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Light level rivals halogens – at MUCH less power! 10W LED Floodlight Design by Branko Justic* Words by Ross Tester LEDs have come a long, long way in recent times. Who would have thought that you could have a LED floodlight with brightness which rivals that that of incandescent lamps of yesterday? This compact LED floodlight is efficient, simple to build and cheap! W even those figures can be quite deceiving! (See the panel hen the Australian government announced “How Bright?”). its ban on incandescent lamps in 2007, one of our first thoughts was “what are we going LED array to do for floodlights?”. The majority of high-power LEDs these days are made Mainly powered by halogen lamps of 150 and 500W from a number of individual LEDs forming an “array”. In ratings, these floodlights have become incredibly this case, it’s a 3x3 matrix of pure white LEDs, each one popular in domestic, industrial and public lighting rated at 1.2W. The net result is a single LED light source installations. rated at roughly 10W (there are some losses). Until quite recently, there simply wasn’t a viable The array itself measures about 1cm square but with alternative to the halogen lamp, often called a QI lamp, mounting, the whole assembly measures about 2cm square which stands for Quartz Iodine (the construction and – still pretty small compared to a halogen lamp. Attached gas inside), not Quite Interesting (the TV show). to each side are tabs for soldering power leads. But with the recent spectacular developments in The good news is that in this kit, the LED array is already LEDs, there is now a very effective replacement for fitted to the lamp housing (which acts as a heatsink) and power-hungry halogen lamps. a reflector drops into place around the LED array. So the To get this into perspective, halogen floodlights hardware side is easy! comparable in size to this LED floodlight generally use 150W lamps; 15 times the power! Their Driver circuit light output varies depending on type but The downside of a LED, especially a typical figure is about 2300 Lumens, or an ultra-high-brightness type, is that about 15 Lumens per watt (2300/150). And you cannot simply connect power to that really only happens with a new lamp it. LEDs need to be “driven” by an apas light output drops with age. propriate supply or they will be burnt The light output from this LED floodlight out very quickly. output is not as high, at 720 Lumens and With low-power LEDs, it’s easy; a suittherefore, 72 Lumens per watt. able current-limiting resistor will do the OK, so that’s about one third the light A close-up view of the LED array, output of the Halogen but almost five times already mounted in the lamp case. job. But high-power LEDs need a driver circuit to suit the type of LED/number of as efficient. You can quite clearly see the 3x3 LEDs. And this project has the answer But as we show in our measurements, pattern of LEDs in the centre. 20  Silicon Chip siliconchip.com.au to this question as well: a tiny (30 x 23mm) PCB which contains the constant current driver circuit. It’s a simple circuit but perfectly adequate for the purpose. Many (probably most) high-power LED drivers use a switch-mode driver but they are more complicated and usually generate some (and some a LOT!) radio-frequency interference, which must be suppressed. This two-transistor circuit shown in Fig.1 doesn’t have this drawback yet still manages about 80% efficiency, when used with a 12V source. It has only two connections, power in and power out and it can be connected in series with the positive or negative side of the LED array. Ideally though, it should be in the negative side (ie, between the LED array and the negative supply) because that way the collector of the main regulator transistor (a TIP42C) will not need to be insulated from the lamp housing (the collector and the lamp housing will both be at the negative potential). How it works As mentioned, the TIP42C is the current control transistor, biased on by a BC327. It works in the following way: the base-emitter junction of the BC327 effectively monitors the voltage developed across the two 1.2 resistors connected in parallel. These act as a sensing resistor for the current passed by the TIP42C and therefore, the LED array. Since the two resistors in parallel give an effective resistance of 0.6and the base-emitter junction of the BC327 has a nominal voltage across it of 0.6V, this sets the emitter current of Q1 to 1A – exactly what we want for this array. You may ask why there are two 100 resistors connected in series with the collector of the BC327. There is no magic in this; these two values provide sufficient base current for the TIP42C under all voltage conditions to which it is likely to be connected. You may also wonder why we are presenting an analog regulator when, in this very same issue, we are