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Adding Voltage and Current to the Bits’n’Pieces Battery T here are three easy ways to add voltage and current meters to our battery charger. One is relatively expensive, one is dirt cheap and one is in the middle. Let’s look at these in turn: (a) Using Mechanical (Moving Coil) Panel Meters This is arguably the easiest way to go because you can buy panel meters already set up to read exactly what you want. For example, the Altronics Q0421A panel meter reads 0-20A “straight out of the box”, while their Q0523A model reads 0-20V. The other big advantage of these panel meters is that they don’t require power to 86  Silicon Chip operate so that also simplifies things somewhat. All you need to do with these meters is cut suitable holes in the charger case, mount the meters and then connect the ammeter in series with the output and the voltmeter in parallel with the output. Bingo – simple. But this is the most expensive way to go and it’s not all that accurate, simply because the meter scale only lets you read to about the nearest amp or volt. In many cases this might be all you need but sometimes, you want more accuracy than that. And unless you can snaffle a couple of suitable meters from junked equipment, you’re going to be up for around $15 per meter. That’s a significant proportion of what the charger without meters would cost! When we said “suitable” meters a moment ago, you’d probably be aware that just about all meters can be set up to read whatever you want them to. The same basic meter, with an appropriate shunt (a low-value resistance in parallel) will read whatever amperes (or parts thereof) you set it up to read, or with appropriate multiplier (a much higher resistance in series with the meter), whatever voltage you want. A typical moving coil meter without shunt or multiplier might read, say, 1mA full scale. This meter would have a resistance of 210Ω. But if you put siliconchip.com.au Meters Charger Last month, when we put together our bits’n’pieces battery charger, we promised to show how to add meters to show both current and voltage. Sure, it’s getting out of the realms of a dirt cheap charger but, what the heck . . . another 210Ω resistor in parallel with it, it will read 2mA full scale – half the current flows through the meter, half through the shunt. The lower value you make that resistor in parallel, the more current flows through it but the current through the meter movement (and therefore the reading) will stay in proportion. To read high currents, the vast majority of current needs to flow through the shunt so the values of shunt resistance become very low indeed – fractions of an ohm. It’s similar with voltage: the resistance of the meter movement is still 210Ω so if you put, say, a 20kΩ resistor in series with it, the voltage will divide in the ratio of 210:20,210 and if there is 20V across both the meter and multiplier, the meter will read 20V (or very close to it). So as you can see, if you can find a couple of old meters and (carefully!) work out what their resistance is by slowly increasing the current through them until they read full scale (also called their sensitivity), you can use Ohm’s law to work out their resistance (ie, R=V/I), you can then make up your own shunts and multipliers to make the meters read what you want. Before we finish with mechanical panel meters, you might have heard of “expanded scale” meters. These are invariably standard meters which have shunts and multipliers set up so that they read only a limited range of values – for example, 10-15V – which means that they don’t start reading until the voltage exceeds 10V and it One of these moving-coil meters reads 0-20V, the other 0-50µA. But they are exactly the same meter movement – to read voltage, you add a series multiplier; for current, you add a parallel shunt. Obviously scales are changed to reflect the different measurements. siliconchip.com.au by Ross Tester These little multimeters from Jaycar are so cheap you could justify using one as a dedicated panel meter. Powering on and off could be a problem – unless you use the tricky little circuit elsewhere in this issue! reaches maximum scale at 15V. This gives much greater accuracy as the divisions on the scale are further apart (or there are many more of them). (b) Using cheap DMMs This is the lowest-cost method and for many people, it will be more than sufficient. Elsewhere in this issue we show how to add auto power-down to cheap ($4.95) digital multimeters from Jaycar (QM1502). That’s right, the whole DMM is just $4.95! You can set it to read what you want (eg, 20V or 10A) and measure the voltage or the current in the normal way – voltage in parallel, current in series. At the price, you could afford to have two of these meters dedicated to read voltage and current, simply by leaving the dial set and turning the meters on when needed. But it doesn’t matter if the meters are turned on or not when using the battery charger, charging current will still flow through the shunt in the meter so it will make precious little difference and as a voltmeter, it is in