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Salvage It! By BRUCE PIERSON Wrecking a Dead PC Power Supply for Parts In previous issues, we’ve talked about the goodies you can salvage from dead (or old!) computers, including a handy 5V/12V power supply. But what if the power supply itself is dead? S o, your computer’s power supply has died and you’ve fitted a new one – and now you are going to bin the old one. Hang on a minute! There’s a lot of good parts in that power supply and it could be well worth wrecking it for parts before you toss the rest out. So, what could be useful inside there? Let’s have a look inside and see. But wait! Isn’t it dangerous to open a computer power supply? Yes, it can be if you don’t take proper precautions! The first rule of safety is to never open a computer power supply while it is plugged in to power (even if you think the power is turned off!). That is a sure way to risk death or serious injury. So before you do anything else, make absolutely sure that the power supply lead is removed from the computer completely. The vast majority will be fitted with an IEC plug which simply unplugs. The next thing to be aware of, is that in some cases, there can be a potentially lethal charge stored on some of the capacitors. Therefore, extreme care needs to be taken to ensure that these capacitors do not have any charge on them before proceeding to handle the circuit board further. That’s the first thing we’ll do after we open the power supply itself. 46  Silicon Chip Opening the case Before that, though, you need to work out how to open the computer case (if you haven’t done so already). Depending on brand, type (and age) this can be anywhere from delightfully simple to mind-bogglingly difficult. Invariably, there will be at least a screw or two (perhaps quite a few more) which – theoretically – will allow you to slide a side panel along and off. Even if you accomplish that task easily, Murphy’s law dictates that you have taken the wrong one off so you don’t A typical power supply as removed from the case. Most of the sockets and plugs will be useless these days as times have changed – as have computer connections! siliconchip.com.au have access to the power supply. No harm done, you’re junking the computer anyway. But keep the screws (they’re always handy). Now that you have access to the inside, the next thing to do is probably run a vacuum cleaner over it to get rid of years of accumulated dust. Even after removing the supply, you’ll probably want to remove lots of bits from the PC itself, although many will be useless due to changes in computer design. Back to the supply So, now to open the power supply and have a look inside. Typically, there will be four screws securing the “lid” (which may even be half of the case), one of which is usually under a sticker of some sort. Occasionally, there will be additional screw(s) on the side(s) as well, while rarer units may have a different type of case with screws in different places. However, most computer power supplies are fairly standard in construction and are similar to the unit pictured above. Three screws can be seen near the edges of the box and the fourth screw is under the green sticker on the left-hand side, near the back of the box. Undo these four screws and remove the lid. other components on the other side of the circuit board. It’s not immediately obvious in this photo but the electrolytic on the far right has failed in typical electrolytic capacitor style, with the obvious bulged top. This is a sure sign of a defective electrolytic capacitor which should be discarded. Safely getting into it! In order to proceed safely, the following instructions need to be followed very carefully. Firstly, remove the four screws securing the circuit board to the bottom of the case. Then, carefully remove the circuit board from the case, being very careful not to touch any part of the underside of the circuit board. With the circuit board turned over, measure the voltage on the two large capacitors with your multimeter on the 500V DC range. In most cases, there will be no voltage present, but if there is any reading above, say, 20V or so, the capacitors need to be discharged before proceeding. This can be accomplished by using a 230V 100W incandescent light globe (if you still have such a beast!) in a holder with two insulated wires connected to it or a 5W resistor of around 1.5k. Check again with the multimeter to make sure there is no charge left and then the circuit board is safe to handle. Don’t worry about any smaller capacitors, because if there is any charge remaining on them (which is unlikely anyway) it won’t be any more than 12V, which of course is not harmful. With the circuit board now safe to handle, either unplug or if necessary cut any wires that are connecting it to any part(s) still attached to the case and it will then be free to remove and ready to dismantle. Now we can see what can be salvaged from it for the parts box. To make handling the circuit board easier, remove the hookup wire from the circuit board first. The hookup wire is often held together with cable ties. These can be removed in such a way that they are re-usable, always handy for securing wire and other items. Simply cut the cable tie as shown in the picture below and pull out the small piece of the end that was cut off and you have a re-usable cable tie. Looking at the photo above, we can see what’s