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Introducing: Part 1: by Duraid Madina and Tim Blythman Our all-new 800W Uninterruptible Power Supply (+) We’ll say it right up front: this will not be a cheap project to build. But if you do build it, we believe you will end up with a UPS that is a better performer than anything else on the market at even two or three times the price. And even then (unlike most commercial units), the design is quite flexible if you wish to expand its already exceptional capabilities. So if you’re in the market for a UPS (and who isn’t, with the quality [?!] of mains power these days?) you will go a long way to find better value than this. 28 Silicon Chip Celebrating 30 Years siliconchip.com.au We searched high and low for a high quality, low-cost case to house the UPS but proved the two terms are mutually exclusive! However, this case is ideal for the job, being pre-drilled and slotted for excellent ventilation. I f you aren’t familiar with the concept of an Uninterruptible Power Supply (UPS), it provides a back-up for the mains supply to an important piece of equipment (such as a corporate server or other mission-critical system), so that it won’t shut down during blackouts. The typical use for a UPS is to give plenty of time – a few minutes to perhaps an hour or so – to save work before it’s lost – or in large organisations, long enough for a mains generator to be fired up and take over. Some UPSes are designed to give hours, and occasionally days or even more, of power to enable an enterprise to keep working as if the blackout didn’t exist. But these are VERY expensive systems, relying on a large (and even more expensive!) battery bank to keep them supplied. A UPS is now standard equipment for computer systems in commerce or industry but they are becoming more popular for home or small business. Laptop computers don’t need a UPS as their internal battery does the same job. But the printer or large monitor connected to a laptop will obviously cease working during a blackout. Powered via a UPS, work can continue. Disaster power A UPS like the one we are describing could be used in a lot more situations. ifications Features & spnsec plug For example, perhaps with a few modifications, it could power all your computers, your modem and all your entertainment equipment in the event that you experience a blackout for many hours. It could keep some or most lights working, allowing an orderly (and safe) exit from deep within otherwise-dark office blocks (sorry, you’ll have to use the stairs as the lifts won’t be working!). Or if you have a much longer blackout, it could perhaps run your refrigerator for several days during a long power outage which could occur after a bush fire, a big storm or a flood. That would mean you would not lose any food due to spoilage. And of course, it means that you can keep your mobile phones and notebooks and tablets fully charged so you can stay in contact with the outside world. Just how long you could run a refrigerator would depend on the power rating of its compressor and the temperature setting. Or perhaps it could allow you to also run a gas heater or oven which requires 230VAC at low power to run the igniter and the control electronics. So a UPS is an important accessory for a variety of reasons in both business and in the home. But why would you want to build this one instead of buying a commercial unit? Wouldn’t that be cheaper? Not in this case. • Power input: 10A mai s • Output socket: four switched GPO 800W er: pow ut • Continuous outp • Peak output power: 1200W • Battery capacity: 588Wh • Inverter type: pure sinewave , 4h <at> 135W, 5h <at> 110W , 1h <at> 500W, 2h <at> 260W, 3h <at> 175W • Approximate runtime: 35m <at> 800W <40ms (two mains cycles) • Response time after mains failure: cycle) g back to mains: ~20ms (one mains • Power interruption when switchin ustable) • Brownout threshold: 200VAC (adj AC (adjustable) 260V d: shol thre out cut• Over-voltage y 5 hours from flat • Battery charging time: approximatel • Quiescent current: 19W battery ing off inverter, battery charging, low • Status indicators: mains good, runn UPS software) rce ng interface (compatible with open-sou • PC interface: USB serial monitori s are nearly flat erie batt n kout; continuous tone sounded whe • Audible alert: beeps during a blac e harg battery cut-out with zero battery disc • Protection: 10A mains fuse, lowe) harg disc es (full • Battery longevity: at least 1500 cycl siliconchip.com.au Celebrating 30 Years LiFePO4 batteries This UPS has high capacity, very safe LiFePO4 batteries which can be deep discharged without damage – something that can easily occur in a long blackout. Plus it uses an Arduino to monitor and control it. And while this UPS is conservatively rated at 800W, it actually employs a 24V DC to 240VAC true sinewave inverter which is rated to deliver 1200W or up to 2400W