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Here's a way that anyone with a working computer can get a handy fixed rail power supply with several outputs. It's a little card to slot into your computer to give fixed +12V, +9V, +6V and +5V rails. You can use it to power equipment that would normally run from plugpacks or as an experimenter's power supply. PC POWERHOU Get four fixed rail supplies for minimum dollars Design by BARRY HUBBLE There are two good reasons to build this project. First, if you are plagued with the problem of having too many plugpacks to be fitted into a 240VAC power board, the PC Power Extender may enable to eliminate some or all of them. Second, perhaps you've often been working on a project and you want a power supply with a fixed output of 12V, 9V or whatever. Well, if you have a PC you already have a thumping big power supply builtin, so why not use it? Just getting back to those plugpacks for a moment, they really are a problem aren't they? You can't fit two plugpacks next to each other in any standard double power point or power-board and you often can't fit a standard power plug into a double power point if you already have a large plugpack running from it. They are just too big and bulky. Wouldn't it be good if you could get rid of them altogether? For example, if you have an external modem, a scanner and a printer all running from plugpacks, you might be able to run them all from your PC's built-in power supply. This is one of those ideas that is so 32  Silicon Chip obvious that you may well be thinking "Why didn't I think of that?" or "How come it's taken this long for someone to think of this?" Well, while we might contemplate our (collective) navels about why some ideas take such a long time to become obvious, let's not muck about. Let's just look at the circuit of Fig.1. As you can see, it takes power via a standard DC connector in an IBM-compatible comp u t e r. T h e +12V and +5V rails are connected via 1A fuses to standard 2.1mm sockets. At the same time, the +12V rail is also fed to two 3-terminal regulators which then provide a fixed +9V and +6V and their outputs also connect to 2.1mm sockets. Of course, you might want to vary it a bit. You might want an 8V output instead of 6V and this is easily fixed by fitting an 8V regulator instead of a 6V type. Or you might decide you want to two 9V outputs and this is easy: just fit two 9V regulators. Each regulator has an effective current limit of about 1A, so you get four fixed supply rails each with an output capability of about 1A. The circuit is so simple that there is really not much more to say. But perhaps you are worried about the extra load that might be placed on your computer's power supply. Can it handle it? Well, think of it this way. If your machine has a standard 200W or 250W power supply, you would not think twice about adding another hard disk drive, maybe another CD or DVD-ROM drive, a ZIP drive or whatever. Most computers have buckets of spare capacity, so the extra drain due to this small circuit is hardly worth thinking about. As well, PC power supplies are well designed and will normally shut down without damage if they are overloaded; normal operation being resumed after removal of the overload. A typical 200W computer power supply would have a capacity of 8A for the +12V rail and 20A for the +5V rail but how much of that your system uses is your guess. And whether the supply really can deliver a full 200W is probably doubtful but suffice to say that most computers still have oodles of spare power supply capacity. By having a power supply extender with four outputs of +12V, +9V, +6V and +5V, all at 1A capacity, we are unlikely to embarrass the vast majority of computer power supplies. After all, you are unlikely to use all output simultaneously at their full outputs a more likely scenario is that the PC Power Extender would only draw about 10W or less, depending on what you connect to it. By the way, computer power supplies also have -12V and -5V outputs but these have only a limited capacity and we do not recommend that you attempt to use these outputs as well. Leave them strictly alone. USE PC board mounting The PC board for this project is mounted in the computer with the aid of a PC mounting plate with integral Fig.1: using the existing +12V and +5V rails in your computer, this circuit uses two 3-terminal regulators to provide +9V and +6V. mounting brackets. You may have to salvage one from an obsolete card. If you can't obtain a PC mounting plate with these mounting brackets you may have to make up a couple of righthand brackets yourself; not an onerous task. Another part that you may be able to salvage would be a 4-pin PC-mounted power connector from a scrapped disk drive. Due to the mounting plate being screwed to the computer's chassis and therefore at mains Earth