presenting + A A  K 10W LED ARRAY K A  A K A C 100  0.5W  K 1.2  0.5W K 1.2  0.5W B 4.7nF 100  0.5W BC327 E C B Q1 TIP42C E 4.7nF 10W LED DRIVER C E B 1.2 1.2 C + TIP42C NOTE: SCREW TAB OF Q1 TO LAMP CASE FOR HEATSINKING E B C Fig.1: the driver circuit, which is a simple constantcurrent regulator, drives the 3 x 3 LED array with a current of about 1A. siliconchip.com.au C E B 4.7nF Q1 100 100 Q2 BC327 C TIP42C – SC A K 1 Hardware Pack, consisting of lamp housing, gland, cable and pre-mounted 3 x 3 LED array 1 PCB, coded K318, 30 x 23mm 1 two-way screw terminal block, PCB mounting 1 TIP42C PNP power transistor (Q1) 1 BC327 PNP transistor (Q2) 2 4.7nF ceramic capacitors 2 100Ω 0.5W resistors 2 1.2Ω 0.5W resistors 1 length 2-core insulated power cable (length to suit) 1 M3 x 10-15mm screw with nut and washer. 4.7nF E B 2012  K  K A – 12V BATTERY Q2 BC327 A   +  K a highly efficient switching regulator, the MiniSwitcher. In that article, Nicholas Vinen has poured scorn on analog regulators. But the MiniSwitcher is a voltage regulator and we need a current regulator for this application. The analog current regulator has several advantages; cheaper, smaller and simpler. And in any case, we are not too worried about efficiency which, as already noted, is above 80%. That means that it will dissipate between 2 and 3W but Parts list – 10W LED Floodlight A  The photo doesn’t really do it justice: it’s so bright, it’s dazzling! – TO LED ARRAY (– TERMINAL) K318 TO BATTERY (0V) Fig.2: PCB component overlay with a same-size photograph at right. February 2012  21 In this and the photo at right, we’ve disassembled the lamp housing to show how it all goes together. The reflector “drops into” the space above the LED array – but be careful that it doesn’t short the two solder connections (on each side of the array). If there is any doubt, we’d be inclined to put a washer or two under the reflector where the screws hold it in place. This photo shows the disassembled lamp housing from the rear. Note that in this shot, neither the holes for the PCB mounting screw nor the cable gland have been drilled (the cable gland hole can be seen in the pic at left). The blue item second from front is the reflector, again seen in the photo at left. Don’t be tempted to leave out the gaskets – they keep the whole thing waterproof when used outside. that is not an issue since we have a good heatsink available in the form of the lamp housing; fastening the TIP42C to the case will provide the cooling required. Some current regulators of this configuration can be prone to oscillation, so 4.7nF capacitors are included between the collectors and bases of both transistors; as they say in Ireland, to be sure, to be sure. because it’s big. Solder this in so that the lead access is to the edge of the PCB. Construction The PCB component overlay clearly identifies the location and where appropriate, the orientation of polarised components. Of the latter, there are only two, the transistors and of these only one might cause any confusion at all. This is the TIP42C power transistor, which must be soldered into the board with maximum length of legs emerging, then folded down 90° so that it can be screwed to the case/ heatsink. It should be obvious which way around it goes, even if you don’t identify the legs: when laid flat, its metal tab should be in direct contact with the case. The other (smaller) transistor is soldered in so its orientation matches the overlay on the PCB. Leave the PCB-mounting terminal block until last, if only The LED array As noted earlier, this should be supplied already mounted in its heatsink (complete with heatsink compound), with two terminals ready for soldering the power leads onto. The + and - terminals are clearly marked, though may not be immediately obvious in some light. Ensure that you get them correct and you don’t make the joins too high. PCB mounting As mentioned earlier, the driver PCB can be mounted between the +12V (power) terminal and the LED array or between the LED array and the 0V power terminal. Because the tab of the power transistor (collector) is connected to 0V anyway, it makes sense to mount it in the negative line. Therefore the case itself will be at 0V and no insulating washer will be needed between the collector and the case. Obviously, if you do want to mount the PCB in the positive line, an insulating washer and bush WILL be required A comparison between the 10W LED floodlight featured here and a typical mains floodlight fitted with a 150W QI lamp. These unretouched photos of my fishpond (ignore spiders on bird net!) were shot within moments of each other late at night, at the same speed and aperture (2sec, f4.0), with lamps in the same spot. Inset top right are the images of the two floods. Voltage on the LED was 12.4V while the mains voltage on the QI was 237V. Incidentally, the QI attracted many more fish than the LED! 