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DC Power Supply for Desktop Computers? 27

cmowire asks: "Given that inverters waste a lot of power and using solar and battery power is getting to be more attractive given power shortages, are there any DC power supplies that can be bolted in place of a standard ATX power supply and tied to a battery/solar panel combo? I know that it's pretty trivial for a laptop to work like that, but I'm more curious about doing that for a desktop system."
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DC Power Supply for Desktop Computers?

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  • by Anonymous Coward
    Amtrade [amtrade.com] has several DC power supplies available... they ain't cheap, though... the cheapest one listed is $85.50, and prices go UP, UP, UP from there...
  • Uninterruptable power supplies have a battery and circuitry to convert that battery's current to 120V AC. Instead of feeding that battery from wall socket AC that's been stepped down, rectified and filtered (not necessarily in that order), just feed the battery from whatever source of DC you were planning on using (solar, etc.) Then you can just plug your computer and monitor into the AC outlets of the UPS.

    Of course, you'll need to be able to feed that UPS battery with a slightly higher DC Voltage than the battery's own voltage to keep it trickle charged, and that source of DC will have to be able to supply enough current to drive the computer and monitor and keep the battery trickle-charged, and it's probably at this point that you'll realize distributing power via 120 Volt AC around the average home and letting each device convert it according to that device's needs is much more practical.

    Another possibility is to rebuild your computer's power supply, eliminating the AC side of it and replacing the isolation/step down/switching transformer and associated switching transistors with something that's the same on the secondary side but uses lower voltage-higher current transistors and fewer windings of heavier wire on the primary side, unless you can put enough solar cells in series to achieve about 170 to 200 Volts DC.

    You may have realized by now that I'm trying to point out that although what you want is possible and an interesting thought exercise, it suffers from a pretty high lack of practicality.

    Using DC is going to almost invariably mean lower voltages than the wall socket, which means higher amperages for the same wattage, which means thicker cables keeping the current source and the computer near each other or even thicker cables to prevent voltage drop.

  • Avoiding the conversion to 110 Vac seems attractive, but (as has been mentioned above) modern power inverters are more efficient than you might think. The main advantage of reproducing the ac line voltage is the ease of just plugging things in, with no modification to their wiring.

    I bought a "Smart UPS 1000", made by APC, on Ebay for $65. To provide 300 V-A (see comment #14 above concerning power factor) for fifteen minutes, this unit uses two 12V sealed lead-acid batteries that weigh about ten pounds apiece, connected in series.

    A power supply that ran on 12V would be more versatile (solar cells, vehicle power, etc.). I think a practical approach would be this: switch the 12V through a transformer (higher frequency for a smaller transformer) and rectify it to make around 270Vdc. It would not have to be very well regulated, by power supply standards.

    Since monitors and ATX power supplies start out by directly rectifying the line voltage to make 270Vdc, (which they use as the input to their various switching power supply circuits to produce the different output voltages), their line cords can simple be plugged your 270Vdc output. You don't have to worry about polarity, because the full-wave line-voltage rectifiers will direct your 270Vdc properly to their reservoir capacitors.

    Thus, you don't need an inverter, and you don't have to listen to the BUZZ of an inverter.

    You'd have to check for any line-operated fans in the equipment, though, and replace them with dc fans.

  • by dutky ( 20510 ) on Wednesday April 18, 2001 @04:49AM (#283367) Homepage Journal

    The entire point of the power supply in most computers is to convert AC to DC: the components in the computer take DC power directly. If you already have the DC power, you could almost (modulo deriving the right voltage levels and some power conditioning) hook up all the components directly.

    The item (or items) you are looking for is called a DC-DC power converter, possibly combined with a moderately sized capacitor to smooth out any sags or dips (you'll also need something to filter out surges, maybe a zener diode or some such). For most of the computer all you are worried about is producing 5 V and 12 V feeds, which shouldn't be a big deal, but for some components (CRTs and the flourescent backlight on most LCDs) you will need to produce some very high voltages (in the kV range, at least). For high voltages you will need to go back to a real power supply and an intermediate AC stage (aka, a switching power supply, which is what you already have in all such applications). Then the question is, do you replace the one that you already have in your CRT or LCD monitor, which is expecting 50-60 Hz, 110V (or 220 V, or whatever the local flavor is) AC power, or do you roll your own that can take whatever DC voltage you happen to have handy?

    Ultimately, it is probably much simpler just to convert your local DC source into a nice 60 Hz, 110 V AC signal and continue using all your equipment in an unmodified manner. The conversion, both in your DC-AC converter and in the computers' power supplies, is pretty efficient these days, so don't worry too much about lost power there.

  • Not counting the monitor problem, the best bet might be to go with a DC-DC power supply. A friend and I recently had the opportunity to put our server in a telco grade colo and we had to scour the net to find such a beast. We were able to get a -48V DC input power supply with a standard form factor that dropped right into an ATX case. It has the standard voltages and connectors of a regular ATX supply on one side and a -48V DC input on the other. Unfortunately, our company went out of business a week after we got it hooked up but it worked great in the meantime.

