, in theory, applying a suitable dc voltage, instead of an ac one, would work. However, in practice, there are usually extra components involved that rely on the input supply being ac, which makes your idea impractical.

Something similar is done - many UPS use several 12v batteries in series. However, a bank of series connected batteries will still need some electronics to keep its output voltage constant. Once you have to have this electronics, other factors dominate the design. A few hundred volts dc from a bank of batteries is quite difficult to design for - especially when considering safety. Keeping it under 100v is a lot easier.
-- Sue
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abu wrote:

Better to replace your PS with one designed for a 48Vdc input (they exist for telco and other uses). Then all you need is a battery charger.
Or, get a laptop.
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There are a bunch of "maybes" involved. One typical design uses full-wave rectification on 240 VAC and voltage doubling on 120 VAC. It would likely work with 300-340 V applies, since that's the peak voltage of the 240 VAC line (though it might overstress an input diode, since half the diodes would carry all the current). It would *not* work on the 120 V setting with 170 VDC, because the voltage doubler configuration requires AC in.

First, plenty of people do *not* use expensive sine-output UPSes, since the square wave (or so-called modified sine wave, which is really a modified square wave) type works just fine. Sine output is good for motors and other devices that care about harmonic content, but computer power supplies mostly don't.
Second, how much would it cost you to get 300+ V worth of batteries, even if each battery is only a few Ah capacity? How much would it weigh? You'd need voltage equalizing resistors to keep the voltage correct across each battery when charging. You'd need a 300 V power supply as a charger. When all this is factored in, you might find that it's easier after all to use an inverter to convert a moderate DC voltage (12 or 24 V) to ~140 V stepped square wave.
    Dave
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| There are a bunch of "maybes" involved. One typical design uses | full-wave rectification on 240 VAC and voltage doubling on 120 VAC. | It would likely work with 300-340 V applies, since that's the peak | voltage of the 240 VAC line (though it might overstress an input diode, | since half the diodes would carry all the current). It would *not* work | on the 120 V setting with 170 VDC, because the voltage doubler | configuration requires AC in.
What happens in the PSUs that work on the full 100-240 volt range? There are a lot of those around. Do they still double the voltage at some point?
|>If yes: why everybody wants an expensive sinusoidal UPS, with a big |>expensive inverters in it, instead of using some 20 car batteries in a |>series (+ a rectifier to keep the charge), in parallel to the computers? | | First, plenty of people do *not* use expensive sine-output UPSes, since | the square wave (or so-called modified sine wave, which is really a | modified square wave) type works just fine. Sine output is good for | motors and other devices that care about harmonic content, but computer | power supplies mostly don't. | | Second, how much would it cost you to get 300+ V worth of batteries, | even if each battery is only a few Ah capacity? How much would it | weigh? You'd need voltage equalizing resistors to keep the voltage | correct across each battery when charging. You'd need a 300 V power | supply as a charger. When all this is factored in, you might find that | it's easier after all to use an inverter to convert a moderate DC | voltage (12 or 24 V) to ~140 V stepped square wave.
If a PSU can handle the full 100-240 volt range, AND if it can also handle DC, then if the DC voltage varies within that range, how is that a problem?
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     snipped-for-privacy@ipal.net writes:

Many electronic ballasts for fluorescent tubes are designed to run from both AC and DC. (These are basically switched mode PSU's too.) DC operation is generally provided so they can operate directly from emergency battery supply. One of the issues with this is as the battery voltage drops at end of charge, it was found the ballasts were burning out. The reason for this is that as the supply voltage decreases, the current in the primary of the high frequency transformer increases in order to transfer the same power through. When the voltage gets too low, the transformer overheats. Most electronic ballasts contain shutoff circuitry nowadays to avoid this.
In the case of a 100-240 volt AC range, this maps onto a 140V-340V DC capacitor voltage, so I would caution against trying to run a 100V AC PSU on less than 140V DC, as you might again burn out the primary if you are drawing any significant load. I would also caution against operating at the 340V top end for the reason another poster gave - protection circuitry such as fuses and thermal trips may not be able to break a DC current.
Also note that many SPMSU's now have more complex input stages to get the power factor up well above 0.9. These may not like DC at all, unless specifically designed for it. I haven't looked in to how they work.
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Andrew Gabriel
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| snipped-for-privacy@ipal.net writes: |> If a PSU can handle the full 100-240 volt range, AND if it can also handle DC, |> then if the DC voltage varies within that range, how is that a problem? | | Many electronic ballasts for fluorescent tubes are designed to run | from both AC and DC. (These are basically switched mode PSU's too.) | DC operation is generally provided so they can operate directly from | emergency battery supply. One of the issues with this is as the | battery voltage drops at end of charge, it was found the ballasts | were burning out. The reason for this is that as the supply voltage | decreases, the current in the primary of the high frequency | transformer increases in order to transfer the same power through. | When the voltage gets too low, the transformer overheats. Most | electronic ballasts contain shutoff circuitry nowadays to avoid | this. | | In the case of a 100-240 volt AC range, this maps onto a 140V-340V DC | capacitor voltage, so I would caution against trying to run a 100V AC | PSU on less than 140V DC, as you might again burn out the primary if | you are drawing any significant load. I would also caution against | operating at the 340V top end for the reason another poster gave - | protection circuitry such as fuses and thermal trips may not be able | to break a DC current. | | Also note that many SPMSU's now have more complex input stages to | get the power factor up well above 0.9. These may not like DC at all, | unless specifically designed for it. I haven't looked in to how they | work.
Then we might agree that the OP's safe choice is to do 48VDC and get the appropriate PSU.
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| would the PSU of a computer work if I'd provide 220V DC current to it? | | If yes: why everybody wants an expensive sinusoidal UPS, with a big | expensive inverters in it, instead of using some 20 car batteries in a | series (+ a rectifier to keep the charge), in parallel to the computers?
It has been suggested that you might need as much as 340 volts DC to make a PSU work based on the typical internal designs they have. Be warned that DC has very different requirements for fuse or circuit breaker protection. An opening fuse or breaker contact that could extinguish the arc at some AC voltage will not be able to do so as easily with DC. The reason is that AC has a point in time where the voltage drops to zero while DC does not. So you can typically see fuses and breakers rated for both AC and DC, but with the maximum voltage being less, typically half as much, for DC than for AC.
20 or more car batteries in series can create a very dangerous electrical setup, including possible issues inside the batteries themselves when one cell goes bad. It's not that this can't be done. Rather, it needs to be done with a substantial amount of engineering for reliability and safety, including batteries designed for such setups (not ordinary car batteries).
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abu wrote:

Thank you all for your suggestions. I understand that it's not that easy: one big problem problem I see after your comments is that all PSUs are made differently and I would probably not be able to find a DC voltage that is ok for all (also some have suggested that active PFC ones wouldn't work with any DC voltage) and in most cases I cannot replace the PSUs that are embedded in the servers we have.
I know 48v DC-to-DC PSUs exist, however 1) their cost is high ($130 at least) 2) I would need a lot of relatively thick wires for the high amperes due to the low voltage 3) I would need to replace all PSUs in our servers, and as I said most servers come bundled with a PSU and I don't think I can replace those 4) more importantly, I seem not able to find big 24v UPSs around, such as a 15kVA 24v UPS.
It is interesting what Dave has said: square wave or modified sinewave UPSs should be cheaper (possibly much cheaper?) than sinewave UPSs, unfortunately I cannot find a >10kVA UPS with modified sine wave or square wave, all big ones are true sine wave and are very expensive. Or do you have a link?
I kinda guess that modified sinewave ones are not so much cheaper, presumably because they still want to keep output voltages very precise, so there is expensive power electronics for that, after which making the true sinewave is not much more expensive. In reality I think computers don't need such precise voltages at their input, I think that a +-2v precision should be OK for computers. 2v is the voltage of a battery cell, so that means that a cheap & big UPS should just have a large array of 2v cells in a cascade and then add or remove a few cells from the cascade in real time so to keep an output of 310 (340?) +-2 volts. Then it could use this voltage to do the 3-levels stepped sine wave. Don't you think? A 15kVA UPS made like this I think doesn't exist, or does it? (link?)
300 cells (car-like, like 50 car batteries) in a series could produce 360V with cranking amperes (CA, according to car batteries specs that is about 600 CA each battery maintaining 7.2v per battery so 300x50 30,000A at 7.2*5060V), that is about 220*30000 = 6.6 MVA with little more than the cost of 50 car batteries (some $4000...)
Wouldn't that work? I understand there would be some safety issues putting 50 car batteries together... probably this should be kept in a separate building
Thanks for all your comments
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| 300 cells (car-like, like 50 car batteries) in a series could produce | 360V with cranking amperes (CA, according to car batteries specs that is | about 600 CA each battery maintaining 7.2v per battery so 300x50 | 30,000A at 7.2*5060V), that is about 220*30000 = 6.6 MVA with little | more than the cost of 50 car batteries (some $4000...) | | Wouldn't that work? | I understand there would be some safety issues putting 50 car batteries | together... probably this should be kept in a separate building
And keep people away. Maybe even yourself.
When a cell gets weak, it gets "back charged" by the rest of the system under the forward current flow. That weak cell presents more of its own internal resistance instead of a forward potential in these cases. The higher the overall system voltage, the more it can continue to push the current through the dead cell. Backwards charging a lead-acid cell will damage it and make it even weaker still. Eventually, you get to a point with a high voltage system that the voltage is able to arc across between the plates inside the cell. The temperature would already be rising from the back charging, but the arc will now add to that. The cell could soon explode. If it did not have pressure release, it could be very violent (think: hot sulfuric acid rapidly spraying everywhere, along with toxic gasses). It can ruin your day.
I would not put car batteries in series beyond 48 volts. And even then I have some worries. Marine batteries may be better designed for this. You might be better off getting surplus submarine batteries.
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