parallel with the output so might as well not be there. Unfortunately, you cannot power the multimeter from the same source as being measured but with the simple modification mentioned, the 12V battery in these cheapies should last for quite a long time. If you got really enthusiastic, you could work out a way to mount these DMMs inside the charger case and bring the “power” pushbutton out to the case. Incidentally, the rotary switch on a multimeter merely selects various shunts and multipliers to read amps and volts over various ranges. To measure resistance (Ohms) it uses an internal battery to push a small current through the resistor and reads that current but displays it as resistance. To measure AC voltage or current, in the vast majority of cases the AC is rectified inside the multimeter and the resulting DC voltage or current is displayed on an “AC” scale. (c) Using Digital Panel Meters This is the preferred approach – it will cost more than using cheap DMMs but not as much as using mechanical meters. In our case, we are using a couple of Oatley Electronics’ 3.5-digit Digital Panel Meters (DPM1). One is set up to read volts, the other amps – just the same as the panel meters above. However, these digital panel meters do need power and, once again, you cannot simply power them from the device being measured. But the power they need to operate is “flea power” – May 2013  87 Preparing the box Now we’re talking! These $9.95 Digital Panel Meters from Oatley Electronics (Cat no DPM1) can be set up to read current or voltage – which is exactly what we’re after. just a couple of milliamps. If you add Oatley’s K-265 Interface Kit (K265) it will supply all the power you need from the battery charger itself. 3.5 or 4 digits? Before we get into it, though, we can already hear the question: 3.5 digit? I can see four digits! It’s long been a source of confusion – but the explanation is pretty simple. It’s more expensive to produce a meter which reads 9999 (a 4-digit display), so many are made to read 1999 instead. Therefore, a 3.5-digit display can show any value up to 1.999 (or 19.99, 199.9, 1999). In our case, we want it to display up to 20A and 20V – well, it can just about do that – it can never quite get there (it’s 1mV or 1mA short!). OK, so how do you use them? 9.1V zener – the circuit won’t work if you do – and also note that the zener mounts in the opposite direction to the other four diodes. One other point to note: the overlay on the PCB was different to that supplied in the instructions – the overlay is correct, with a 2k trimpot (VR3) instead of a fixed resistor (R6 – 390Ω). Once completed and before the interface is connected to the digital panel meters, though, we need to adjust the output voltage (using VR3) to get 9V. Using a 12V battery, connect power to the interface board and adjust VR3 to get as close as you can to 9V at the output “V” and “I” DPM terminals (they should be identical). Once done, disconnect the battery and put the interface board aside until you’re ready to assemble everything. From here on, we are assuming that you are using the preferred approach. The first task is to determine where you want to mount the DPMs. The main thing to remember is to keep them away from the “bitey bits” on the left side of the box – we chose a spot on the top right. Mark the positions of your meters remembering that there is an escutcheon which is larger than the meter itself. There should be around 20mm between the meters if mounting them side-by-side. Mark the two cutouts, which should be 50 x 25mm, in your chosen positions, and cut them out. Whether you use the tried and true method of drilling a lot of small holes and cutting out the panel (filing it smooth), drilling a larger hole and nibbling out the panel or perhaps using a metal blade in a jigsaw, make sure that you don’t get any swarf in the case. In fact, it’s a good idea to open the case right out – that means your blade or drill can’t do any damage either. Mark the four holes for each of the mounting screws (attached to the escutcheon) and drill them out to 3mm. These holes are centred around the display, 60mm wide and 24mm deep. Remove the nuts from the displays and separate the back halves from their escutcheons. Make sure the four bolts attached to the escutcheons fit easily through the mounting holes and that Basically, using a digital panel meter is very similar to using a mechanical panel meter, as detailed above. The instructions supplied with the meter show how to set it up as a voltmeter, with a series multiplier, or an ammeter, with a parallel shunt (now where have we heard those terms before?). Building the interface board This is simply a matter of following the diagrams supplied with the kit and on the PCB component overlay. Just a couple of tips: three miniature transformers are supplied; it doesn’t matter which one goes where. However, you will find there are three pins on one side and two on the other – which determines which orientation they have! And before soldering the PCB-mounting terminal blocks in, slide them together so they link. Finally, don’t mix up the four 