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Power the brightest LEDs on Earth with this simple linear supply! Want to run one or more Luxeon 1W Star white LEDs from a 12V battery or a DC plugpack? This circuit allows you to do it and allows for dimming as well. It uses bogstandard parts, including a 555 timer and two 3-terminal regulators. By PETER SMITH Back in the May 2003 edition, we described two of the brightest LEDs available anywhere – Lumileds’ 1W and 5W Luxeon Stars. Understandably, the article generated lots of interest, with many readers resolved to wiring up their own Stars and seeing this dazzling new technology first hand. Unlike the (much) smaller 3mm and 34  Silicon Chip 5mm LEDs that we’re all familiar with, driving these new devices with just a series current-limiting resistor can be a bit risky. A better way is to power them from a constant current source, to achieve full brightness without exceeding maximum ratings. This simple circuit will allow you to drive the 1W version (any colour) with the maximum rated current and keep it cool as well. It also gives you control over LED brightness, which can be varied from about 10% to 100% with an on-board potentiometer. How it works The circuit diagram for the power supply appears in Fig.1. It consists of two main elements – a current source and a variable duty cycle oscillator. Let’s examine the current source first – it uses a LM317 3-terminal regulator (REG1). Commonly, these regulators are programmed with two resistors to provide a particular output voltage, as shown in Fig.2. To maintain the programmed output voltage, the regulator keeps the difference between its ‘ADJ’ and ‘OUT’ terminals equal to an internal 1.25V reference. Fig.3 shows that without the resiswww.siliconchip.com.au Fig.1: the circuit is based on an LM317 regulator and 555 timer. The regulator is connected as a 350mA constant current source, with its ‘on’ time varied by the 555 to control LED brightness. tor to ground (R2), the regulator still maintains 1.25V across R1. But rather than a regulated voltage, we now have a constant current source proportional to 1.25V/R1. Calculating R1 for our 350mA Star is easy: R1 = 1.25V/350mA = 3.57Ω Referring again to the main circuit (Fig.1), you can see that ‘R1’ consists of 3.9Ω and 47Ω resistors in parallel, for a total resistance of 3.6Ω. Unlike the simple schematic in Fig.3, the output is connected back to the ‘ADJ’ pin via a 120Ω resistor. This additional resistor has virtually no effect on the programmed current and its purpose will become clear in a moment. For our description thus far, we’ve assumed that JP1 is open circuit. But what happens when it’s shorted? Well, when transistor Q2 switches on, the LM317 begins to regulate the output voltage (instead of current), with the 120Ω and 47Ω resistors forming ‘R1’ & ‘R2’ as depicted in Fig.2. The output voltage will be: VOUT = 1.25V(1 + 47Ω/120Ω) = 1.7V Taking into account Q2’s collector to emitter saturation voltage, the output voltage is slightly higher than our calculated value. However, it’s still less than the minimum forward voltage of www.siliconchip.com.au the red/amber and white/blue Stars (about 2.3V and 2.8V respectively), so the LED will be switched off. Pulse-width modulation Rather than reducing drive current, Luxeon recommends using pulse width modulation (PWM) switching to reduce the brightness of the Star. This results in a much more colour-uniform light output, right down to minimum brightness. If you just vary the drive voltage in a linear fashion, the Star’s light output tends to become yellowish as the drive voltage is reduced. PWM switching is just a matter of switching the LED on and off at a fixed frequency and varying the duty cycle (on/off time) to vary brightness. With a high enough frequency, the switching Fig.2: the LM317’s output voltage is set with two resistors. Main Features • • • • Simple construction Variable LED brightness Plugpack or battery powered Drives 1 to 4 x 1W Luxeon Stars effects are invisible. This is due to the long persistence of the phosphors (in white LEDs) and the natural light integration of the human eye. As you’ve probably guessed, transistor Q2 in our circuit is responsible for switching the current source (REG1) to give PWM control. Q2 is driven by Q1, which is simply a buffer and inverter stage. The real work is performed by IC1, an old 555 workhorse. IC1 is configured as a free-running oscillator (or “astable multivibrator”) with a nominal frequency