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What do you do when you have stuff left over from another project? You think of uses for it, of course! Here we make some surplus halogen down-light transformers the heart of a simple car battery charger. Rugged Battery Charger from es Bi ts’n’Piec R eaders will recall the feature in February this year where we replaced some power-hungry 12V halogen down-lights in our office with much more efficient and brighter LED fittings from Tenrod. We’re absolutely delighted with the result (you will be too if you follow our lead). But then we started thinking what we could do with the now-surplus 12VAC transformers and sundry light fittings/globes. The light fittings and holders were consigned to the “round file” – they were discoloured with age, the wiring was brittle and we certainly didn’t want to put in any more halogen down-lights (that was the point of the exercise, after all). But at least the transformers were functional and it seemed such a 74  Silicon Chip shame to bin them. What could we do with them? We quickly came up with a number of ideas and this is the first: a basic car battery charger that can put out a good 10A or so, with three of these trannys in parallel. Commercial chargers with this rating are expensive so if we could cobble up a cheap equivalent, so much the better. We’re assuming that the transformers you remove are iron-cored and not the so-called electronic type. “Electronic” transformers cannot be used for our purpose. Typically the iron-cored transformers are labelled 4A (or close to it) and By Ross Tester 11.4-11.6VAC. That means they’re intended for 12V 50W halogen downlights. If you’re removing them all from one area, the chances are they will be identical. This is quite important as far as this project is concerned – you should not mix’n’match brands otherwise one of them may tend to take the lion’s share of the load. If your transformers are identical in brand and style, the chances are also good that they were installed at the same time and are all part of the same batch, wound on the same machine, so the output voltages should be the same. You can check this out before use with a DMM if you wish – ours were within a couple of millivolts of each other. siliconchip.com.au We’re using three transformers in parallel which gives us a nominal output of just on 12A (ie, 3 x 3.95A). You won’t quite get that much (we’ll explain why shortly) but as we mentioned earlier, it should be good for 10A or so. If you only need (say) 6A or so, or if you only have two identical transformers, go right ahead. Using identical transformers in parallel is not too dissimilar to paralleling windings on the one transformer to give higher current. For example, you might have a transformer which has two separate secondary windings rated at 6V, 1A – you can connect these (in the right phase) in series to give 12V <at> 1A or in parallel to give 6V <at> 2A. That is effectively what we are doing here. It’s not quite according to Hoyle but One of the three identical halogen downlight transformers we removed. They are each rated at 11.4V, 3.95A. siliconchip.com.au we’ve done it before and it works. Again, though, we must emphasise that they must be identical transformers – and definitely not electronic versions! What else do you need? Basically, all you need is a hefty bridge rectifier to convert the AC output of the transformers to unsmoothed DC, to charge a battery. Naturally, you’re also going to need hardware to safely connect the transformer primaries together (and thence to the mains) plus connections from the secondaries to the bridge rectifier and thence output cables for connection to the battery to be charged. Add a case to put it all in and Bob’s your uncle. Well, nearly so. It will also need a mains cable, mains switch and fuse. We elected to use an IEC mains socket with integrated fuseholder – saves having a mains lead dangling out the back to get damaged and we also have plenty of IEC mains cords left over from other devices – you probably have a few as well. You can get an IEC mains (male) socket with both integrated fuseholder AND mains switch but we didn’t want the mains switch on the back of the case, so elected to use a separate switch up front. And because it’s now getting rather difficult to buy a round-hole-mounting mains switch with an integrated neon indicator, we added a separate neon bezel. The case This presented something of a prob- The Supercheap Auto Storage Box. Remove the seven plastic trays and presto! A case complete with handle! April 2013  75 POWER S1 T1 12V 230V BR1 F1 5A CON1 INTEGRATED IEC MAINS CONNECTOR AND FUSE HOLDER ~ 35A/400V T2 A E – ~ T3 N 230V SC BITS’N’PIECES BATTERY CHARGER lem. We wanted a metal case, preferably steel, to house the charger but once again, suitable cases are starting to become as rare as the proverbial. And those that are available are worth a fortune – definitely not what we wanted for a “surplus parts” project. So instead of a purpose-made case, we purchased a steel storage box from Supercheap Auto for less than $20. It’s N E + THERMAL SWITCH NC – 90o OUTPUT TO BATTERY UNDER CHARGE 12V T1-T3: 230V – 12V AC HALOGEN LIGHT TRANSFORMERS 2013 CON2 + 12V 230V  NEON BEZEL – Fig.1: the circuit diagram of our battery charger shows that it is a conventional full-wave rectified supply. What is not conventional is driving the bridge rectifier with three transformers in a parallel. It’s not strictly-speaking according to the rule book – but we’ve proved that it works! called an “SCA Multistorage Case, 7 Compartment”. It’s more than strong enough, about the right size and it has a couple of nice features such as a carry handle and provision for semi-permanent locking. If you’re not in a hurry, Supercheap Auto regularly have “20% off everything sales” so it could be yours for even less. But if you happen to have a suitable case on hand, so much the better. Output leads We could have made up a set of charger leads from heavy-duty cable and large alligator clips but “why reinvent wheels?”. Cheap jumper leads already have the heavy-duty cable and large alligator clips – all we had to do was remove the clips from one end. We’ve seen these before in bargain A A HEATSHRINK SLEEVING OVER MAINS CONNECTIONS T1 ELEPHANTIDE (OR BLANK PCB) INSULATION SHEET CASE EARTH CONNECTION ALUMINIUM HEATSINK SHEET T2 POWER S1 NEON BEZEL BRIDGE RECT – + HEATSHRINK SLEEVING OVER SWITCH CONNECTIONS 76  Silicon Chip T3 THERMAL SWITCH TO BATTERY siliconchip.com.au stores for less than $10 but of course, when we went to get them there weren’t any. Supercheap Auto had some but they weren’t super cheap. However, we managed to get a set from Repco for less than $20. You may even have a surplus set of leads that you can sacrifice for this project – they don’t have to be superhigh current leads. If you have to buy some, get the cheapest you can find. Normally, we’d never recommend these – as jumper leads they make great shoelaces but for our purpose, they’re more than adequate. By the way, most jumper leads have rather extravagant claims for current rating – like 400A and so on. But if you look at the leads closely, you’ll see that they are probably about 80% insulation and 20% (or less) wire. Given the fact that they are intended for 12V (or perhaps 24V) usage, we wonder why they need insulation rated at, what, kilovolts and grossly exaggerated wire “capacity”? Hmm! ZR-1324 <at> $4.95). We don’t need either the 35A or 400V ratings but they give a nice margin for safety. Following this is a normally closed (NC) 90° thermal cutout (Jaycar ST3825 <at> $5.75) to protect against shorting the output leads. At the same time, we also grabbed a strip of 12-way ultra-large terminal strip (HM-3198 <at> $2.95) and a couple of metres of 25A hookup wire (WH3080/3082 <at> $2.20/m). Finally, we wanted some large output terminals and Jaycar had a polarised heavy-duty pair (PT-0457 <at> $6.95). We could have saved this cost by bringing the charger leads out through a gland but again, we didn’t want to have leads permanently hanging from the case. Apart from nuts and bolts to mount everything (see parts list), rubber feet, some heatshrink tubing and scraps of thick aluminium (to act as a heatsink) and PCB material (for an insulator), that was it. What else did we use? OK, so if you have to buy everything (except the transformers!) it all adds up to $60-ish but we couldn’t find a The main item is the rectifier – we used a metal 35A/400V bridge (Jaycar siliconchip.com.au The cost charger of this power for under $100. If you have a lot of what’s needed in your junk box – and many hobbyists will – the cost will obviously come down. How it works See the circuit of Fig.1. This one is definitely not rocket science! It’s a typical full-wave rectified supply producing pulsating DC at the output. What’s not typical is that we’ve used three transformers, all wired in parallel so all contribute their share of the nominal 12VAC <at> 12A output to the bridge rectifier. (We mentioned earlier that the transformers are labelled 11.4V but this would be at the full 3.95A output. Unloaded or not fully loaded, the voltage is at least 12V, perhaps a bit more). Once rectified, the pulsating DC voltage will be 12 x 1.4142 or 17V, less the voltage drop across the two diodes in the bridge rectifier conducting at the time (2 x 0.6 or 1.2V) = 15.8V. This is the peak voltage. Because it is unsmoothed (ie, pulsating) DC, the voltage you read with your multimeter will be less than this, actually peak x 0.707. So the output should measure around 15.8 x 0.707 or April 2013  77 This set of four scope wave-forms demonstrates the operation of this car battery charger. The yellow trace shows the unsmoothed DC output of the battery charger with no battery connected but with a load of 1kΩ (to give a clean waveform). The green trace shows the output of the battery charger when connected to a battery which is being charged at about 3A. The humps in the green waveform occur each time the battery gets a pulse of current (ie, 100 times a second or 100Hz). The flat portions of the green trace represent the battery voltage at times between each current pulse while the pink trace (partly obscured by the green trace) represents the battery voltage when charger is turned off. Naturally the average voltage when it is being charged will be slightly higher than when the charger is turned off. Hence the green trace is slightly above the pink trace. The peaks of the yellow trace are slightly above the peaks of the green trace (battery voltage under charge). This is to be expected because the battery places a considerable load on the charger output. 11.17V. But aren’t we trying to charge a 12V battery? How can we do this if the output voltage is less than the nominal battery voltage? The reason is that current flows into the battery whenever the peak voltage exceeds the battery’s nominal voltage. Remember a moment ago we said that the peak voltage was about 15.8V?. So when the charger voltage rises above 12V (or whatever the battery voltage is at the time) current will flow into the battery, charging it. And this happens 100 times every The twin output terminals (binding posts) we used – these have large holes which easily fit the jumper lead cables. Some binding posts can be a real pain to connect to! 78  Silicon Chip The blue trace shows the amplitude of the 100Hz current pulses being fed the battery. It represents the voltage across a 0.1Ω resistor in series with negative lead from the battery charger and has a peak-to-peak voltage of 958mV (across 0.1Ω). This means that the current pulses are peaking at 9.58A; much higher than might be thought with an average current of about 3A. Note that the maximum current delivered by the charger will depend on both the mains voltage at your location and the state of the battery being charged. second as the pulsating DC voltage starts at zero, rises up to 15.8V then falls to zero again. See the scope grab above. How much current? We mentioned earlier that you wouldn’t expect to get the full 12A from three 4A transformers. There are We chose an IEC socket with integral fuseholder (at the bottom) – it means the fuse is before the mains switch but this isn’t a great problem. The latches on this case have a screw hole right through them which means you can semi-permanently lock the case. (See screw & nut at bottom of latch). That’s important if there are young hands around . . . siliconchip.com.au losses in the system – for example, the voltage losses in the rectifier and also due to the resistance of the wiring and leads. But we’d be surprised if you didn’t get at least 10A peak into a “flat” battery as ours did. This reduces, of course, as the battery charges. The one big disadvantage of a simple battery charger like this is that it will continue to try to “charge” the battery, even though the battery is nominally “charged”. So beware of this – if the battery fluid starts to bubble (gas), turn off the charger and disconnect it (not the other way around – that bubbling is hydrogen gas and you really don’t want to have any sparks around that!). Construction When you open the SCA case, you’ll find there are seven plastic compartments inside. You don’t need them for this project (in fact, they won’t fit!) but they make dandy little parts holders for your workbench! Layout within the case is not critical but the main thing to remember is that this is a mains device – care must be taken with the mains wiring and the output wiring must be kept completely separated from the mains, with no possibility of connection should a wire work its way loose. One advantage of the transformers we used is that they have nice, big holes for cable connection – even the 25A auto cable fits easily. We marked all hole positions before drilling any. That way you can easily move something if necessary! Start by placing the transformers in the case. If you’re using three, as we did, it makes sense to locate one right in the middle (ie, on the centreline) and