1N4148 small signal diodes with the 88  Silicon Chip Oatley’s K265 Digital Panel Meter Interface Board is specifically designed to supply power to the panel meters and also make adjustment of voltage and current really simple. It sells for $16.50 siliconchip.com.au POWER S1 BR1 CON1: INTEGRATED IEC MAINS SOCKET A AND FUSE HOLDER N 230V F1 5A 1N4004 ~ 35A/400V 12V + E – NEON BEZEL ~ 2013 Modifying the DPMs As supplied, the DPMs are set up to read 200mV (well, actually 199.9mV). To make them read 20V we need to change the multipler and move the decimal point. Rather than try to disassemble the panel meter PCB (which is not easy) and reassemble it (which is almost impossible!) provision is made on the interface board. You would have earlier (during construction of the interface board) selected a 1MΩ resistor (R1) so the DPM would read 20V; all you need do is connect the DPM to the interface board (both power and voltage input), connect a known voltage source of, say, 12-20V DC to the BAT+ and BATterminals of the interface board and adjust the “V CAL” trimpot (VR1) to + THERMAL SWITCH NC – 90o OUTPUT TO BATTERY UNDER CHARGE SHUNT 0.011 – CON2 BITS’N’PIECES 10A BATTERY CHARGER the escutcheons cover the edges of the cutouts. Before we mount the DPMs we need to modify them slightly to act as the 0-20V and 0-20A meters. 100uF 25V 90 T1-T3: 230V – 12V AC HALOGEN LIGHT TRANSFORMERS SC  + TO INTERFACE PCB (POWER) T1-T3 – – + TO TO VOLTMETER AMMETER + (VIA INTERFACE PCB) that voltage. For example, you could use a 12V SLA battery and your digital multimeter to get the known voltage. Changing the decimal point is not quite so simple. It is wired to suit a “199.9” reading; we want it to suit a “19.99” reading. Theoretically, that’s just a matter of changing a link on the PCB from P3 to P2 – but as we said earlier, disassembling the PCB to get at the P3 link is not a good idea. Instead, we are suggesting you carefully cut a track on the PCB and solder a link between that cut track and the right-hand pair of P2 pads – the photo below shows the detail. To solder to a solder-masked track, carefully scrape some of the green mask off the track to reveal bright copper and equally carefully solder to that. Be careful – it doesn’t take much heat to lift thin tracks. The current meter needs to have the same decimal point modification as we want it to read up to 19.99A. Once again, the interface board is set up to Fig.1: the main differences between this and last month’s charger circuit is the addition of the 0.011Ω shunt resistor, adding connections for the voltmeter and ammeter panel meters and a smoothed DC supply. allow it to read this with a suitable shunt connected. The shunt is actually two parallelconnected 1.5m lengths of resistance wire (supplied in the interface kit). A single length of this wire has a resistance of 0.0146 ohms per metre, so 2x 1.5m lengths in parallel will have a resistance of 0.011Ω. If reading 20A, this will result in a voltage drop of 0.22V. While this is slightly too high (it should be 0.199V) this error can be corrected via the use of the “I CAL” trimpot, VR2. Connecting the shunt The shunt is simply wired in series with the charger output. You need to break the connection between the bridge rectifier and the negative output terminal and wire the shunt in its place. A pair of much thinner wires (as thin as you like!) connect from each end of the shunt to the “SHT” and “BAT-” terminals on the interface board. We wound the shunt into a pretty CUT THIS TRACK AND BARE COPPER JOIN siliconchip.com.au The panel meter is supplied with P3 joined, which means it will read 199.9. To make it read 19.99, P2 must be joined instead. As it is very difficult to disassemble and reassemble the PCB, we suggest cutting the track shown, baring some copper and soldering a link between the point shown and the bare track. May 2013  89 Fig.2: adding the meters is quite simple, especially when using the K265 interface board. Effectively, all you need to do is to cut the connection between the bridge rectifier “–” terminal and the output post and replace it with the coiled shunt wire. Connections to the meters themselves is all via terminal blocks on the interface board. We also added a small smoothing circuit (on the 4-way terminal block) to ensure the meters weren’t trying to work with a pulsating DC supply. Refer to the first article (last month) for the remainder of the wiring details. CURRENT METER FROM TRANSFORMERS BRIDGE RECT – TO BATTERY – + VOLTAGE METER + "I" DPM – + – "V" IN "I" IN + IN+IN– IN+ IN– + – + 9V A + "V" DPM – 9V – K BAT– BAT+ – + SHT IN+IN– IN+ IN– 1N4004 100F 25VW 0.011 SHUNT (SEE TEXT) K265 small coil and placed it near the output terminals. A two-way terminal block is provided in the kit but we replaced this with a much larger 4-way block – this is much easier to connect to as the shunt wires (and the output wires) are quite thick and securing them in the small terminal block is not the easiest thing in the world. Besides, we wanted another two terminals for some more components. Connecting the multiplier It’s already done for you – on the interface