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A Regulated 12V DC Plugpack By Ross Tester As a hobbyist, the chances are you have collected several plugpack power supplies over the years because, well, they’re too good to throw away, aren’t they? Here’s how to turn a surplus plugpack into a fully regulated supply for next to nothing! First, let’s look at these ubiquitous plugpacks. By definition, these are supplies which are designed to hang off a standard 240VAC power point. They come in a wide range of voltage and current ratings; some are AC output and some are DC. They’re compact, safe and convenient. But they’re not perfect. They have a couple of disadvantages. For a start, most DC plugpacks suffer badly from hum (or should that read the devices to which they connect can suffer badly...). The reason is simple – they usually only have a bare minimum of filter capacitance for their rated load current. So while they may be perfectly adequate for many jobs, if you use them to power a small tape deck, DC player or amplifier, hum on their output will be audible – and annoying! The other major drawback of plugpacks is poor regulation. This means is that there is a wide discrepancy between their open‑circuit (ie, no‑load) voltage and their full rated load voltage. A typical DC plugpack might be rated at 12V and 1A but when you measure its voltage without a load you’re likely to find it is 17V or more. Sure, when it’s supplying the rated current the output might fall to nearly 12V but most electronic devices don’t draw their full rated current all the time. So the supply voltage could be varying all over the shop. Making the situation worse is the input voltage. While the mains voltage is a nominal 240VAC it can vary quite widely. Here at the SILICON CHIP office it seldom falls below 252‑253V. One of our staff members regularly cops 260V (he’s at the top end of a very long power feeder). My house, only a few kilometres away, averages about 243VAC. The problem is that if the mains voltage is high, so will be the output of the plugpack. Combine this with poor regulation and a plugpack rated at 12V DC could easily deliver more than 18V if the mains voltage is high! Aren’t most devices designed to cope with variations in input voltage? for a Couple of Bucks. . . 40  Silicon Chip Here's what we started with: this Nokia plugpack from Oatley Electronics is rated at 13.8V 1A but measured over 17V no-load. Poor regulation is typical of plugpacks, as is an excess of hum in the DC output. Well, yes and no. But feeding more than 17V to a device calling for 12V DC can be risky; you could blow it up. As a matter of fact, if you have a switchable plugpack with outputs of say 6V, 9V and 12V, it is generally better to switch it to 9V when powering something calling for 12V; it is better to be safe than sorry. But wouldn’t it be better still to have a 12V plugpack which delivered a genuine 12V DC all the time, regardless of the load current and input mains voltage? And wouldn’t it be better again if it had very low hum output? You can achieve this fairly easily by putting a regulator circuit inside the plugpack itself. This can be done with the good old garden‑variety 78xx 3‑terminal regulator. There are a couple of wrinkles one has to take into account – for example, the 78xx series of regulators in TO‑220 cases all need an input voltage about 2.5V above their rated output voltage to regulate properly. But under normal circumstances they’re almost indestructible ‑ overloading/overheating and even shorting will simply cause them to shut down. The regulated plugpack You can presently buy a regulated plugpack without too much drama. But they’re not cheap ‑ at least thirty dollars or so and typically about ten dollars dearer than an equivalent unregulated model. But this article is intended for those who have a plugpack or two lying around – possibly once connected to something which has failed but you’ve kept the plugpack. For the cost of a regulator (less than $1.50), a small capacitor (no more than 50c if you have to buy one) and perhaps a LED and resistor (another 50c or less) you can turn that plugpack into a regulated supply in an hour or less. Note that we are not talking about piddly little 200mA or even 500mA plugpacks – there isn’t a great deal of room in them at the best of times. No, this article is aimed at the larger plugpacks, typically rated at 1A or 1.5A. Usually these plugpacks have enough space inside the case and also have benefit of a larger filter capacitor into the bargain. What we are going to do here is show you how to get the plugpack case open without destroying it, fit the extra components required and close it up again. Before we start though, a word on legalities: to be sold in Australia, plugpacks must be type‑approved –that is, they must meet certain standards on safety (mainly insulation) and construction. Opening the case of the supply will almost certainly void that type approval; fitting new components OPENING UP THE WELDED PLASTIC CASE Gently but firmly squeeze the join just nipped in a vice. Tap gently with a ball peen hammer as you tighten the vice. Repeat for the opposite end then the sides. Here you can see the join just opening up under the pressure. Once the join is cracked, a flat blade such as a table knife will help break the weld. Fig. 1: this 'scope screen dramatically illustrates one of the major drawbacks of plug-packs: hum. The top trace shows the plugpack supply output before modification with severe 100Hz hum – 700mV peak-to-peak. This would play havoc with an amplifier. The bottom trace shows the output after the regulator with just 1.5mV pk-pk of hum and noise . The load in both cases was 600mA. Finally, lever the two halves apart out of the vice, again using the knife (or even two knives). December 1998  41 This is what we found when the case was opened: a 6A bridge rectifier and, importantly, a nice, big smoothing capacitor (4700µF). The 3.9kΩ resistor just visible under the bridge is probably there to give some minimum loading but it is redundant after our mods. We've ignored it for the sake of clarity in this article. certainly will, if only because the device hasn’t been tested. However, we are only adding components to