surge (useful to start motors or run a microwave oven for a short period). May 2018  29 To a large degree, our 800W UPS is based on existing modules which we connect together in an appropriate manner. The photos above show two of the main components: at left is the pair of Drypower 12V, 23Ah batteries which we connected in series for 24V DC, while at right is the Giandel pure sinewave inverter, which is used to power equipment from the batteries when mains power goes down. The conservative limitation to 800W is determined by the batteries but you could possibly run at the full 1200W continuous output of the inverter for short periods without any problems. So let’s talk about the batteries. Most commercial UPSes come with sealed lead-acid (SLA) batteries. The problem with SLA batteries, apart from being very heavy and bulky, is that they are easily damaged or even destroyed if you allow them to discharge below 11V – and that can easily occur in a typical UPS. We speak from experience – and we’ve heard that our experience is not uncommon. We used to have a UPS on the SILICON CHIP office server, because blackouts are fairly common in the northern beaches of Sydney (we’ve had quite a few in the last decade). But the one we were using failed because its lead-acid battery was deeply discharged by a long blackout over a weekend. We replaced it but only a few months later, it went bad again after yet another extended blackout so we just gave up and removed it. As it uses Lithium iron-phosphate batteries our new UPS design is a lot more robust than that commercial unit so it won’t fail in the same manner. They will survive hundreds, if not thousands of blackouts (perish the thought!). Why did we use lithium iron-phosphate batteries instead of lithium-ion or lithium-polymer? In a word: safety! They are much less likely to catch fire! While a fire is unlikely with a Lithium-ion or Lithiumpolymer battery, it isn’t unheard of – and the sudden failure of a battery of this size could be very dramatic. And since this is a DIY project, we can’t rule out mistakes being made during construction. So we wanted the safest possible option. While some UPSes are able to guarantee no loss of power at all during a blackout, most operate by feeding the incoming mains directly to the load, as long as the mains voltage is OK, but then switching over to inverter operation if the mains waveform goes bad or disappears entirely. Normally this switching is done with a relay or relays and so there is a very brief switch-over period where the load gets no power. But most devices will not be affected by this. For example, 30 Silicon Chip all desktop and server computers these days run from a switchmode power supply, which rectifies the mains to charge a large capacitor or capacitors to around 350V, which then power the switching circuitry. It takes some time for the filter capacitor bank to discharge to the point where the output voltages are affected. So as long as the switch-over time is short, the supply and thus computer will operate uninterrupted. Similarly, a motor-driven appliance such as a refrigerator will have some inertia and the loss of mains for a fraction of a second will likely not affect its normal operation. And most low-cost UPSes do not have a sinewave output when running off the battery. They usually have a “modified square wave” or even a square wave output, since it’s easier to produce and the switchmode supply in a computer will run just fine off a square wave (or even high-voltage DC, for that matter). Our design uses a “proper” sinewave inverter so is usable with a much wider range of devices. Want more grunt? Now before we go on to discuss the design philosophy behind this project, we should point out that many aspects can be modified or greatly expanded to suit your particular application. Want higher power output or much longer run for more extended blackouts? No problem, just substitute a bigger inverter and a bigger (much bigger) battery bank. Want to operate from solar panels to use it for off-grid power? Again, no problem (we will discuss these various possibilities in a later article). 12V or 24V operation? Our initial design brief for this project was to have a rated output of at least 500W. So what would be the right battery voltage? To deliver 500W, a 12V inverter would require an input current of over 40A, which would be harsh on the battery and inverter and require very thick cables. So we started looking for inverters and batteries in the range of 24-48V. It quickly became apparent that 24V batteries and in- Celebrating 30 Years siliconchip.com.au These three photos show some of the other modules we used – while not so fundamental as those shown opposite, they’re important nevertheless! At left is the Victron Battery Balancer, required because we used (for economy reasons) two 12V DC batteries instead of a single 24V DC. Even when adding in the