the others lined up, about 10mm in from the edge of the case. When you’re happy with their positions, mark their screw hole positions with a fine felt pen. The two lengths of terminal strip (one 3-way, one 2-way) also sit on the centreline. The 3-way length, the one that connects mains power, has two screws holding it in while the 2-way obviously can have only one screw. At the “mains” end, you’ll need to mark a hole position for the earth screw. We positioned the mains switch and neon on the end of the case, equal distance from top and bottom. The bezel (7mm hole) is 25mm in from siliconchip.com.au Parts list – Rugged Battery Charger 3 (or 1 or 2 – see text) 230V to “12V” <at> ~4A downlight transformers, same brand & type (not electronic type) 1 suitable steel or aluminium case, approx. 330 x 225 x 68mm (eg, “SCA” brand multistorage 7-compartment carry case from Supercheap Auto, $19.95) 1 IEC male chassis connector with integral fuse holder and 5A fuse 1 SPST mains switch 1 Neon bezel (230V) 1 BR354 (or similar) 35A/400V bridge rectifier 1 90°C thermal switch, normally closed 2 large red & black terminals (binding posts) 1 12-way large terminal block (eg Jaycar HM3198) 1 earth lug crimp terminal 1m 25A Auto cable – red and black 1m twin-core mains cable 1 set economy jumper leads heatshrink to cover mains socket and switch, all exposed terminals 5 M3 x 10mm screws with nuts & washers 3 M3 x 20mm screws with nuts & washers 8 M4 x 10mm screws with nuts & washers 1 M3 x 30mm screw with nut & washer 4 rubber feet, self-adhesive 1 aluminium offcut for heatsink, roughly 100 x 60mm 1 blank PCB or plastic offcut for mains terminal block insulator, roughly 50 x 50mm Small cable ties the front and the mains switch (12mm hole) another 20mm further in. The only other hole in this end of the case is the cutout on the rear for the fused IEC socket. Mark its position and cutout carefully – there’s not a great deal of “meat” on the edges of the socket. The cutout can be made by either drilling a series of small holes and finishing off with a file, or using a nibbler. Note that there are two chamfered corners on the bottom of the cutout. At the opposite (output) end in the bottom of the case there are holes required for the 2-way terminal strip mentioned above and the bridge rectifier and thermal cutout. We mounted the two latter components on a small piece of thick aluminium to act as a heatsink, with screws going through both the case and heatsink. We worked out the positions of both components on the heatsink then used that as a template to drill the holes through the case. The pair of output binding posts needs careful drilling to ensure it fits and sits correctly – it has two 10mm holes 19mm apart. Again, this socket was mounted at the midline of the side of the case, the first hole 25mm from the front edge and the second (obviously!) 19mm further in. We used M4 screws for the transformers, earthing point and bridge rectifier; M3 for the rest. You will need to remove paint around the earthing point so that the screw is guaranteed to contact bare metal. This screw needs to have, from the case up, a star washer, nut, crimped earth wire lug, shakeproof washer and finally another nut to ensure the earth wire is held securely in place. Because there are screwheads emerging from the bottom of the case, it makes sense to place some rubber feet on the underside – because the chances are someone will “rest” it on a car bonnet. Self-adhesive feet are easiest – you don’t need to drill any holes. Connecting it up Once all the holes are drilled/cut, it’s quite a simple matter to connect it all together using our photos and diagrams as a guide. Ideally, we would have used spade (quick-connect) connectors to attach to the various terminals but there are two problems here – the different sizes of lugs (I think there are five!) and second, the thickness of the wire on the secondary side makes getting the connectors on and crimped a bit of a chore. OK, there was a third problem – I forgot to buy any! So I elected to solder all connections. Just make sure before you solder the wires make a good mechanical conApril 2013  79 nection (ie, they won’t pop off even without solder). Pre-tinning any connectors also makes sense because it’s sometimes difficult to solder thick wire – it really sucks the heat away from the iron. With pre-tinning you have a much better chance for a really good solder joint. The connections between the transformers and bridge rate special mention. We already said that we obtained some thick (ie, high current) wire for these but we haven’t mentioned they should all be cut to exactly the same length. This is to ensure, as far as