    I can't remember the exact output wattage, but we had it hooked up to a 5 amp fuse running an average number of components w/o trouble. I'm no EE but I imagine you could easily run it off a battery bank if you had an existing photovoltaic array and distribution system. If you didn't need a monitor, you'd be all set, otherwise, you might as well stick w/inverting to AC and using standard components as an earlier poster had suggested.

    Unfortunately, I don't have the link w/me but I'll put it up when I find it.

    I'm trying to sell it (moved back to an AC colo) so if anyone is interested, drop me an email.
  • Dan Bernstein wonders why power supplies don't come with an internal UPS [cr.yp.to].
  • Sound and serial ports (sometimes)...

    (quoted from somewhere)
    "RS232 data is bi-polar.... +3 TO +12 volts indicates an "ON or 0-state (SPACE) condition" while A -3 to -12 volts indicates an "OFF" 1-state (MARK) condition.... Modern computer equipment ignores the negative level and accepts a zero voltage level as the "OFF" state. In fact, the "ON" state may be achieved with lesser positive potential. This means circuits powered by 5 VDC are capable of driving RS232 circuits directly, however, the overall range that the RS232 signal may be transmitted/received may be dramatically reduced.

    The output signal level usually swings between +12V and -12V. The "dead area" between +3v and -3v is designed to absorb line noise. In the various RS-232-like definitions this dead area may vary. For instance, the definition for V.10 has a dead area from +0.3v to -0.3v. Many receivers designed for RS-232 are sensitive to differentials of 1v or less."

  • The -12v isn't used by most components, but a lot of sound cards DO use it. (I happen to know because my -12v lead came loose, and the only thing that went wrong was my audio out picked up a whole lot of noise and reduced its volume 90%.)
  • The conversion of DC-AC has a bout a 30% loss
    a 1400VA yeilds 950 Watts 32% loss
    a 280VA yeilds 180 watts 36% loss

    on things like pc's not including monitors I feel that the DC-DC power converters could lead to more run time off the same batteries.
    Not to mention I have not see any real plans for hooking up soalr panels directly to a battery backup units.

    -- Tim

  • Whatever you try, don't use those cheapy seventy dollar inverter critters from the local Rat Shack. They make enough interference that some laptops and handheld radios cease to function near them.

    crap and i just bought one of those off ebay - oh well, luckily it was only 15$ - guess we'll find out if it was a good investment when i hook my $3000 system with all my crucial data with no backups to it
  • I think you're doing it wrong.

    An easy but crude way of _estimating_ losses is to feel how much heat your power supply is giving off.

    You implied a 100watt loss in the second case (you said 280VA , 180W means 36% loss ).

    I doubt you'll feel 100 watts of heat coming out of your power supply unless it's in serious trouble...

    OK so I'm one of those lazy engineers who can't bother to calculate stuff to the decimal point.

    But hey this does have some basis in science...

  • Solar/Battery backup, try Applied Power Corporation http://www.appliedpower.com or any other company that deals with remote solar solutions (what do you think those solar sites do when there is no sun? :)). You get 2 benefits, free energy, and free battery backup recharge. We deal with 12v (deep cycle car battery) solar sites. Solar produces a native DC voltage and 12V solar setups aren't uncommon, SOO, just get 300W worth of Solar output at 12v and you're ready to rock and roll. In addition the same companies that package the cells can sell you a regulator to allow battery charging when voltage is sufficient on the solar cells as well as to disconnect the solar cells from the battery system when voltage is too low.
  • Are you sure about that? AC watts are usually measured as RMS power, whereas for AC volt-amps are peak (which for sinusoidal AC is a factor of square-root 2 larger than RMS). In DC, volt-amps and watts are equivalent.

    950 Watts x 1.414 = 1340 AC VA
    180 Watts x 1.414 = 255 AC VA

    1400 VA -> 1340 VA is 4% loss
    280 VA -> 255 VA is 9% loss

    There are still losses, but I don't think they're as bad as you're saying.

  • errr... dude, you wouldnt want to read the post first would you? thats what he wants to do... skip the middle man, use a dc atx powersupply.. he said the inverters use too much power.. read it again.
  • . The only way to raise DC voltage is to chop it up into AC and run it through a transformer.

    Not so! One can directly raise DC voltages with a switched-capacitor circuit. You can also reduce and invert DC voltages with clever switched-capacitor circuits.

    Now, getting the voltage necessary to drive a CRT out of a 5V power supply this way would be a royal pain.

    Unless you really enjoy screwing around with electronics, I'd recommend just using an inverter and plugging in your hardware unaltered.

    But if you're willing to go to the trouble and expense of modifying your computer, why not just shell out the extra bucks and use a laptop?

  • A standard PC power supply puts out +5 V, -5 V, and +12 V --- All DC. For an alternative supply, you need two isolated 5 V power supplies and a 12 V supply, and bypass the entire power supply on the PC.

