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Your old PC might be past it... but its power supply might not be! Cheap, High-Current Bench Supplies by Nicholas Vinen If you’ve ever had to buy a highcurrent bench supply, you’ll know they don’t come cheap. But you may well have such a supply sitting unloved and (until now!) unwanted in the back of a cupboard. It’s the power supply in that old computer you never quite got around to throwing away! H ere at SILICON CHIP we are big fans of re-using and recycling old electronics. We’re loathe to throw away anything which is still operational, even if it’s obsolete. Manufacturing these devices involves much effort, so just throwing them away when they still work would be a shame. This means that, among other things, we have a number of computer power supplies lying around, gathering dust. Some of these are still inside old computers which are too slow to be useful while others are left over from upgrades (where the old supply wasn’t up to the task of powering a new motherboard or CPU). Others were rescued from machines that were recycled or thrown away. Even if you don’t have a spare computer power supply, these days they are cheaper to buy than an equivalent bench supply. They don’t have particularly good voltage regulation, 44  Silicon Chip either in terms of absolute output voltage or ripple but they do have multiple voltage rails, in some cases capable of delivering upwards of 30A. If all you need is a high current fixed voltage supply (12V, 5V and possibly 3.3V), using a computer supply is a cheap and efficient option. Note that we are not modifying the supply to provide different output voltages than those offered. Of course that can be done (see the articles in SILICON CHIP, December 1998 and October 2003) but here we are just making it much easier to use the existing rails for a bench supply. Choosing a supply Our first task was to decide which supply to adapt. We have some of the old “AT” supplies as well as the newer “ATX” supplies. The latter are far more common these days and safer to work with since there is no external mains power switch. As ATX supplies are now pretty much universal (and also more powerful), that is what most constructors would use. In the end we chose a 600W Shaw brand supply. We decided against two others with higher current delivery because they are still useful for running a modern computer; and quiet and efficient to boot. Of the rest, the Shaw delivers the most current at 12V (18A) as well as a healthy 35A at 5V and 30A at 3.3V. It also has both negative outputs (-5V and -12V; some supplies lack the -5V), rated at 0.5A each. This particular supply was bundled siliconchip.com.au DANGER! A few terminals, a power switch and a LED turn a redundant PC power supply into a really useful high-current bench supply. with an ATX case but it was noisy and inefficient at idle so we replaced it with a more expensive but much better unit, leaving this one spare. In the role of a bench supply, these issues are quite minor, as it will only be operated intermittently. Any ATX supply is suitable for conversion but before you start, check the ratings, which are usually printed on a label attached to the side of the supply. Once you are happy that the current ratings are sufficient for your uses, you can begin the conversion, which should take no more than a few hours. Parts Which parts you need will vary slightly depending upon your supply and how many voltage rails you want to access. Here are the parts that we used: 8 binding post terminals (three black, the rest different colours) 1 SPDT miniature toggle switch 1 3mm LED 1 3mm LED bezel 1 390Ω 0.25W resistor 4 stick-on rubber feet 2 M3 x 10mm machine screws, nuts & shakeproof washers small piece of aluminium plate, ~35 x 35mm short lengths of 2.5mm and 4mm diameter heat shrink tubing adhesive labels siliconchip.com.au In addition you will need the following tools: Screwdriver Side-cutters Soldering iron Wire stripper Needle-nose pliers Centre punch Drill and drill bits (3-8mm) Construction First, a word on safety. Computer power supplies can kill: they rectify the 230V AC mains without the benefit of an isolation transformer and many sections of the circuitry are at full mains potential. Never operate the supply with the lid open and always wait at least five minutes after switching off before opening it up again. There are capacitors in the switch-mode supply which, even with the supply turned off, can hold their possibly lethal charge for a couple of minutes or more. In our supply there are exposed live mains conductors just below the lid which could easily produce a fatal shock. Other computer power supplies will certainly have similar hazards inside. Make sure that your modifications do not interfere with the mains isolation of the PC board inside. As you can see from the internal photo of our supply, there is a row of transformers and Internally, computer power supplies carry the full 230V