cost of the balancer, two batteries are a much better proposition. Centre is the 12V switchmode power supply used to power the Arduino, while at right is the 24V, 5A mains charger for the batteries. verters were less common and more expensive than 12V types, and 36V/48V batteries and inverters even more so. Two 12V batteries (24V) seemed like the best compromise. We decided to use two Drypower 23Ah 12.8V LiFePO4 batteries in series, which were supplied by Master Instruments (Cat No IFM12-230E2). We considered using a 25.6V LiFePO4 battery but a similar capacity model cost significantly more than twice as much as the two 12V batteries. Using two batteries meant that we would need a charge balancer, to ensure that the two battery voltages are kept similar – but even when we include the cost of the balancer, the two 12V batteries are still significantly cheaper. This battery bank then drives a Giandel 24V/1.2kW pure sinewave inverter which we bought from the Giandel Australia website for $138 plus postage (Cat No PS1200DAR/24). This is excellent value. It comes with a pair of battery cables with eyelet lugs and also a remote control that attaches to the unit using telephone-style flat cabling. We hooked this up to an Arduino, which is then able to monitor the inverter status and switch it on and off. This inverter has a typical efficiency figure of around 90% and it includes a cooling fan and substantial heatsinks so it can deal with the approximately 100W of dissipation at full power. As already noted, the inverter is rated at 1.2kW (2.4kW peak) but the specified batteries can’t supply sufficient current to allow such a high power delivery. They are rated at 35A continuous which works out to around 800W at the output when you take inverter losses into account. That’s still handily above the target we had set ourselves for this project. The 588Wh nominal capacity of the battery bank is specified at a 5-hour discharge rate, which is what our specification of five hours battery life for a 110W load is based on. Curves are not provided to show how capacity diminishes at higher discharge rates but lithium-chemistry batteries normally have a low internal impedance so we believe our moderate de-rating of capacity with increasing load should be approximately right. We also considered designing a “line interactive” or “onsiliconchip.com.au line” UPS, where the load is always powered by the inverter and the charger provides the DC current to operate it when mains is available. This avoids the need to switch the load between mains and the inverter and also, poor mains power quality (ie, distorted waveform) is not transferred through to the load. However, that approach would require a charger capable of around 30A which would be large and quite expensive and it would also be less efficient due to the constant conversion from 230VAC to low-voltage, high-current DC and back to 230VAC. Hence, we decided to design a “standby UPS” instead, as presented here. By the way, the inverter output is specified as 240VAC; somewhat higher than 230VAC. So when the UPS switches the load to the inverter, the supply voltage will typically increase slightly. But this is still well within the Australian mains specification of 230VAC+10%,-6% so it should not present any problems. In many parts of Australia, the mains supply is typically above 240VAC anyway. Charging and mains switching Having decided on the two most important components of our UPS system, ie, the batteries and inverter, there were still other important details to be determined. These included how the batteries are charged once mains returns after the inverter has been operating (and indeed, are kept charged long-term), how we determine when to switch the output sockets from mains to the inverter output and how that switching is performed. Charging is quite simple; we purchased a 5A mains charger designed for LiFePO4 batteries and it’s permanently wired to the incoming mains socket so that whenever mains is present, it’s charging the batteries. Like other Lithiumbased rechargeable batteries, LiFePO4 use a constant-current/constant-voltage (CC/CV) charging scheme. So the charger will deliver 5A to the batteries until the voltage across them reaches 29.2V (14.6V per battery or 3.65V per cell). It will then hold the terminal voltage at 29.2V as the charge current decreases until it reaches a low level, at which point the batteries are considered charged. Celebrating 30 Years May 2018  31 Fig.1: block diagram of the SILICON CHIP 800W UPS. Much of the “magic” is in the Arduino software and shield which will be described in detail next month, along with full circuit and construction details. However, the inverter needs to be kept on constantly so that it’s always ready to take over, should the mains supply cut out. Therefore, it draws several watts from the batteries constantly and the battery voltage will never quite