of about Fig.3: with a single resistor between its ‘OUT’ and ‘ADJ’ terminals, the LM317 acts as constant current source. December 2003  35 Fig.4: these two waveforms were captured at the output of the supply. With the brightness pot (VR1) set to minimum resistance, only 9% of the power is delivered to the LED. 1.1kHz. Diodes D3 & D4 provide independent charge and discharge paths for the 10nF capacitor, allowing the duty cycle to be controlled without much variation in the frequency of oscillation. As a result, trimpot VR1 can vary the duty cycle from 9% to 99% (see Figs.4 & 5), resulting in an average current of between about 30mA and 346mA. Even at minimum brightness, you can still read a book by one of these little marvels! When driving 3 or 4 LEDs in series, the circuit input voltage can exceed 18V (the 555’s max. supply voltage), so we’ve provided a separate +5V supply for the 555 and associated circuitry. This is generated by REG2, a 78L05 low-power regulator. Input to REG1 & REG2 is via series diodes D1 & D2, ensuring nothing bad happens if the supply is accidentally reversed. Input power (single LED) For a single Star, the input voltage should be between 7.5V and 12.5V. This means that you can drive it from a 7.5V or 9V plugpack (min. 500mA rating), or a 12V SLA battery. 12V plugpacks are generally not suitable, Fig.5: when trimpot VR1 is at the maximum setting, a duty cycle of 99% drives the LED at virtually full brilliance. because they put out excessively high voltages when lightly loaded. The maximum input voltage that can be applied is limited by available power dissipation. When properly mounted to the specified heatsink, the temperature rise of regulator REG1 is about 25°C above ambient with a 12.5V input. This is well within the regulator’s rating and the heatsink won’t burn your fingers or start a fire! The minimum input voltage is governed by circuit overhead (about 3.9V) and the LED’s forward voltage (about 3.4V for white or blue Stars). So for a single white or blue Star, about 7.3V minimum is required to obtain full brilliance. input voltage has been established. Alternatively, monitor the voltage drop across the 3.9Ω resistor while slowly increasing the input voltage. When it reaches 1.25V, the LM317 is in regulation and therefore sourcing the full 350mA. Using a lower voltage than recommended will result in less than maximum brightness, whereas higher voltages may (eventually) overheat the assembly. The LM317 regulator has in-built over-temperature protection and can survive short-term abuse. However, extended high temperatures will eventually destroy it and burn (or delamin­ate) the PC board. If the heatsink is too hot to touch, then the input voltage is too high! Note: do not attempt to drive these LEDs in parallel. Although possible, Driving multiple Stars Up to four stars (any colour) can be driven in series. The recommended voltage ranges are shown in Table 3. This should be considered as a rough guide only, as the total voltage across any LED string will vary considerably, according to LED colour and individual device characteristics. The optimum input voltage can be established using a variable power supply. When the LEDs just reach maximum brilliance, the minimum Table 2: Capacitor Codes Value μF Code 220nF 0.22µF 100nF 0.1µF 10nF .01µF   1nF .001µF EIA Code IEC Code   224 220n   104 100n   103   10n   102    1n Table 1: Resistor Colour Codes o o o o o No.   2   2   1   2 36  Silicon Chip Value 3.3kΩ 1kΩ 120Ω 47Ω 4-Band Code (1%) orange orange red brown brown black red brown brown red brown brown yellow violet black brown 5-Band Code (1%) orange orange black brown brown brown black black brown brown brown red black black brown yellow violet black gold brown www.siliconchip.com.au parallel configurations require voltage-matched devices. Power supply board assembly All parts (except for the LED) mount on a single PC board, coded 11112031. Using the overlay diagram in Fig.6 as a guide, begin by installing the two wire links, followed by all of the 0.25W resistors. Diodes D1-D4 can go in next, making sure that you have the cathode (banded) ends oriented as shown. Follow up with the two transistors (Q1 & Q2), 78L05 regulator (REG2) and trimpot (VR1). All remaining components, apart from the LM317 (REG1) and its heatsink, can now be installed. Note that the 555 timer (IC1) and electrolytic capacitors (100µF & 10µF) must go in the right way around. The final step involves mounting the heatsink and installing the