inside the power supply. The two (sometimes only one) large, high voltage capacitors can be clearly seen at the left front of the circuit board. It’s these capacitors that you have to be very cautious of because they can retain a real bite for sometimes weeks (or even months). The next photo shows the low voltage capacitors and Now, with the circuit board easily handled, we can proceed to remove any useful parts from it. A soldering iron and solder sucker can be used for this process, but my method of choice is to use a blowtorch with the flame set on low. The flame is angled across the board in such a way as to melt the solder and free the parts, which are then pulled out from the component side. Care needs to be taken not to overheat the parts during this process but with some practice, the method can be perfected. Be careful not to burn yourself, use safety glasses and gloves and always have a bucket of cold water handy in case of burns. siliconchip.com.au January 2014  47 So, what did we end up with from this salvage exercise? As you can see above, we got the following parts: 2 large transistors 1 medium transistor 6 small transistors 2 large dual switching diodes 2 small dual switching diodes 1 600V 6A bridge rectifier 1 3A diode 1 fuse and clips 1 thermistor 2 250V electrolytic capacitors 1 X2 rated mains capacitor 3 medium sized electrolytic capacitors 12 smaller electrolytic capacitors 3 greencaps 8 ceramic capacitors 2 heatsinks 1 120mm 12V fan and grille (Most power supplies are fitted with an 80mm fan) 4 cable ties Several screws of different types Transistor insulators Several chokes & transformers (limited use?) Several lengths of light and medium hookup wire A 230V switch A 230V IEC socket with X2 filter capacitor and choke A couple of computer connectors which we kept with wire 1 sheet of insulating plastic from under the circuit board Overall, a worthwhile exercise that yielded a variety of useful parts for the junk box. Amongst the parts was a splitter cable consisting of a Molex connector to two SATA connectors. This was kept intact as a spare part for possible use in upgrading a computer from an IDE drive to a SATA drive, or adding extra SATA drives where there weren’t enough SATA connectors on the existing power supply. There was also a P4 connector, which the wire was left on. If needed it can be spliced into an older power supply that is lacking a P4 connector. 48  Silicon Chip As well, there were two standard computer cables consisting of two molex connectors and one floppy connector. These were retained as spares, but probably won’t be needed, as floppy drives are rare these days and most drives are now SATA. It could be handy for an antique computer though! The rest of the hookup wire had the connectors cut off and the hookup wire was sorted into colours and sizes for storing for later use as needed. There is a wide variety of computer power supply manufacturers and you may not find all the mentioned components in all power supplies. In fact, some power supplies will have additional components in them, such as voltage regulators and more transistors and different types of heatsinks. Modular types of power supply will also have a variety of different plugs and sockets that may be of use. There will almost always be one or more integrated circuits in every different type of computer power supply, however, there is a fairly high chance of these ICs being faulty if the power supply no longer works and it’s unlikely that a suitable method of testing them will be available. Therefore, I never bother salvaging them, unless I need a particular type and I can remove it from a known working power supply that isn’t worth keeping, for reasons such as the case being very rusty or the power supply being so old that its rating is too low to be of any use for a reasonably modern computer. I usually don’t bother with salvaging small signal diodes, 1A rectifier diodes, small resistors and small capacitors, as these are harder to remove and of limited use with their short leads. However, these components can be salvaged if they are of any use, as otherwise they will be binned with the circuit board. In some cases, it may be worthwhile saving the case, if there is a need for it for a project, as it has an inbuilt fan and vents. However, I don’t usually bother saving the case in most cases, because it is often rusty anyway. I usually just flatten it and toss it in the recycle bin, along with the circuit board. The last point to be considered when salvaging parts, is that it is very important to test all salvaged parts before they are used in a repair or a project. Remember, they might have caused the fault in the first place! If you can’t test a component (because of a lack of test gear suitable for testing that component), then don’t use it! It’s no use putting a potentially faulty component into a piece of equipment or a project, because you could be just introducing a fault. (Ed note:) As far as semiconductors are concerned, an analyser such as the Peak Atlas DCA is definitely worthwhile (available from Altronics – cat Q2100). It will not only tell you what the device is – ie, transistor, SCR, etc, (including its parameters) it will also identify leads – very handy when you don’t know what you’ve pulled out. SC NEXT MONTH: OK, so you’ve got all the bits from the power supply. But what goodies are worth saving on the motherboard? siliconchip.com.au