reach 29.2V (it sits at around 29.15V). This should not pose a problem; they are effectively float charged. Enter the Arduino controller We’re using an Arduino Uno to monitor the mains voltage, via a small mains transformer. The primary of this transformer is connected across the incoming mains supply and the voltage from the secondary is divided down and fed to one of the Arduino’s analog inputs via a biasing network which keeps the analog pin voltage in the 0-5V range. The Arduino is constantly sampling the mains waveform and if it detects an under-voltage or over-voltage condition, or a significant deviation from a sinewave, it immediately switches the output over to the inverter. It only switches the output back to mains when it determines that the mains waveform and voltage are stable and have been for a few seconds. The switching is accomplished by using three DPDT relays which are controlled via a relay driver shield and the Arduino. Both the Active and Neutral wires are switched. Relay logic for safe switching Now refer to Fig.1 which is the block diagram for our high power UPS. It shows how the three relays are arranged. RLY2 is the mains changeover relay and it is arranged so that there is no possible way that the output of the inverter could be connected to the mains. RLY1 is used to connect mains to RLY2 (and on to the output) while RLY3 is used to connect the inverter to RLY2 (and on to the output). Why do we need three relays when it might seem that only one or two relays might be able to switch the load between incoming mains or the output from the inverter? 32 Silicon Chip The reason for using three relays in this manner is that there is no way to precisely lock the phase of the inverter output waveform to the incoming mains waveform. While both are nominally at 50Hz, they could be in phase, 180° out of phase or anywhere in between. The phase difference between them is likely to slowly drift over time, due to slight differences in the two frequencies. So it’s entirely possible that the momentary mains voltage could be +350V while the momentary inverter output voltage could be -350V. A single 250VAC-rated relay is not designed to handle 700V DC between two terminals on the same pole. There could be an insulation breakdown and/ or major contact arcing and this could destroy the inverter. By having an extra relay between each AC source and ensuring that both RLY1 and RLY3 are off at the time when RLY2 is switching, we avoid applying any more than the normal mains peak voltage across a single relay. When the unit is powered off, all the relays are off and so the output sockets are not connected to anything, except for the Earth pins, which are connected to mains Earth and also the unit’s chassis. When the unit powers on, it checks the mains voltage and waveform and assuming they are good, it switches RLY1 on. This connects mains to the output sockets and load(s). If mains goes bad or disappears altogether, the unit immediately switches RLY1 off. Then, after a short delay, it switches RLY2 and RLY3 on. So the load is briefly disconnected from mains altogether (for around 10ms), then connected to the output of the inverter, which is already running. When mains power comes good again, RLY3 is switched off and after a brief delay, RLY2 is switched off and RLY1 switched on. Again, there is a brief period where the outputs are not connected to either mains or the inverter. This ensures a safe change-over. The unit is also designed to perform a sequenced change- Celebrating 30 Years siliconchip.com.au over in this manner should its own power supply fail or when it is purposefully switched off, using a switch mounted on the rear panel. This allows you to, for example, disconnect the UPS from mains so you can move it to a different location without it discharging its batteries. Indicator LEDs We’ve fitted three indicator LEDs on the front panel, so you can tell what is happening. The green LED at far left is on continuously while the output is connected to mains and flashes if mains is not present or not clean. The yellow LED in the middle lights continuously when the output is being fed from the inverter. While the output is running off mains (and the green LED is solidly lit), the yellow LED will also flash to indicate that if there is a problem with the inverter, such as if the Arduino detects it is not running when it should be. The red LED at right starts flashing when the battery voltage drops. The flashes become faster as the batteries discharge until it is on continuously when the remaining charge is around 10%. The unit is also fitted with a piezo buzzer which beeps intermittently while the output is running off the inverter and it changes to a continuous tone when the batteries are nearly flat. If the battery voltage drops below about 21V, the Arduino switches the inverter and relays off. It also shuts itself down. The drain on