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Why Aren't PC Power Supplies External?

Posted by Cliff on from the modular-is-better dept.

An Anonymous Coward asks: "After years in the computer industry it just occurred to me: Why don't computers have external power supplies? The lions share of the heat and noise from a computer are generated from the power supply. Why not take it out of the case and run just the 12 and 5 volt lines into the case, leaving the heat and noise tucked down under the desk somewhere? Perhaps there is some electrical law that makes that a bad idea, but I can't think of it." One stumbling block is that a power supply brings in more than the 12V and 5V lines, it also provides the power connectors for the motherboard, internal peripherals and external fans. However, these could be provided by the case. If a standard form connector from external PS to the PC is designed, might we be able to avoid further incompatibilities like AT/ATX? It's an interesting idea for future designs, but I wonder if there are other stubling blocks out there. Thoughts?

7 of 67 comments (clear)

  1. most heat & noise from a P/S? Nope. by Tumbleweed · · Score: 5

    Not hardly.

    These days, Most heat is generated from the CPU, and most noise, believe it or not, is generated from those cruddy little CPU fans. A lot of noise is also generated from extra case fans you may have trying to improve airflow in your case. P/S fans are often much quieter than those other fans in your system.

    I've thought of this idea before, and I also think it'd be a fantastic idea. Just keep in mind, the P/S also needs to be cooled, so that external box will have fans in it, and once it's outside the case, it'll be making some noise, too. I remember my Amiga 500 fondly, though, as it's power needs were such the external P/S didn't need any fans...

  2. Cost? by Bazman · · Score: 5

    We had some X-terminals with external power supplies. They had a habit of breaking, so we tried to source some new ones. 80 quid (thats over $100 in USA money). I was stunned. 80 quid for a black box transformer.

    Anyway, the thing had the pinout of the connector printed on the bottom, so I hacked off the cables, soldered on some standard molex connectors, and wired three of them up to a standard PC power supply running in an empty case.

    Back to the question. An Xterminal is a pretty low-power-usage device (no moving parts, and the monitor was on a mains cable) and that seemed to need an expensive PSU. A comparable unit to power a PC as well would be ridiculous...

    Baz

  3. It's a little more complex... by bandgap · · Score: 5

    Wow, something on Slashdot I'm actually semi-qualified to answer! ( I used to design server-class power supplies)

    There have been some good answers, but the biggest reason PC power supplies are internal rather than external boils down to $$. Well, that and safety, but then safety boils down to $$. And then technical, but that's also solved by $$.

    There are at least four voltages produced by PC power supplies, with currents on the 5V line up around 12-20 Amps. The regulation of the 5V line measured on the motherboard is about +-5% or better (+-250mV), which at 20A would work out to about 0.012 Ohms of resistance split between cable and connectors. 14 guage wire (similar to an extension cord) has a resistance of 0.0026 Ohms per foot. So, about 5 feet of wire would chew up all of the allowed resistance with nothing left over for connectors. I've been loose with my numbers, but you get the idea.

    Now imagine a cable from the power supply to the computer with 8-10 wires, all the size of those in a standard extension cord. Now, imagine paying for all of that copper.

    The old AT-class and now the ATX class supplies are commodity products. This means that the design is stable, has been engineered and cost reduced to death, and are now being manufactured in low labor rate locations with component parts being negotiated to sub-fractional pennies.

    Custom power supplies used to be priced in the $1 per watt range (in large volumes). PC power supplies are well down below the $0.10 per watt range and going lower. Anything new will have to survive being compared against a mature $15-20 solution.

    Cheers,
    Jim

    1. Re:It's a little more complex... by pallex · · Score: 5

      "Wow, something on Slashdot I'm actually semi-qualified to answer! "

      If only everyone here thought like that! :)

    2. Re:It's a little more complex... by markmoss · · Score: 5

      Correct, for a given power the current is inversely proportional to voltage, so bringing in the power at 5V instead of 120V takes thick, expensive, and unwieldy wires. I think this arrangement wouldn't make much sense unless the wires were about 5 feet long so you could put the supply on the floor -- if it has to sit next to the mainframe, you've just added more clutter. With a separate case and fan you've added $10-15 to the parts cost of the system, or $20-30 to the street price.

      The other thing is, a long distance between the regulator and the load slows down the response to load variations and degrades the voltage regulation no matter how thick the wires are. High-speed rack-mounted electronics systems often put a regulator on each circuit card, with power distributed at an intermediate voltage, just to get the regulators a couple of feet closer to the loads. It might make sense to do this with PC's -- convert AC to 48 and 12VDC in a separate module, then put simple switching regulators on the motherboard to transform 48VDC to +5, +3.3, and whatever other voltages are needed. One part of this is already in place; most Pentium motherboards power the CPU with a switching regulator located right next to or even under the CPU socket.

  4. Extension cords! by billcopc · · Score: 5

    Why not attack it the redneck way ? Just buy a truckload of molex extensions and chain them together until you have about 12ft of unshielded power wires dangling around.. that way you could hide the case away and screw the cdroms and tape drives directly onto your desk thanks to the extra long power connections.. as a bonus it will automatically kill your roommate's cat in seconds as soon as it sinks its teeth into it.

    --
    -Billco, Fnarg.com
  5. no more dongles! by jonniesmokes · · Score: 5

    I can't say I want anymore power supply devices lying around my desk. So please don't suggest that they make another. Its wonderful and unusual that they make proper UL listed power supplies for computers. Most cheap things don't want to plug into the wall. Currently, I have One AC-DC for the scanner One AC-DC for the external zip drive One AC-AC for the DSL modem One AC-DC for the PIC16F873 board These dongles either take up cumbersome space or they take up way too much space on the powerstips. Happily the printer, monitor, and computer all plug in normally. On the other hand... If they could link all of these peripherals up to the same power supply, that'd be pretty cool. I wouldn't mind having just one external power supply if we could plug all the devices into that. Unfortunately daisy chaining is frowned upon in DC power supply situations. So that means a spidery nest of power cables no matter what we do. Is there anyway out of this? How about a tesla-coil right above the computer ;) I'd give mega frankenstein points to anyone who made a working computer system using a tesla-coil.