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By LEO SIMPSON Got an old PC power supply gathering dust? Want to use it to power your projects? We tell you what to do and how to do it. Use your old PC power supply for high current outputs A S TIME GOES ON, more and more old computers are quiet  ly gathering dust or worse, being thrown on to the tip. Often these are perfectly good machines which still function as well as the day they were purchased. But if you don’t want to use them as computers you can still use their power supplies. Computers have big power supplies in a small box. A typical older 74  Silicon Chip machine will have a 200W power supply capable of delivering +5V at 20A, +12V at 8A, -5V at 0.5A and -12V at 0.5A. You can use this power supply for all sorts of applications pretty well as it is, with no modifications required. And if you want, you can crank up the +12V output to get around +13V which is more appro­priate if you want to power CB or amateur band equipment, audio equipment or bench test car projects. We’ll talk more about this aspect later. First, let’s talk about the PC supply as it stands. Typi­cally it is contained in a small folded metal box with an inbuilt 12V fan and two IEC power sockets, one male and one female. The male socket is for the mains input while the female socket is for the switched output to the video monitor. This is a switched mode supply DANGER: HIGH VOLTAGE Fig.1: the general circuit arrange­ment inside most computer power supplies. The TL494 gives precise regulation of the main +5V rail and the other rails are unregu­lated. Note that all the circuitry on the primary side of the inverter transformer runs at around +340V and is also floating at around half the 240VAC. It is lethal if touched. and typically uses a TL494 switch­ mode controller IC and a couple of transistors driving a transformer at around 40kHz or more to provide the four separate supply rails. Fig.1 shows the general arrangement. We must stress here that Fig.1 shows only the broad outline of the circuit and every one of these power supplies shows great differences in the detail of their circuits. By the way, if you do manage to obtain the circuit of a computer power supply, you have rare treasure indeed. We have yet to see a full circuit and we understand that most serviceman do not have the benefit of circuits either. Back to Fig.1: the mains supply comes in via a filter net­work and is fed to a bridge rectifier to produce around 340V DC. Interestingly, the filter capacitance is usually made up of two 200V capacitors connected in series across the 340V and they typically have a capacitance of around 220-330µF. Each of these capacitors generally has a diode and resistor across it to ensure that they share the total voltage of 340V equally. The 340V DC is then fed to a switchmode circuit involving two transistors (or Mosfets) in a push-pull inverter transformer. By the way, considering the amount of power involved, the transformer is ridiculously small. It looks fairly conventional in construction but instead of using steel laminations it has a ferrite core which enables it to run with switching speeds of 40kHz or more. This enables a very small transformer instead of the very bulky and heavy unit which would be required if the transformer was running at 50Hz. The transformer provides the full isolation between the 340V DC supply on the primary side and the low volt- WARNING! The internal wiring of switch­ mode computer power supplies is dangerous when powered up. Not only do you have bare 240VAC wiring to the IEC sockets but a good portion of the circuitry on the PC board is +340V DC floating at half the mains voltage. IT IS THEREFORE POTENTIALLY LETHAL! Use extreme care if you do decide to make measurements on the supply when the case is open and DO NOT TOUCH ANY PART OF THE CIRCUIT when it is operating. Make sure that it is disconnected from the mains when you are making any modifications to the internal wiring. age supplies on the secondary side. Note that all the circuitry on the primary side of the transformer is at mains potential and must be regarded as lethal. On the output side, the transformer has at least four sec­ondary windings, each centre-tapped. Each secondary feeds two high speed fast recovery diodes in a full-wave rectifier followed by a toroidal inductor and another filter capacitor. The diodes for the +5V and +12V rails are usually clamped to a finned heat­sink. By the way, each pair of diodes are in a three-lead package which usually looks like a plastic power transistor. Block diagram Fig.2 is the block diagram of the TL494 switchmode con­troller used in most of these supplies. If yours does not use a TL494 you will probably find it has a Samsung KA7500B and guess what? It’s identical in function and pin connections to the TL494. This chip provides precise voltage regulation for the +5V rail only and the other supply rails depend on the basic regula­ tion of the transformer for their performance. Typically, if you connect a 5A load across the +5V rail it will drop by only a few millivolts whereas if you connect a 5A load across the +12V rail it will drop by 0.5V or more. Even if you December 1998  75 Fig.2: this is the schematic of the Texas Instruments TL494 and Samsung KA7500B switchmode controllers, used in the big majority of PC computer power supplies. don’t measure the +12V rail when you load it up, you will still know that the voltage has dropped a bit because the fan will sound a little slower. By the way, when installed in a typical computer, the fan does double duty. Not only does it cool the switch­ mode power supply components, it also cools the componentry inside the case of the computer. But its most important job is to cool the power supply itself and so it should not be disconnected, even if your proposed application means that the supply will be lightly loaded most of the time. Minimum load While the TL494 provides very good regulation for the 5V rail, the supply needs a minimum load. If you disconnect all the supply leads from inside your computer and then turn it on, you will probably find that the power supply will not work at all and this is because it does not