the battery becomes almost zero. While these batteries do incorporate their own over-discharge protection, we feel it’s still good practice to minimise the load at low voltages. The unit is able to “bootstrap” itself and power back up when mains returns and this procedure is described below. It can also be manually switched off and powered back on later if necessary. Powering itself We need a source of 12V DC to run the three relays and Parts list – 800W Uninterruptible Power Supply (UPS) 1 vented 3U rack-mount case, 559mm deep [Bud Industries RM-14222+TBC-14253+TBC-14263] [Digi-Key 377-1392-ND; 377-1396-ND; 377-1397-ND] 2 Drypower IFM12-230E2 12.8V 23Ah Lithium Iron Phosphate batteries [Master Instruments] 1 Victron Energy 2x12V Battery Balancer [Master Instruments – www.master-instruments.com.au] 1 Giandel PS-1200DAR/24V Pure Sinewave Inverter with cables [www.giandel.com.au] 1 5-7A LiFePO4 charger [Master Instruments, AliExpress] 1 DETA 6224B Silver Four Outlet Power Point or similar [Bunnings 4430423] 3 12V DC coil, 10A 240VAC cradle relays [Jaycar SY4065] 3 DPDT chassis-mount relay cradles [Jaycar SY4064] 1 12V 1.3A enclosed switchmode power supply [Jaycar MP3296] 1 12.6V CT 7VA transformer [Jaycar MM2013] 4 screw-on equipment feet [Jaycar HP0832] 1 3AG safety fuseholder [Jaycar SZ2025] 1 3AG 10A 250VAC fuse 1 connector to suit battery charger (see text) 1 Arduino Uno or compatible 1 Freetronics 8-Channel Relay Driver Shield [Core Electronics Cat CE04549] 1 Arduino control shield (details next month) 1 green chassis-mount LED with chrome bezel [Altronics Z0265, Jaycar SL2645] 1 yellow chassis-mount LED with chrome bezel [Altronics Z0224] with 1kW series resistor 1 red chassis-mount LED with chrome bezel [Altronics Z0264, Jaycar SL2644] 3 1kΩ 0.25W resistors 1 NO momentary pushbutton switch Fasteners 8 M5 x 90-100mm bolts or machine screws 12 M5 x 10mm machine screws 28 M5 nuts 6 M4 x 10mm machine screws 6 M4 nuts 6 M4 shakeproof washers siliconchip.com.au 4 M3 x 32mm machine screws 6 M3 x 15mm machine screws 28 M3 x 10mm machine screws 1 M3 x 6mm machine screw 34 M3 flat washers 34 M3 nuts 4 25mm-long 3mm ID untapped spacers 8 15mm-long 3mm tapped Nylon spacers 4 M3 x 25mm Nylon machine screws Cables, wires and insulation 1 2-wire mains cable with figure-8 plug* 2 3-wire mains cables with moulded 10A plugs* 1 100mm length of 40A+ rated wire 1 2m length red medium duty hookup wire 1 2m length black medium duty hookup wire 1 2m length yellow medium duty hookup wire 1 1m length white light duty hookup wire 1 1m length yellow light duty hookup wire 1 1m length red light duty hookup wire 1 1m length black light duty hookup wire 1 cable gland to suit 3-wire mains cable [eg, Jaycar HP0732] 1 150mm length 6mm diameter heatshrink tubing 1 50mm length 10mm diameter heatshrink tubing 1 50mm length 16mm diameter heatshrink tubing 1 50mm length 20mm diameter heatshrink tubing * Can be cut from spare power cables, extension cords or similar Other hardware 2 Carinya MABF2101 Make-a-Bracket flat plates, 100 x 200 x 1mm [Bunnings 3975858] 6 Carinya MA0003 25 x 25 x 40 x 1mm angle brackets [Bunnings 3975955] 5 adhesive wire clamps 6 small P-clamps 10 4mm crimp eyelets 2 red 6.3mm insulated crimp spade lugs (for the power switch) 30 small black cable ties Celebrating 30 Years May 2018  33 5V DC for the Arduino. While we could simply run both off one of the batteries, this would not be ideal as it would present an unbalanced load to the overall battery pack. It would also place a load on the batteries when they are nearly flat. To avoid this problem, we have fitted a small mains switchmode power supply inside the case and wired this in parallel with the output sockets. So when mains power is present, this powers the Arduino and relays and when running off the inverter, the inverter powers this switchmode converter instead. When the output is switched off, this totally disconnects the Arduino and relay power supply. So during a short blackout, the Arduino will be powered by the inverter and will simply switch back to mains power once it’s restored. But if there’s a long blackout and it powers down, when mains power comes back, the output is disconnected. So how does it start back up and switch on the inverter (in case it’s needed later) and RLY1? The answer is that we’ve added a small relay on the Arduino shield which normally connects the secondary of the mains-sensing transformer to a diode. Current flows through that diode and into the 12V supply bypass capacitors, providing an initial source of power for the module. (Note that this fourth relay is not shown in the diagram of Fig.1 but it is on the control shield). Once RLY1 is on, that relay is also energised, disconnecting the transformer from the diode. This means that the transformer is not being loaded, so its output is once again a good proxy for the mains voltage. In fact, this relay is briefly