AC mains voltage and should be regarded as potentially lethal. Much of the exposed internal circuitry, heatsinks, etc floats at the mains voltage. NEVER open a computer power supply case or work on the supply with the IEC mains cable plugged in (turning it off is NOT good enough!). Heed the warnings in the text! optocouplers down the middle which form the isolation barrier between the high-voltage and low-voltage circuitry. The high-voltage section contains the large mains filter capacitors. In this case they are rated at 200VDC and are connected in series (with parallel high value resistors) to handle the 325V or so which results from rectifying 230VAC. Do not mount any binding posts, switches or other components over or around this area. It is essential that the low-voltage side of the supply cannot short against a mains conductor and become live. This includes any heatsinks in the mains section; they may be live! Start by opening up the supply (unplugged, of course!). You may need to remove one or more stickers to expose screw heads, before this is possible. The lid will typically be held on using four Phillips head screws – undo them and it should come off. Usually, the lid clamps the grommet which holds the bundle of low-voltage carrying wires where it exits the supply. Lift the bundle out of the case and remove the grommet. Because the wire colour coding can vary between supplies, check yours against the list in Table 1. Now that you can see the PC board, if there are any silk-screened descriptions where the wires are soldered, check that they match this list. Do not proceed January 2011  45 In the original supply, all the low voltage cables emerge through a hole in the case (top right of above pic), held in place by a cord-grip grommet. In the modified supply, this hole is covered by a small piece of aluminum which contains a “standby” power switch and a LED connected via a 390Ω resistor to one of the low voltage terminals until you are sure of the function of each wire. Metalwork With the lid off and the low voltage wires loose you can now determine where to mount the various components and drill the holes. As you can see from the photos, we decided to mount a standby switch and indicator LED on our supply but these parts are optional. In fact the bare minimum supply requires the addition of just two binding posts, although most constructors will want to use at least three (+12V, +5V and ground). Aside from these components you will also need to fit a small metal plate to cover the now empty wire exit hole. This will prevent any accidents involving screwdrivers or fingers going inside the supply and possibly contacting dangerous voltages. Cut a rectangle from an aluminium sheet or off-cut which will cover the opening and provide enough space for two or more mounting screws. File it to fit; remove any burrs or lips at the same time. Then, drill holes in both the panel and the supply case to take M3 (or larger) machine screws. If you like, you can also drill holes to accept a switch and/or LED bezel in the plate, 46  Silicon Chip as we have. Ensure that with the cover in place, the lid closes properly, leaving no large gaps. Now you must decide where to mount the binding posts. As mentioned previously, it is dangerous to locate these above the portion of the board which carries mains potential. Is it for this reason that we decided to mount all our additional components near the now covered wire exit hole, adjacent to the low-voltage side of the PC board. Also, be careful that the bottom ends of the binding posts or the attached wires cannot short against any heat sinks. Once you have selected the appropriate locations, use a centre punch (or a nail and a hammer) to mark them. Don’t put them right up against the edge as that will make assembly tricky. Space them apart sufficiently to give room for access to the binding post wire entry holes once they are in place (at least 16mm, more if possible). Be gentle with the punch as the relatively thin steel can be bent easily. You just want a small depression to guide the drill. You can then proceed to drill the approximately 7mm binding post mounting holes. If you are fitting a switch and/or LED and have not already made holes for them, do so. De-burr all the holes using a larger drill bit. After that, install the binding posts. Unscrew the plastic cap so that you can orientate them for good access to the wire entry holes. This usually means facing the hole towards the nearest edge of the case or, for those posts in the middle, diagonally. When you are satisfied, tighten the binding post nuts very firmly while preventing the posts from rotating. When you have finished, screw the plastic caps back down. Now stick the rubber feet onto the bottom of the supply. Don’t use screwon feet as you would likely have to remove the main board from the case to get them in and it’s possible that the screws could short to the bottom of the PC board and create a shock hazard. Wiring