have a minimum load. How much load is required? Difficult to say really, but we have found that you typically need at least 100mA. That means you need a minimum load consisting of a 47Ω 1W resistor connected across the +5V output. (Having said that, as part of the preparation for this article, we purchased a brand new computer supply and found that it did not need any minimum loading to make it work.) 76  Silicon Chip The other supply rails do not need any loading to make them work but you will generally improve their regulation if you do connect some minimum load across them. The +12V rail already has a load because of the 12V fan but you can improve the regulation by pulling another 100mA or so; use a 100Ω 5W resistor. For the -12V and -5V rails, try a minimum load of around 10mA; use a 1kΩ 0.25W resistor across the -12V and a 470Ω 0.25W resistor across the -5V rail. Regulation explained How does a minimum load make the supply regulation better? To explain that we should first discuss what we mean by the term regulation. There are two types of regulation: load and line. Load regulation refers to how the output voltage varies between the “noload” condition and “full load” and is usually referred to as a percentage. For example, in a typical computer power supply in which the +5V rail can supply up to 20A, the no-load voltage would typically be very close to +5V (eg, 5.02V) and might drop to 4.88V at 20A. The difference in the two voltages is 140mV (5.02 - 4.88 = 140mV) and when divided by the noload voltage of 5.02V, the percentage becomes 2.8% which is pretty good. Since the other supply rails are not regulated (ie, direct­ly controlled by the TL494), their regulation is not as good and will typically be around 6-10% at full load. Line regulation refers to the change in output voltage as the input voltage (ie, the mains supply) is varied. Computer switchmode supplies are really excellent in this respect since the nominal input supply range is typically 115V to 230V. In practice, the mains voltage can be varied from less than 110V to more than 250VAC while the +5V rail stays rock steady. In this respect the switchmode power supplies in computers are vastly superior to any conventional linear regulated supply and they are a great deal more efficient, as well. To answer the question as to how a low level of loading can improve regulation, the main factor is the voltage drop across the diodes. When the supply loading is zero or minimal, the voltage drop across the rectifier diodes becomes quite low, possibly less than 0.5V. But when the supply is loaded up, the voltage drop across the diodes increases to as much as 1V and then stays more or less constant, regardless of increasing current. This minimises the diode voltage drop as a factor in the output regu­ lation and the result is improved performance. Mind you, there is a trade-off and any increase in load leads to an in- crease in power supply ripple and hash. Lead colour coding This talk of regulation and minimum loading is all very well but how do you identify which output wire is which and how do you make the connections? When you look at one of these sup­plies you will find that there is a veritable festoon of wires coming out of it, all terminating in multiple four and six-way plugs of various sizes and configuration. So you don’t just have one really heavy gauge wire coming out for the +5V output; there are multiple 5V wires. Happily there is a consensus on the colour coding and it is generally as follows: +5V red; +12V yellow; -12V blue; -5V white and common (0V) black. There may also be an orange lead which is the Power Good (PG) signal wire. Now if you want to use the supply to provide +12V at 8A, for example, you really need to connect all the yellow wires in parallel to your output. If you try to pull 8A from just one of the yellow wires, you will find that the output regulation is not as good as it could be and in an extreme case, you could end up melting the wire insulation; so connect ‘em up in parallel and the same comment goes for the black (0V) wires. Powering op amps Now while the thrust of this article has been about using the +12V rail to power equipment in a variety of situations, these computer power supplies can also be used to power audio equipment which requires balanced ±15V supplies, most of which employs op amps which are not critical as far as regulation is concerned. Therefore you can use the +12V and -12V rails to power equipment where ±15V is normally required. There is a proviso here and that is that the -12V rail usually can only supply up to 0.5A. Another factor which must be considered in all of this is that computer supplies have switch­ing hash superimposed on their outputs and this could be a prob­lem if you are using it to power sensitive audio equipment. On the other hand, if the equipment has onboard regulators, the problem is solved. Remember that sound cards in computers do have sensitive low level analog circuitry There’s lots of lethal wiring inside every computer switchmode power supply. In particular note the bare wires to the IEC power sockets (240VAC) and all the circuitry above the transformer in this picture which sits at around 340V DC and at half mains potential. Do not touch any part of the circuit while it is operating. and they cope with the hash situation pretty well. And what about the main +5V rail? What can you use that for? This question has us stumped. Perhaps some of our readers can make a few suggestions. Keep them clean please. Boosting the output So far we have discussed using a computer power supply just as it comes but a lot of 12V equipment for use in cars, particu­larly CB radios, amateur transceivers and audio equipment, per­ forms considerably better if the supply is increased to around +13.6V DC. In fact, a lot of nominal 12V equipment is perfor­mance-rated at 13.6V or 13.8V. Can the computer supply be tweaked to deliver this? The answer is maybe. Some supplies can be made to go that high and others wimp out before they get there. To make the supply deliver more than 12V you need to open up the case and here we must stress that this is dangerous terri­tory indeed. Not only do you have bare 240VAC