the secondary side of the transformer, not the bitey side. In fact, we don’t even touch the transformer ‑ it stays locked into place exactly where the manufacturer put it. Getting started As Mrs Beaton’s cook book states, first catch your hare, or in this case, your plugpack. What you need is one nominally rated a bit over 12V DC (13.8V is common; up to 15V DC is OK) rated at 1A or 1.5A; anything larger and the transformer will proba- The surgery The first step is to open the welded plastic case. These cases are made in two halves, one of which fits into a recess in the other. When assembled, they are welded (very occasionally glued) together. What we have to do is break that weld (or glue) without destroying the case. Fortunately, this is fairly easy to do once you know the trick which is to apply just enough pressure to make the weld crack open. We do this by gently squeezing the joins (and just the joins) in a vice. The photos give an indication of how it is placed. Start with the shorter sides as these are easiest to handle. Place the plugpack in the vice using some jaw Fig. 3: the circuit of the retro-fitted plugpack. Only four extra components are needed and these all mount within the existing plugpack. The most difficult part is getting the case apart! 42  Silicon Chip bly take up too much space. We did mention before that the regulator needs an input 2.5V higher than the rated output yet we’re using a supply rated at 13.8V. Yes, we know that 12 + 2.5 doesn’t equal 13.8 but we are relying on the poor regulation of the plugpack. The output voltage will normally be somewhat higher than 14.5V (in fact, ours measured 17.3V with no load and a 240VAC input). protectors (to prevent damage to the case surface) and tighten the handle up to a firm but not tight grip. Each time you slightly tighten the vice grip (no more than about a tenth of a turn at a time) gently tap the join with, say, a ball peen hammer. Before too long, you should hear a reassuring “crack” as the weld gives way. Repeat this for the opposite end, then for the two longer sides. What happens next depends on how lucky you have been. Sometimes you’ll find the two halves of the case can be pulled apart at this stage but more likely than not you’ll need to gently prise apart the two halves. A wide, flat bladed knife such as a kitchen or table knife is best. Anything smaller, such as a screwdriver, is likely to mar or even tear the case and you don’t want that. Once you’ve been able to get one or two knives between the case halves you should be able to gradually work around the case, prising it apart as you go. If all the welds have been cracked, it normally doesn’t take too much effort to separate the halves. Sometimes some of the plastic in the join breaks instead of the weld. If not too much, this is not too serious because you will be gluing it all back together anyway. The transplant Once apart, you can see what work you have to do to include the new components. Again if luck is with you, you will find a PC board which can be slightly modified. However, Fig. 4: we were able to use the existing PC board to mount some of our components. If you think soldering components in mid-air is not ideal, you're right – but some plugpacks are made entirely this way! we have seen some plugpacks where the components are simply soldered to each other. The modifications are then not quite as simple but possible nonetheless. Inside most plugpacks, all you will find is a rectifier and a filter capacitor. The rectifier could be either a four‑terminal bridge or it could be four individual diodes forming a bridge. Very occasionally, you’ll find a centre‑tapped transformer has been used with two diodes in full‑wave centre tapped configuration. Regardless of the type, we don’t have to modify the rectifier in any way. Following the rectifier will normally be an electrolytic filter capacitor. In our plugpack there was also a 3.9kΩ resistor but this can be ignored because its effect is minimal. In the case we pulled apart for this article, we were delighted to find a 4700µF 25V type which provides a good level of filtering. In some plugpacks, though, we’ve found capacitors as small as 470µF – barely adequate and voltage ratings down to 16V – certainly inadequate. 16V is sailing very close to the wind, with the capacitor operating right on (or more likely slightly over) its limits. It has no margin for safety – for example, to handle any voltage spikes. If you find a low value, low voltage capacitor it is be a good idea to replace it (if possible) with a more suitable type. At a minimum, we would suggest 2200µF 25VW; anything larger is a bonus if it will fit. (There’s no point in fitting one with a high voltage rating; Parts List 1 Plugpack power supply rated approx 13.8-15VDC <at> 1A 1 7812 positive voltage regulator 1 5mm LED, any type 1 10µF 16VW electroyltic capacitor 1 2.2kΩ 1/4W resistor all else being equal, go for increased capacitance). Speaking of space, some of that is going to be needed for the regulator and one or two other components we haven’t mentioned yet. First of all, we need to put a small electrolytic capacitor across the output of the regulator to make sure that it does not oscillate supersonically. Secondly, the regulator doesn’t like being left unloaded – it needs a small output current at all times. One way to do this is simply place a resistor across the output to draw a few milliamps at all times. 