it up Referring to Table 1, cut off any wires which are no longer necessary. Do this as close to the PC board as possible so that the wire stubs are not free to flex and contact any other wires or components. Ideally, there should be no more than about 5mm of each wire left. Cut the connectors off the wires you will be keeping, as close to the connector as possible (to ensure the wires are long enough). siliconchip.com.au In the modified supply, all those low voltage wires now remain inside the case and connect to appropriately labelled terminals fitted to the case “lid”, as shown in these two photogreaphs. The reason that we suggest retaining thirteen black wires is that one will be used for the on/off switch and the other twelve can be split into three groups of four and soldered to the ground posts that correspond to +12V, +5V and +3.3V. These are high current outputs and this prevents the return current from one from affecting the other voltages. If you are not providing all three outputs, you don’t need as many ground wires. If you are installing a switch, cut the green wire and one of the black wires so that they are just long enough to reach its terminals, strip the ends and solder them to it. They should be attached so that when the switch is in the “on” position (ie, down for Australia and New Zealand), these wires will be connected. Otherwise, cut the wires short, solder them together and heatshrink the junction. If you are installing a LED, trim its anode (the longer lead) and solder the 390# resistor to it. Then trim the grey and purple wires so that they will reach the LED leads and strip the ends. Solder the purple wire to the 390Ω resistor and the grey wire to the cathode lead, then heatshrink both and push the LED and bezel through the hole you made earlier. With this arrangement, the LED lights when the supply is in standby (ie, it has mains power but it is off) and when it is on but overloaded; otherwise it is off. You can arrange for it to light under other circumstances. For example if you want it to be on whenever mains power is applied, connect siliconchip.com.au the cathode to a black wire (ground) rather than grey. We shall leave other possibilities up to the reader. Now place the lid upside-down, with the binding posts near to where the low voltage wires exit the PC board (see photo). Remove any small nuts which may be screwed onto the exposed binding posts shafts. Trim the remaining wires so that they will reach the appropriate binding posts. If you are not sure which wires go where, refer to Table 1. There are a couple of tricks here. Firstly, make them about 20mm longer than necessary to allow for stripping the ends. Secondly, you need to check to make sure that once the wires are soldered to the binding posts, you can actually manoeuvre the lid into place. This requires leaving a little slack in them. You can see from our photos how much extra length we allowed. Before proceeding, check if your supply has pink (+5V sense) or brown (+3.3V sense) wires. If so, they must be soldered to the same point as the red (+5V) and orange (+3.3V) wires respectively. The easiest way to do this is to twist them together as explained below. If you are not using one or both of these rails for an output, you must still connect the corresponding sense wires to at least one output wire (and heatshrink the junction). Strip 20mm of insulation off the end of each wire and twist the strands together tightly. Wrap them around the binding post as many times as possible and flow solder onto the junction. For binding posts where more than one wire is attached, twist all the wires together into a single, large bundle before wrapping it around the post; this is much easier than trying to solder them individually. Table 1 – ATX power supply wire colour codes Colour Meaning Number to keep (if possible) Black.............................................. Ground........................................ 13 Yellow............................................... +12V........................................... 4 Red................................................... +5V............................................ 4 Pink......................................+5V sense (optional).............................. 1 Orange.............................................+3.3V.......................................... 4 Brown................................. +3.3V sense (optional)............................. 1 Blue.................................................. –12V........................................... 1 White......................................... -5V (optional).................................... 1 Green...........................On/off switch (input, active low)...................... 1 Purple........................................ +5V standby..................................... 1 Grey...............................Power good (5V, active high)........................ 