wiring to the IEC sockets but a good portion of the circuitry on the PC board is +340V DC floating at half the mains voltage. You have been warned. Once you open the case, you have a lethal supply. With the warning out of the way, how do you go about making the supply deliver more than 12V? The answer is to tweak the feedback circuit to the TL494 which monitors the +5V rail. First, you must identify the feedback resistor which connects to pin 1 of the TL494 as this is almost always the op amp input used for this purpose. To make the identification, make sure that the power supply is disconnected from the 240VAC mains. Then switch your multimeter to its lowest “ohms” range or the audible continuity test and find out which resistor adjacent to pin 1 is actually connected to pin 1. You must find the resistor pigtail which is a short circuit to pin 1. Now before you go any further, you might have struck it lucky and you may find that also close to pin 1 of the TL494 is a small trimpot. Bingo! You can tweak that to increase the +12V supply. Remember here that you will actually be increasing the +5V rail and all the other DC rails will increase in the same proportion. If you want to December 1998  77 If you are going to boost the output of the +12V rail, you need to identify the feedback resistor connected to pin 1 of the TL494 switchmode controller. It is shown arrowed here but you have to go through the exercise with your supply. get to +13.8V you will need to in­crease the 5V rail by 15% or to +5.75V. That is quite a big increase and in practice, some supplies cannot be pushed that far; they will get to around +5.5V and then audibly “squeal”, possibly because of an overvoltage protection circuit. If that happens, back off on the adjustment until it settles down. Even so, you should be able to get more than 13V on the +12V output. Making the adjustment Adjusting the trimpot while the supply is powered and with the case open is a dangerous procedure because you will almost certainly find that the trimpot is right underneath the 240VAC wires to the IEC sockets. You can do the adjustment but you will need an electrician’s Phillips head screwdriver with a completely insulated shaft. We used an electrician’s screwdriver with a label rating of 1000V. Don’t even think of using an ordinary screwdriver – we don’t want to lose any readers! To make the adjustment, connect your minimum load to the +5V rail. We used a 12V 50W halogen lamp as it could easily be plugged into one of the output sockets. Connect your multimeter to measure the +5V rail or the 12V rail. Make sure you have someone with you when you do the adjust­ment. If the worst comes to the worst, and you get an electric shock, you want 78  Silicon Chip someone next to you to kill the power immediate­ly. Position your electrician’s screwdriver in the trimpot and have your companion switch on the power. Rotate the trimpot in the direction to increase the supply as desired. Note that the fan will become louder as the +12V rail increases. When satisfied that the adjustment is what you want, have your companion switch off the supply, unplug it from the mains and then replace the lid. Finding the feedback resistor Back to the continuity testing: if your power supply does not have a trimpot you still have to find the 5V feedback resis­tor. Having found the end that connects to pin 1 of the TL494, now check whether the other end connects to the +5V output. You can do this by connecting one of your meter prods to a red wire in one of the multi-way output plugs. Again, you should have a short circuit between the red +5V wire at one end and the +5V end of the feedback resistor. Once you have clearly identified the resistor in question, you can measure its value. More often than not you will find that it is labelled 4.7kΩ but will measure half that value. That means that another resistor is shunting it somewhere in the circuit. Now it is unlikely that you will want to trace the circuit out but you don’t have to do it anyway. All you have to do is to increase the resistance of the identi- fied feedback resistor. Unfortunately, to do this, you have to gain access to the underside of the PC board. Remove the four screws securing the board inside the case and then you can manoeuvre it to access the underside. Unsolder one end of the resistor and then solder a 560Ω resistor in series with it. That done, replace the PC board and the four screws. Be warned: don’t take a shortcut and just sit the board back where it was without fitting the screws. If you do that, there is the danger of a short circuit underneath when you turn it back on and the whole power supply could be destroyed. When you turn the supply on, measure the +5V and +12V rails and note the increase. If the +12V rail is now over +13V, you probably have gone far enough. If not, note the increase in voltage and then calculate the required additional series feedback resistor to get the increase you want. Current rating There are a couple of other points we need to make concern­ ing this boosting of the +12V rail. While you can increase the voltage, you cannot increase the overall power output. Any in­crease in voltage must result in a proportional decrease in current. So if your supply is rated +12V at 8A and you increase it to +13.6V, the overall current will be reduced to around 7A. Remember also that none of the DC outputs has any short circuit protection so if you overload the supply, you are liable to damage it. For that reason you cannot use the supply for battery charging unless you put in a suitable current limiting resistor. On/off switch If you want to remove the power supply from the computer case, you will no doubt want to change the on/off switch which is normally on the computer’s front panel. Very old computers had their power switch on the back of the supply. An easy way of installing a power switch would be to remove the IEC female socket and install a large illuminated rocker switch instead. Most of these switches snap into a standard cutout and a little work with a file or a chassis nibbler will do the trick. However, make sure that there are no metal particles floating around inside the case when you have finished. SC