    DC voltage conversion is a subject in itself. Reducing DC voltage is fairly easy, but involves losses and inefficiencies. The only way to raise DC voltage is to chop it up into AC and run it through a transformer. Chop it up right and you get an inverter.

    Anyone with a bit of experience in power supplies should be able to figure out what is needed, it is fairly basic electricity. The main concerns I see are ensuring that the supply has enough capacity and getting isolated supplies so you can get both + and - 5 Volts.

    And, oh yes, the monitor. Not so easy here. Traditional CRTs require a high voltage that can only be generated from AC. Even DC powered TVs have a built in inverter.

    So are the new LCD panel screens DC powered? No particular reason why they shouldn't be. Just the cost consideration there.

    So it can be done fairly easily, just a case of bodging the parts together and getting an inverter for the monitor or a DC powered monitor.


  • Your calculations assume a power factor of one. In an AC circuit, real power is RMS Volts * RMS Current * Power Factor. Power factor is the cosine of the phase angle between the voltage and current waveforms. When the voltage and current are in phase, which only occurs with a resistive load, the power factor is one. The power factor of the power supply, which probably has an inductive or capacitive component, is less that 1, although probably not much less.

  • try the following:

    Find a couple of older 386 laptops and strip the dc converter/regulator boards out of them, wire the outputs in parallel, and mount appropriately.

    This should get you by, although if your PC has a large power draw you may need more than two. If you don't have spare laptops laying around you can find similar circuits available from Marlin P. Jones or All electronics.

    Whatever you try, don't use those cheapy seventy dollar inverter critters from the local Rat Shack. They make enough interference that some laptops and handheld radios cease to function near them.

  • Look over at www.mp3car.com
    http://www.mp3car.com/hardwarecats.asp?Type=H&Ca t= P

    There you go.
  • Actually, the ATX spec has +5v (x3) +3.3v +12v (x2) and -12v plus another 5v for standby power. this is not including the +5 and +12v for hard drives etc...
  • First of all you don't need another PSU, just a filter as others have mentioned.

    You can purchase large solar panels which can produce 400 W 12V in direct sun, but these are large and often cost several thousand dollars. And, the very important question of night is also an issue.

    A good alternative is a solar battery. It's a solar cell connected to a large DC NiMh or NiCd battery, which is continually trickle-charged (batteries don't care too much about current/voltage variations). This way, you have non-sunlight or low light level access and relativly consistent power. These units often come with a DC-AC converter, which means all you would have to do is plug in your computer (or any other appliance for that matter). These are also expensive, around $500+ for anything good.

    I worked on a project which used a small version of a solar battery in remote monitoring embedded computers. We used the LART Intel SA-based MIPPs 200 MHz devices with 802.11 communications. They worked very well with solar batteries, and uptime for each node was almost 100%.

    Of course, there is a question of why you would want to do this when you have plenty of juice piped in your home. For me at least, power interuptions are negligable and for the other times when I really need 100% I use APS. This approach would be difficult to scale, as I don't see any solar powered datacenters and I'd question the reliability of solar devices powering critical systems as compared to a good, multiple grid power input and backup systems.

    You don't live in California, do you? ;-)
  • Zippy Technologies makes DC-DC powersupplies that can take -24v DC and N+1
    redundant power suppluies that do -48DC.

    Sold under the EMACS [emacs.com] brand (no kidding!) I've seen the ATX for 190$ on the web,
    this is probably the extreme end of the spectrum (though if you need DC applications,
    you got $$$.)

  • Just found this list [total-web.net] of power supply manufacturers.
  • Found a site, Arise Computer Inc [arisecomputer.com], who have a whole page full of ATX power supplies [arisecomputer.com] that should suit your needs. Looks like they've got 5 on this page which will take DC input - just select one that's got the input voltage you need and away you go!
  • You don't seem to realize that volt-amperes are not the same as watts. You can literally have 1000 volt-amps of draw and have zero (real) power flowing. To convert VA's to watts you have to multiply by the power factor, which (for sinusoidal waveforms) is the cosine of the angle between current and voltage. Total volt-amps times the sine of the angle gives you volt-amps reactive (VARs).

    Most PC power supplies are switchers which operate by rectifying the line voltage and running it through a chopper to drive the primary coil of a small toroidal transformer. The secondary coils yield +5, +12, -12 and the other outputs required by the computer; the chopper duty cycle is regulated to keep the output voltage correct. It wouldn't be much of a trick to use a different primary coil (heavier wire, fewer turns) and chopper designed to operate off 12 VDC instead of 135-350 VDC. Voila, 12-volt-capable PC power supply. Mind the input connector, that thing is going to draw some amps.
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  • http://www.keypower.com/
  • I bought one of those surplus laptop supplies from Marlin P Jones. It was DOA. Nice looking unit, though. So I used an inverter. Much easier to hook up, and now I can blend margaritas in the back of my Jeep at field parties =|> .

    Those purpose-built 12V ATX power supplies are nice but too pricey for a homebrew system.

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