1 January 2011  47 Make sure that the wires do not move as you solder them and use enough solder to fully envelope the joint. Stop heating as soon as the joint has been made or else you risk damaging the wire insulation. Use small cable ties to hold the switch and LED wires in place, so that they can not possibly come loose and contact any high voltage components. Wrap another cable tie around the bundle of wires connecting to the binding posts so that if one comes loose, it can not flap around inside the supply. Minimum load Some ATX power supplies will not regulate their outputs correctly if there is no external load. This is not universal, the supply we used does not have this requirement. If yours does and you do not attach a dummy load, either the output voltages will be too high or the supply will not start up properly. If you are not sure about your supply, you can proceed to the testing step and return here if either condition occurs. The 5V rail is the most likely to require a dummy load. Usually, this rail is regulated and the others just trackit. However it is possible that some supplies regulate the rails separately and in this case each positive output will require a load. While minimum load requirements will vary, the following 5W resistors between the output and ground should be sufficient in most cases: for the 5V rail, 27Ω; for the 12V rail, 150Ω and for the 3.3V rail, 15Ω. These resistors can be soldered between the binding post terminals. Testing and completion Before proceeding, check that all your solder joints are solid and that they are either insulated or can not possibly contact any exposed metal inside the supply. If you have placed the binding posts correctly you will not need to insulate them but all other joints should be heatshrinked. Having checked that, screw the lid in place. As you fit it, take care that the wire bundles are not squashed up against any components. Connect a multimeter set on volts mode between the +5V output and ground. Banana plug-to-banana plug leads come in very handy in this type of situation. If you have several mul- timeters, connect them to the other outputs. Plug an IEC power lead into the supply, flip the standby switch to on (if fitted) and then plug the mains in and switch it on. Check that the output quickly rises to 5V (or thereabouts) and stays there. If it does not, immediately switch the supply off, disconnect it from mains and check your work. If you did not attach a load to the 5V rail then it is possible your supply requires a load; if so, follow the preceding instructions. Assuming that all is well, you can check the other outputs and make sure they are correct. If you installed a standby switch you can also check that it works and that the LED (if installed) behaves as expected. Finally, it is a good idea to attach adhesive labels to indicate the voltage and current available at each output. You may remember the colour coding now but it’s easy to forget in future. A label printing machine will result in a neat and legible result although we found we had to cut the labels up to get the spacing correct. An alternative would be a label prepared on your computer and possibly laminated to protect it. SC Radio, Television & Hobbies: the COMPLETE archive on DVD YES! NA R MO E THA URY ENT QUARTER C NICS O OF ELECTR ! Y R O T IS H This remarkable collection of PDFs covers every issue of R & H, as it was known from the beginning (April 1939 – price sixpence!) right through to the final edition of R, TV & H in March 1965, before it disappeared forever with the change of name to EA. For the first time ever, complete and in one handy DVD, every article and every issue is covered. If you’re an old timer (or even young timer!) into vintage radio, it doesn’t get much more vintage than this. If you’re a student of history, this archive gives an extraordinary insight into the amazing breakthroughs made in radio and electronics technology following the war years. And speaking of the war years, R & H had some of the best propaganda imaginable! Even if you’re just an electronics dabbler, there’s something here to interest you. • Every issue individually archived, by month and year • Complete with index for each year • A must-have for everyone interested in electronics Please note: this archive is in PDF format on DVD for PC. Your computer will need a DVD-ROM or DVD-recorder (not a CD!) and Acrobat reader (free download) to enable you to view this archive. This DVD is NOT playable through a standard A/V-type DVD player. Exclusive to SILICON CHIP ONLY 62 $ 00 +$10 P&P HERE’S HOW TO ORDER YOUR COPY: BY PHONE:* (02) 9939 3295 9-4 Mon-Fri BY FAX:# (02) 9939 2648 24 Hours 7 Days <at> BY EMAIL:# silchip<at>siliconchip.com.au 24 Hours 7 Days BY MAIL:# PO Box 139, Collaroy NSW 2097 * Please have your credit card handy! # Don’t forget to include your name, address, phone no and credit card details. 48  Silicon Chip BY INTERNET:^ siliconchip.com.au 24 Hours 7 Days ^ You will be prompted for required information siliconchip.com.au