energised before RLY1, giving the Arduino the chance to verify that the mains waveform is clean before the load is connected. The inverter can not necessarily be used at this stage because the batteries are probably flat. But they will start charging as soon as mains returns and will soon be ready for use. Switching it on without a mains source We have considered that this unit may also be useful as a source of emergency power. For example, you could use it to back up the power to your fridge so that the contents don’t go off during a blackout but you might later decide to unplug your fridge and move it to power some other equipment such as lights, a TV and so on. In this case, during an extended blackout, you may need to switch the UPS off and then later switch it back on but unless you have a generator, you won’t have a source of 230VAC to “bootstrap” it. So we have added a momentary pushbutton switch to the front panel which briefly connects the nominally 24V battery bank to the input of a 12V regulator which then feeds the Arduino and relays. Holding this button for a few seconds gives the unit enough time to switch the inverter on and power the load from the inverter. You can then release the button and the unit will continue to run until it is switched off or the battery goes flat. We’ve also fitted a rocker switch on the rear panel which allows you to shut down the internal switchmode supply that powers the Arduino and relays. This means you can unplug the UPS from the mains, flick the switch and it will gracefully shut down. The batteries will remain charged and it can be powered back on later by flicking the switch again and plugging it 34 Silicon Chip back into mains, or alternatively, using the pushbutton method described above. Inverter control The inverter has a “soft start” feature which ramps its output voltage up over a few seconds when it’s switched on. This would be handy in many situations but is unwanted in a UPS because you need to be able to switch over to inverter power in a very short time. But there’s also a delay of around 0.5-1 second between pressing the on/off button and the inverter powering up, so clearly we have no choice but to run it constantly, ready to switch over. We do need to ensure it’s shut down when the batteries go flat. While it has an internal under-voltage lockout that’s actually very close to the minimum specified voltage for these batteries (20V total, 10V per battery), it isn’t that accurate. We should ideally switch the inverter off before the battery voltage drops that low. And we also need to ensure it’s switched on when the unit is starting up. The inverter we’ve specified is supplied with a small “remote control” box that has a single LED and a pushbutton switch. It’s attached to the inverter via a 4-wire telephone style flat cable. The same controls (LED and button) are provided on the inverter itself. The LED and button share one common connection, with the LED wired between the common terminal and a second wire. A small current flows through this loop when the inverter is powered. The button briefly connects this common wire to a third wire. If the button is held down for around half a second, the inverter starts up or shuts down. We’ve interfaced the inverter with the Arduino using two optocouplers. The Arduino drives one to simulate a button press, shorting the two wires to switch power. The second optocoupler LED is connected in place of the LED on the remote control box and pulls an Arduino pin low when the inverter is operating. A software routine on the Arduino compares the inverter status to the desired status and “presses” the button when necessary to turn it on or off. This isolation allows the Arduino ground to be connected to the battery negative terminal and it can then monitor the battery voltage using a simple resistive divider (100Ω/10kΩ) to one of its analog pins, allowing it to determine the charge state, both for display purposes and to decide when to shut the inverter down. Choosing a case Commercial UPSes of this size are often housed in rackmounting cases. This is convenient since they can then mounted in a server rack, along with the servers they are protecting. But rack-mount cases can also be fitted with feet and used in a standalone manner. We spent some time trying to find a low-cost metal box to build the UPS into but in the end, couldn’t find a good solution. It was also difficult to find a rack-mount case which would fit all the required hardware (due to the required depth of at least 450mm) but we eventually located one at a reasonable price. It’s three rack units tall (3RU = 133.5mm), the standard 19-inch width and made from aluminium by a US company called Bud Industries. It is supplied as a kit which includes the front, back, sides and hardware while the top, bottom, rack rails and handles are available separately. Celebrating 30 Years siliconchip.com.au