2.2kΩ will give us about 5mA. But if we’re going to throw away a few milliamps, why not feed it through a LED which will also give us a power on indicator. Gilding the lily? Perhaps – but there was a convenient hole in the case for the LED and LEDs cost only 30 cents, so why not! Fitting it all in Your next step, as was ours, is to decide how to mount the regulator, capacitor and resistor for the LED (the LED itself was on the case top, connected by two strands of rainbow cable). The 3‑terminal regulator is mounted effectively in series with the output of the plugpack. We already had two holes in the PC board for the output leads – plus and minus. Removing the output leads gave us two of the three mounting points we needed for the regulator – input and ground. It was a simple matter to drill a new hole on the negative supply track of the PC board for the negative output lead. The output terminal of the regulator and with it the connection of the positive output lead, the positive side of the extra capacitor and connection for the LED proved to be not quite so simple. So we cheated a bit. Instead of trying to mount all of the above on the PC board, we bent the output lead of the regulator back up through 180 degrees and used this as a terminal point. The extra 10µF capacitor across the output was mounted with its negative lead going through a hole drilled into a suitably close point on the negative track and the positive lead was bent back up the capacitor body and soldered to the regulator output lead. These two leads were rigid so they stayed in position to solder. The other connections, the output positive lead and the positive going to the LED were first twisted together and soldered to make them easier to solder to the regulator output. We’ve already mentioned that we drilled two new holes in the negative track for the negative output lead connection and These photos show the front and back of the PC board after the new components were added. Exact placement isn't too important – as long as everything fits and the assembled board fits back in the case. We were lucky – there was just enough room between the bridge rectifier (black component on board edge) and the main filter electrolytic capacitor. December 1998  43 negative end of the 10µF capacitor. We also drilled a new hole in the same track for the 2.2kΩ resistor to stand end‑on, with the other pair of the rainbow cable leads to the LED soldered to the top of this resistor. There probably won’t be a hole in your case for the LED – this will have to be drilled. If you’re careful with the size you can make the LED a tight fit in the hole. A tiny drop of super glue will then hold the LED in place. Where there was any danger of flying leads coming off ‑ eg, on the LED, and the top of the resistor – we covered them with short lengths of heatshrink sleeving. Last of all, we fitted a small U‑shaped heatsink to the regulator, using a small amount of heatsink compound to improve thermal conductivity. There is no need to use insulating washers or bushes unless there is any danger of the heatsink contacting anything else. That brings us to the final check – making sure that nothing is touching anything that it shouldn’t be and that nothing will be pushed out of position when the two halves of the case are recombined. If there is any danger of this happening, fit insulation between the offending components. back into its appropriate slot. Push the two halves of the case together just to make sure it all goes back together and then pull them apart slightly, ready for gluing. Which glue? It doesn’t really matter as long as the glue is made to adhere to plastics. We’ve found a few drops of super glue placed judiciously around the seam work very well and it has  the advantage of drying very quickly. And that’s just about all there is to it. All up, it should only take an hour or so from beginning to end. Plug polarity This front panel artwork fits the case recess on the Oatley (Nokia) plugpack and may be adaptable to other models. Putting it back together If everything checks out OK, it’s time to put the case back together. First, make sure that none of the cables protrude from the case and any captive cord anchor on the output cable fits Testing it It is quite safe to plug in the supply without assembling the case because the transformer completely shields the 240VAC connections. Nevertheless, turn the power point off before plugging it in. The first check is to make sure that the LED lights. If it does, measure the output voltage – it should be very close to 12V. Due to manufacturing tolerances, the actual output voltage could be anywhere from 11.5 to 12.5 but in practice, we’ve found the regulators to be much more accurate than that. In our case, actual output voltage was 11.97V. Leave the supply on for, say, half an hour or so and confirm that neither the transformer nor the regulator get hot. With no load they should stay fairly cool. If you run the supply at its full rated load, though, it’s a different story. The transformer will probably become quite hot to touch and the regulator/ heatsink may well be bordering on the too‑hot‑to‑be‑held. 44  Silicon Chip Unless you fluked a plugpack with a plug already fitted you will need to solder a DC power plug onto the end of the lead. Naturally, you should use the plug which suits the equipment you’re going to power. There is a wide variety of plugs used but if you have the choice you should aim for one of the larger variety  – after all, the supply can pass 1A or more on peaks. As far as polarity is concerned, there is a standard: whatever the manufacturer decided on that particular day. Originally it was intended that the centre of the DC plug be the positive and the outside the negative (just the same as the tip on a 3.5mm or 6.5mm jack plug is positive, body negative). Unfortunately, this standard has gone out the window so now you have to fit the plug to suit. Some plugpacks have a polarity reversing plug and socket moulded into the cable – if this is the case make the centre positive when the symbol on the reversing plug (+, ‑ or o are often used) is lined up with same symbol on the socket. Other voltages And here's how it all did fit back into the plug-pack case – actually there's a fair bit of room to spare. The LED leads are insulated with heatshrink to make sure they don't short to each other or to anything else when the case is assembled. The technique described here can be used to turn virtually any plugpack into a regulated type – just as long as there is enough room inside the case to fit the extra components. For example, a 9V plugpack makes an ideal candidate to make a 6V or 5V regulated supply (naturally, you’d use a 7806 6V or 7805 5V regulator). An 18V version can make a 15V regulated supply with a 7815. If you want to get really tricky (and if there’s enough space), you could even use an adjustable regulator such as a the LM317 to make an adjustable, regulated supply. You’ll find the circuit on the SILICON CHIP web site SC – www.siliconchip.com.au