22  Silicon Chip siliconchip.com.au The PCB mounts in the “bottom” of the rear of the lamp housing by means of a single screw and nut through the tab on the power transistor. There is an insulating washer in this photo – this is only necessary if you want to mount the driver PCB between +12V and the LED array. Place some dollops of neutral-cure silicone sealant underneath the PCB to prevent any short-circuits to the case. And here it is completely assembled and ready for use. It’s close to the same size as a 150W halogen floodlight but has the advantage of using much less power. Another big advantage over halogen lamps is that LEDs aren’t fussed which way you angle them (halogen lamps need to operate very close to horizontal for longest life). The bracket on the rear can be rotated to suit any mounting position. if you want to avoid having the case at +12V. Our photo shows how the PCB is mounted flat in the rear portion of the case. A single 3mm screw and nut through the power transistor tab is all that is necessary to hold the board in place (there are no mounting holes on the PCB itself). The hole for this screw will need to be drilled in the case but position is not overly important (as long as the PCB fits!). To prevent the bottom of the PCB shorting to the case, place a few dollops of neutral-cure silicone sealant underneath the PCB. A waterproof cable gland (which also requires a hole drilled through the case) secures the 12V power cable. In early Oatley kits, you may be supplied a length of 3-core mains flex but 2-core (red and black) flex would be preferable – you never know when someone, sometime, somewhere might accidentally connect mains flex to 230V. If you do use 3-core cable, the green/yellow is not used; the brown lead is used as the +12V lead and the blue becomes the 0V. Remove about 150mm of outer insulation from the cable and cut off (but retain!) all but about 40mm of the red(brown) wire. Bare about 5mm of wire from both the red(brown) and black(blue), pass the cable through the gland so there is about 15mm or so of outer insulation inside the gland. Connect the short red(brown) wire to the + terminal on the PCB. There are two holes already drilled in the lamp case which line up pretty well with the two terminals on the LED array. Pass the black(blue) wire through the hole which lines up with the – terminal on the LED array and carefully solder it on. The length of red(brown) wire which you previously removed goes through the other hole and solders to the + terminal on the LED array. Make sure there are no stray strands of wire which can short to the case. The other end of this red(brown) wire connects to the – terminal on the PCB. That’s right, the “–” terminal. All you need do is connect to a 12V power source, preferably with a switch to turn on and off. And that’s it: the lamp housing comes with a rotatable bracket if you wish to mount the LED Floodlight permanently. With a rather modest current draw of just over 1A, a solar-backed battery supply makes a lot of sense – and the amount of light you get would be rather more than other “solar” systems. SC How Bright Is It? Halogen floodlights are popular because they are so bright; much brighter than “traditional” incandescents and streets ahead of anything fluorescent. That might be about to change! Late at night on a fishpond we set up two mini floodlights – the one described here and a standard 150W halogen. These luminaires are roughly the same physical size, hence the choice. The first observation was just how yellow the halogen was in comparision to the LED – and we had always thought that the halogen lamps gave a nice, white light, especially compared to standard incandescents (see photos opposite for comparison). But the second observation really surprised us. Using our Nikon DSLR as a light meter, we measured the output from both at the same distance and axis. To ensure accuracy of reading, we set the speed to 1/1000s and filled the frame with the floodlight from a distance of 2m. Guess what! The in-camera meter read exactly the same with both floodlights. That’s to within plus and minus half a stop. Given the fact that the LED floodlight draws 10W and the halogen 150W, that’s a pretty powerful message! Finally, after about 15 minutes (the time it took us to make the measurements), the LED Floodlight was warm, but not uncomfortably so. The halogen floodlight? Anyone got any eggs to fry? siliconchip.com.au Where from, how much? This kit comes from Oatley Electronics who hold the copyright on the PCB design. A complete kit of parts which includes all those components listed in the parts list is available from Oatley Electronics for $29.00 + $7.00 P&P. Contact Oatley Electronics on (02) 9584 3561, via email (sales<at>oatleyelectronics.com) or via their website (www. oatleyelectronics.com). * Branko Justic is manager of Oatley Electronics. February 2012  23