The completed UPS (sans lid!) showing the internal layout. The two batteries are clamped under the punched metal plates at the right while the pure sinewave inverter is on the left. We’ll show the layout and construction detail next month. While you might think the silver Deta four-outlet power point on the rear panel seems like gilding the lily somewhat, they’re only a couple of dollars dearer than a boring old white one . . . and it really looks the part, matching the aluminium case! We haven’t bothered fitting the rack rails or handles to our prototype but they aren’t expensive or difficult to obtain. Luckily, availability is good; the case is available from US electronics retailers Digi-Key and Mouser and they both offer free express international delivery if you order the required items together (see parts list). We also fitted it with instrument feet from Jaycar as there are quite a few exposed screw heads on the underside. We’ve opted for a solid base and vented lid as the inverter and batteries can get quite warm during operation. The side panels have many drilled holes which provides decent ventilation and also makes fitting cable clamps quite easy. One of the good aspects of using a natural aluminium case such as this one is that it’s quite easy to Earth the entire chassis. This is critical for safety; if a mains wire comes loose inside and contacts the case, it will cause the fuse to blow. Otherwise, the case could become live which would be very dangerous. We have Earthed the rear and bottom panels separately, with the other panels electrically connected via common screws and also direct panel contact. Sourcing a battery charger Master Instruments can supply two suitable battery chargers, the Fuyuan FY2902000 (2A) or FY2907000 (7A). The 2A version has a standard 2.1mm inner diameter DC plug so you just need a matching socket while the 7A version uses an XLR plug; suitable sockets are readily available (eg, from Jaycar). Other chargers are available but they may come with a different plug and so you will need to find a matching socket. Or alternatively, cut the plug off and crimp some eyelet terminals onto the bare wires for direct connection to the battery terminals. Regardless of which charger you use, it must be designed specifically for LiFePO4 batteries and have a charge termination voltage of 29.2V. While these batteries are quite robust, they may not last very long if regularly charged to the wrong voltage. Control algorithm The most critical part of the Arduino software is the “mains-good” detection algorithm. The transformer secondary voltage, which is a proxy for the mains voltage, is siliconchip.com.au sampled 1000 times per second, ie, 20 times per cycle for a 50Hz supply. These samples go into a 32-sample buffer, so there is just over one full cycle worth in the buffer at all times. To convert this into a meaningful number, we calculate both a root-mean-squared (RMS) average and measure the peak-to-peak voltage. For a sinusoidal signal, the RMS value is exactly equal to the peak-to-peak value divided by 2 x √2, or approximately 2.8284. The peak-to-peak calculation is usually quicker to pick up excessively high mains voltage while the RMS calculation is faster at detecting a brownout or blackout, the latter often being detected within a quarter of a cycle (5ms). The RMS calculation starts by taking the average of the ADC readings to establish a ‘mean’ that we can reference the values to. We then add the squares of the differences between our values and the mean. Then we divide the sum by the number of samples – this is our mean of squares, and its square root is the RMS value, after which the scaling factor is applied to get our actual RMS value in volts. As soon as the mains voltage reading is found to be outof-bounds, the relay switching sequence begins, to transfer the load(s) over to the inverter. The unit will not switch the load back to mains operation unless the mains voltage stays within a tighter set of bounds for several seconds. This increase in the strictness acts as a kind of hysteresis, preventing the unit from switching back and forth if the mains voltage is on the cusp of being too high or too low. The unit will simply switch to the inverter in this case and won’t switch back until the mains voltage goes back to a more normal value. The transformer introduces quite a bit of error into the voltage measurements made by the Arduino (and to a lesser extent, resistor and regulator tolerances). We will provide a calibration process to allow you to set the thresholds more accurately in a later article. Construction There will be detailed construction and wiring details in the second article in this series, to be published in the June issue. That article will also have details on the control shield circuitry, including assembly instructions required to build the driver shield. SC Celebrating 30 Years May 2018  35