board! Connecting power from the charger As you would realise, the output from the bridge rectifier is pulsating DC and there is very little in the way of smoothing on the interface board. To make use of the charger output, 90  Silicon Chip we used a diode in series (to isolate the supply from the charger output) and a small electrolytic capacitor to give a smooth supply for the interface. Again, this can be placed wherever it will fit – the same terminal block can Parts List – Adding Meters to the Bits’n’Pieces Battery Charger 2 3.5-digit panel meters (eg, Oatley Electronics DPM1) 1 DPM Interface Kit (Oatley Electronics K265) 1 100µF 25V electrolytic capacitor 1 1N4004 power diode 1 4-way large terminal block Hookup wire Nuts, screws and washers as required. hold these two components. Connecting the modules These are pretty-much self explanatory. You have four terminal blocks on the interface board – the ones labelled “V” are for the voltmeter and the ones labelled “I” are for the current meter. Fig.2 shows the connections – “V” & “I” DPM supply power; + to + and – to – respectively. “V IN” and “I IN” are the connections for the measurement terminals (again, + to + and – to –). The only slight wrinkle here is that the solder pads on the modules are very small. Be careful soldering to them (ignore the centre terminal in all cases). It will probably be easier if you connect the modules and shunt before screwing everything into place. Don’t forget, the wire between the interface siliconchip.com.au (Above): here’s how we “wound” the shunt resistor. It consists of two 1.5m lengths of insulated resistance wire, wound together. Final resistance is 0.011Ω. Actual number of turns is immaterial – just make it as small as practical! The 2-way terminal block (which comes with the kit) was later replaced with a much larger 4-way block, which also connects the power supply components. (Right): the new components to drive the panel meters are all mounted on the right side of the case, as seen here. Ensure there is plenty of clearance between the meters and interface board/terminal block when the lid is closed and that there is enough wire to avoid them being stretched. and panel meters doesn’t have to be at all thick. We used rainbow cable. The only thick cables needed are those required to pass the battery charging current – most of what you need should already be in place from the “meterless” version of last month. In fact, the only extra length of heavy duty cable we needed was to connect the terminal block (shunt connection) back to the negative output terminal. Use cable ties to ensure all cables are secured and won’t come adrift, especially when the case lid is opened. Where do you mount the interface? Wherever you can! There should be enough space for it (and the terminal blocks for both shunt and diode/electro) near the rectifier. You might have to move things around a little bit but there should be tons of room. We mounted ours on the end of the case and stood it off the surface by the thickness of one nut and washer (see photo above). Connect everything up, check your wiring twice and you’re ready for the smoke test. If you don’t get any, you’ve passed! To finish off, mark the case with a couple of labels showing which is the voltmeter and which is the ammeter. siliconchip.com.au Got an extra transformer? Not long after the April issue went on sale we received a note from one of our readers, Charles Tivendale, who told us that he had made a similar charger some years ago but he used an extra transformer to give improved performance. It wasn’t, as you might expect, simply in parallel with the other transformers. He used the fourth transformer to boost the primary voltage slightly to the other three, thus giving slightly higher secondary voltages. This was done as shown in the circuit below, with the secondary winding of one transformer connected in series with the primary and used as an auto-transformer. In other words, the 230V mains voltage was applied to the primary with the output taken from the 230V + 12V winding, resulting in a nominal 242V output. This slightly higher voltage was then applied to the primaries of the other transformers, resulting in a slightly higher output voltage to the bridge rectifier. Naturally, this gave more output from the charger – not a huge amount but enough to make the whole exercise worthwhile (especially if the transformer cost you nothing!). The phasing of the new transformer windings is important – if you connect them up incorrectly, you’ll get less than 220V out. If this happens, simply reverse the connections to the 12V winding. One point to note: as there is no current control on this simple charger, if the battery is fully charged (ie, it’s gassing) the extra voltage might be enough to cause an overcharge. Just something to keep your eye on! SC POWER S1 F1 NEON 5A BEZEL A BR1 35A/400V 230V 230V ~ 12V 12V + E – N ~ T4 CON1: INTEGRATED IEC MAINS SOCKET AND FUSE HOLDER T1-T4: 230V – 12V AC HALOGEN LIGHT TRANSFORMERS Here’s how to add an extra transformer (in autotransformer mode) to give a slightly higher output voltage. May 2013  91