potential, all the jack output sockets MUST have the centre pin wired as the positive connection, to prevent the possibility of short circuiting an output. This may not conform to some of the devices you want to power. For example, powered multi-media speak- Fig.2: the component layout for the PC board. You will need to bolt down the regulators and their heatsinks to the board. Low profile heatsinks must be used. The additional holes alongside C1 & C2 are to accomodate physically larger capacitors if needed. DECEMBER 1999  33 The prototype PC Power Extender, looking towards the four output sockets. Note that these photos do not show the two 0.1µF capacitors that we have added to the input sides of the regulators. ers seem to have the centre pin as the negative connection. In such cases it will be necessary to reverse the leads on one end of the cable connecting the output to these devices. These leads must then be clearly labelled as being a reversed configuration. You could do this by using white heatshrink sleeving and a permanent marking pen to label the cable. While most devices these days are protected against reverse voltage, they are not necessarily protected against over-voltage and the outputs of the PC Power Extender must be clearly labelled. This problem was overcome with the prototype by having small printed labels, laminated them on both sides with an adhesive laminate to stiffen them. They were attached so they cover the outlets. By using an additional nut on each DC socket, the label can be mounted 34  Silicon Chip so that it may swivel freely to one side when a plug is inserted. Assembly You can begin construction by drilling the holes in the mounting plate, as shown in Fig.3. If necessary you will have to fabricate and attach two small `L' brackets to secure the blanking plate to the PC board. Install the four DC sockets with their output voltage labels. Next, mount the PC board so that its output pads are adjacent to their respective sockets. If you don't do this correctly, the PC board may be mounted too low and will liable to foul the bus expansion sockets on the motherboard. The two 3-terminal regulators and their heatsinks are bolted to the PC board. Mount all components on the PC board observing the polarity of the electrolytic capacitors. You can make the connections to the DC sockets Note: if this project is used to drive powered loudspeakers for a PC, you should first check that the ground of the signal line (ie, shield connection) is not connected to the positive supply for the speakers. While we have not encountered this situation, it is not suitable for the PC Powerhouse as it would cause a short circuit to occur across the 5V output. with tin-coated copper wire. In each case, the positive lead must go to the centre-pin of the socket. To make the connection from your computer's power supply to the PC board your will need to find a spare 4-pin disk drive power cable which will normally be dangling inside the machine. If there are no spares, you will have to install a disk drive power splitter cable. Before you make any connections, check the voltages at the pins of your disk drive connector. The yellow lead be +12V and the red lead +5V. The two black leads in the centre are the 0V returns. If you have a DC power supply which can deliver around 12V, it is a good idea to hook it up to the PC board and check the outputs of the two regulators. When installing the PC Power Extender board unit in your computer, it Fig.3: you will need to drill a PC card mounting bracket to take the four 2.1mm DC power sockets. Compare this to the photograph at left. The photo also shows small swivelling voltage labels which the author attached to the output socket mounting screws. Parts List is a good idea to mount it with a free slot on either side to help with heat dissipation. After installation and before putting the case back on, test the whole system to ensure the computer power supply isn't showing any signs of distress (strange whistles, groans, smells, or SMOKE!) and that your programs are SC operating normally. 1 PC board mounting bracket with rightangle lugs (see text) 1 PC board, 89 x 89mm, code 12112991 4 2.1mm rectangular panel mount DC sockets 2 M205 1A slow blow fuses 4 M205 PC-mount fuse clips 1 PC rightangle mount disk drive 4-pin power connector (Altronics Cat P-5671 or equivalent) 2 low profile TO220 heatsinks 2 'L' mounting brackets (see text) 12 2.5mm x 9mm screws 18 2.5mm nuts Capacitors 2 470uF 25VW electrolytic 2 0.1uF MKT polyester 2 .01uF MKT polyester Fig.4: actual size artwork for the PC board. Use this to check your board for defects before installing any of the components. Semiconductors 1 7809 9V regulator (REG1) 1 7806 6V regulator (REG2) Miscellaneous Hookup wire if required for power connector Disk drive power splitter cable if required DECEMBER 1999  35