possible, that the load is shared between the transformers – even a few milliohms of difference could matter. We used red and black wire simply because we had some and that made the phasing of the transformers easy – it’s essential that the three (or even two) transformer outputs are connected in phase, otherwise they will see a short circuit in each other. Ideally, you should check that the outputs are in phase by comparing the waveforms on a ’scope. But if you don’t have one, don’t worry too much – again, with three identical transformers you’d expect the terminals to be connected the same way. Now you’ll find out why we used an ultra-large terminal strip – you need to connect the three wires together and anything smaller simply won’t have room to fit them in. As it is, they’re a tight fit – but they do. Fit, that is! We’ve only run one length of wire from the terminal strip to each of the bridge terminals – we would have liked to use a larger cable but didn’t have any. Again, wrap the bare wire around the bridge terminals before soldering – that’s after you take note which terminals are which. One of the AC (input) terminals is always identified, as is the + terminal. The diagonally opposite terminals are the other AC input and the – terminal, respectively. A thick black wire connects directly from the – bridge terminal to the black output terminal, while a thick red wire connects from the bridge + terminal to the thermal cutout, with the same from the thermal cutout to the red output terminal. We covered all exposed terminals (ie, on the IEC socket and the switch) with heatshrink tubing and shrunk it to fit when finished. The same treatment was given to all soldered connections on the output side – the bridge recti80  Silicon Chip All closed up and ready to go. You’ve even got a handy carry handle to handily carry your charger to where it’s needed! fier, the thermal cutout and the output terminals. And finally, we used a few small cable ties to bundle the wires together. Is it finished? Once you’ve checked all your wiring – and especially checked that no strands of wire poke out from your terminal strips – you can test that it works. Don’t connect any output leads yet but connect a, say, 1kΩ resistor (any wattage) across the output terminals to give the rectifier a small load (that’s all it needs at the moment). Plug in power and turn it on. The neon should glow, telling you that so far all is well. Measure the AC voltage at the terminal strip where the three transformer leads join. It should read just on or over 12VAC. Measure the DC voltage at the output terminals and it should be something similar – perhaps 11.5V (again, if you’re wondering why, read the explanation earlier on). Turn it off, remove the resistor and connect your output leads. While monitoring a 12V car battery voltage, connect the clips to the battery and turn it on again. Unless your battery is fully charged, you should find the voltage rises a little and keeps rising. You should also find that the voltage is somewhat higher than your previous check without the output leads because the battery acts like a giant capacitor or reservoir, smoothing out the peaks of the waveform and thus increasing the average voltage. Leave the charger on for, say, half an hour or so and check the temperature of the transformers. They will probably be quite warm but not excessively hot (they get pretty hot to the touch when operating in your ceiling!). Likewise the bridge rectifier. If that gets too hot, the thermal cutout will trip and cut power to the output. Closing ’er up If you’re happy that everything works as it should, close the case up and snap the locks closed. If you look closely at the bottom of the locks, you’ll note that there is provision for inserting a 3mm screw (with nut), about 30mm long, through the whole thing, which stops the locks being opened. We’d be inclined to do this – despite covering all the bitey bits with heatshrink, you don’t want anyone’s fingers (especially little ones!) inside the case. What’s the charging current? Next month, we’ll show you how to add both a digital ammeter and a digital voltmeter so you know exactly what’s going on. Having set out to produce a lowcost, surplus parts battery charger this could be regarded as “gilding the lily” somewhat! They do add to the cost of the project but also add significantly to the value and we think both are worthwhile additions (of course, you could choose to add only one meter instead of two – and/or leave it as is!). An alternative would be to use a couple of dirt-cheap digital multimeters. Jaycar’s QM-1502 DMMs are just $4.95 each – even cheaper than panel meters! SC siliconchip.com.au