COMPUTING ON 25 WATTS John C. Osborne copyright1990 John C. Osborne ENIAC, the first electronic computer, ran on 174,000 Watts of electricity. Today's desktop systems need 700 times less. As a home power user, you can do 10 times better yet by building your own computer. This column tells how to build a computer drawing about 25 Watts. BUILD A COMPUTER? ARE YOU KIDDING? No, computer stores do it all the time. All the parts are available from hundreds of suppliers as commodity items. Monitors, printers and keyboards simply plug into the system box. Inside are a few subassemblies installed with just a screw driver. SCOPE These items are needed for a minimally functional system. 12 Volt DC input is assumed, but other DC voltages are practical, also. Scanners, printers, modems, FAXs and other fun stuff are not the subject of this article. This discussion is limited to IBM clones because they are the most common and offer the largest choice of parts. STRATEGIES There are three main ways to achieve operation from 12 Volts. A fourth method is a combines the others. Use an inverter The most common and obvious method, but also the least efficient at 400- 600 watts. If you already have an inverter, this is the cheapest from the standpoint of initial cost, but the ongoing operating cost is the highest. Exchange the power supply A few minutes with a screwdriver and your old PC power supply can be replaced with a high efficiency 12 VDC unit. This has moderately low initial cost if you already have a computer and ongoing costs are reduced. You still have the problem of running your monitor and printer, however. Select and modify components By researching the hundreds of available parts, it is possible to find some with very low power consumption. Some can be modified for further gains. For instance: when a monitor rated at 42 Watts at 120 VAC was converted to 12VDC, it needed just 12 Watts. The "select and modify" strategy is the only way to approach our 25 Watt goal. It may be the cheapest overall cost as well. No inverter is needed, the power supply is small and only the very smallest of home power systems would need expansion to carry the extra load. Use a hybrid approach Run the items that are used for long periods (the computer and monitor) from 12 Volts and those devices used occasionally (printers & scanners) from an inverter. This way, you get many of the benefits without undue effort. ITEM BY ITEM ADVICE Lets examine the pieces needed for the "selection and modification" approach. I will give the typical Wattage, the percentage of the total and some alternatives for dramatically lowering the Wattage. Things get a little technical from here on but if the guts of a computer put you off, let someone build the computer to your specs. Monitor This is usually the worst single item at 40 to 200 Watts or as high as 40% of the total. Color, screen size and resolution eat up the power. Staying with a conventional CRT monitor is cheapest, but a 12 Volt, B&W, 12" screen at 640 x 200 resolution takes the power down to just 12 Watts. A 12 inch, B&W, 640 x 480 is 25 Watts and a 12", color, 640 x 480 is 35 Watts. More demanding users can choose LCD screens at about 3 Watts, but about 5 times the cost. For the radical among you, a head mounted device that rests in front of one eye floats a 12" image about 18" in front of you. Its like the Head Up Displays in fighter jets. It uses one third of one Watt. Motherboard The motherboard characterizes the entire system. It uses from 20 to 50 Watts typically, or about 10% of the total. This is far lower than the monitor, but when we get the monitor down to 12 Watts, the it's draw becomes more significant. Some new boards are down to 7 Watts. Look for boards called "baby AT" size. Since more functions are forced into fewer chips to fit the smaller boards, designers have to reduce heat buildup and therefore the power is lower. Short of checking the specs, fewer chips normally means lower power. For the very lowest power (3 Watts and under) special purpose industrial grade boards are available, but at 3 times the cost. Disk drives Hard and floppy disk drives often use 20 to 40 Watts, another 10% of the total. But newer floppy drives need only 2 Watts when running and 1 thousandth of that when idle. The only floppy drive to consider are the 3 1/2" style. The older 5 1/4" need 5 Watts running and 1.5 Watts even when idle. Also, they are less reliable and lower capacity. For entry level computers, two 3 1/2" drives at 1.44 MB each may obviate the need for a hard drive. For those applications that need them, 40 MB units drawing only 4 Watts for are quite affordable. I recommend the IDE (Integrated Drive Electronics) interface. This means the disk controller is built into the drive. While it raises the cost of the drive, it lowers power consumption, increases performance and makes installation much easier than previous generation drives. For extremists, drives drawing a mere .9 Watts are available at twice the cost. Plug-in cards Multiple plug in cards use 20 to 60 Watts, or 12% of the total. Some are necessary for the computer to function, like video, floppy disk, hard disk, printer and serial controllers. Other cards are accessories, like modems and FAX cards. To reduce power consumption here, find cards that combine as many functions as possible on the same card. As with the motherboard, packing more functions into a smaller space means using denser chips, forcing the designer to limit the heat and therefore the power. This strategy can save money and open slots in the motherboard. Power supply Its odd to think of a power supply using power - it's job is to convert it. But the process may consume up to half the power sent to the computer boards. This could be 140 Watts or 30% of total power consumption. A common perception is that switching type power supplies in PCs are efficient. While they certainly could be, they are often designed to be cheap to buy, not cheap to operate. Converting everything inside the case to 12 Volts is a simple matter of dropping in a power supply that takes 12 Volts as input. Because power consumption matters to us, power supplies for this purpose are often 70 to 80% efficient. As we have seen, however, we need far less power than normal PCs and this offers new opportunities. For example, it is possible to run from a single 5 Volt supply by using floppy drives only, no serial ports and no accessory cards that might need other voltages. Since the power drain can be low, a simple 5-Volt regulator chip available from most electronics stores can form the complete power supply. For those needing multiple voltages, power supplies normally used in laptop computers can be used. These provide only 25 to 50 Watts and while they would be hopelessly overloaded by typical PC parts, our specially selected units work fine. Making your own 5 Volt regulator or using the laptop supply require some experience with electronics. PUTTING IT ALL TOGETHER A few more pieces are needed. The keyboard simply plugs into a connector on the motherboard and doesn't draw enough power to worry about. You may need cables for disk drives, though they often come with the controllers. Mounting kits are needed for adapting 3 1/2" drives to fit the 5 1/4" spaces. The case can be the smaller size "XT" case if a baby motherboard is used. Don't try to do without a regular case. It provides grounding and shielding. The shielding works both ways - preventing the computer from radiating interference to radio and TV receivers and also prevent outside noise from reaching the computer. I often find that a monitor's high voltage supply is picked up by the disk drive unless the case is closed. The last "piece" is software - you need an operating system, usually DOS. Starter system A good 25 Watt system for the newcomer to computing consists of an "XT" motherboard with 640 KB of memory, dual high capacity floppy disks, a 640 x 200 B&W monitor and a 9-pin printer. It is usable for word processing, spreadsheets, data base management and most applications that do not require a hard disk. Cost of the complete system is about $860. "Windows" system Recently, a Graphical User Interface, Windows by Microsoft, has exploded in popularity. It is highly demanding of the disk, processor, memory and graphics display. This leads many to ask what is a good setup for running Windows. Start with a fast 286 and 4 MB of memory, add a floppy and 40 MB hard disk, 640 x 480 VGA display and controller, printer port, serial port and mouse and Windows software. With printer, the cost is about $1,710. At 40 Watts, it is a modest load on a home power system. Wrapping up Remember the 174,000 Watt ENIAC? We now have computers that need 7,000 times less power while running thousands of times faster, as well. ACCESS John C. Osborne, AirCastle Enterprises, 15926 Northville Road, Plymouth, Michigan 48170, (313) 348-7135.