regulator. To do this, first secure the heatsink firmly to the PC board with two M3 x 6mm screws, nuts and flat washers. Next, bend the regulator’s leads at 90° about 3mm from the body and temporarily slip it into position. Verify that the hole in the regulator’s tab lines up with the hole in the heatsink, which should in turn match the hole in the PC board underneath. If all is well, you can now remove the regulator and apply a thin smear of heatsink compound to both the rear of the metal tab and the mating area on the heatsink surface. Finally, slip the regulator back into position and fasten it securely to the heatsink & PC board with an M3 x 10mm screw, nut and washer. Solder and trim the leads to complete the job. Note: the metal tab of the regulator is internally connected to the ‘OUT’ terminal, so the heatsink will be live. The LED (and any other uninsulated wiring) must not be allowed to make contact with the heatsink! If you don’t like this idea, then you can mount the regulator to the heatsink using an insulating pad and washer. The down-side to this arrangement is higher regulator temperature. Fig.6: follow this diagram closely when assembling your boards. To make the job easier, leave the heatsink and regulator (REG1) until last. This view shows the completed power supply PC board, prior to fitting the LED carrier board. The heatsink keeps REG1 cool. LED mounting The Star’s emitter and collimating optics are mounted directly onto an aluminium-cored PC board. In most cases, no additional heatsinking is required. However, a small heatsink www.siliconchip.com.au reduces junction temperature significantly and ensures maximum LED life. Just about any small aluminium heatsink with a flat area large enough to accommodate the Star’s 25mm footprint can be pressed into service. For example, an old 486 PC processor December 2003  37 The 1W Star LED is available in seven colours: white, green, cyan, blue, royal blue, red and amber. They can all be driven by this power supply. Fig.7: here are the full-size etching patterns for the two PC boards. Check your etched boards carefully before installing the parts. heatsink would probably be ideal! For experimentation purposes, an area of PC board copper also does the job nicely. This is the purpose of our simple “carrier” board, which also provides a convenient mounting and terminating method for the LED module. LED carrier board assembly Before mounting the LED module, make sure that the mating surface is completely smooth. If there are any “lumps” of solder, then they must be removed using desoldering braid. Apply a thin smear of heatsink compound to the rear of the LED module as well as to the mating surface (copper side) of the PC board. The module can then be attached to the PC board using two M3 x 6mm screws, nuts & washers. With opposing corner holes, the module could be mounted one of two ways. To determine the correct orientation, look for a tiny copper “dot” next to one of the corner solder pads. This indicates positive (+) and should be aligned as shown on the overlay diagram (Fig.6). Once mounted, all that remains is to wire up the LED anode (+) and cathode (-) terminals, provided in the form of two solder pads on opposite corners of the module’s PC board. Solder a short length (about 15cm) of wire to one of the pads and pass it through the neighbouring hole in the carrier board. Repeat for the opposite pad and then twist the two wires together under the board. Secure at the end of the carrier board with a small cable tie to ensure that no tension can be applied to the solder joints. Before connecting your LED to the power supply output terminals, it’s important to verify that the supply is working properly. A faulty supply could destroy your $30+ investment in a blinding flash! Testing Connect a 10Ω 5W resistor directly across the power supply output terminals. Position the body of the resistor so that it is clear of your workbench Table 3: A Rough Guide To Input Voltage Ranges No. of Stars Min. Voltage Max. Voltage 1 2 3 4 7.3V 10.7V 14.1V 17.5V 12.5V 15.9V 19.3V 22.7V 38  Silicon Chip (and your pinkies!), as it could get extremely hot. If you fitted a jumper shunt on JP1 earlier, remove it for now. Plug in your chosen DC power source and hit the “go” switch. Assuming there are no ominous bangs or puffs of smoke, use your multimeter to measure the voltage drop across the 10Ω resistor. If the supply is sourcing the expected 350mA (nominal) of current, your measurement should fall within the 3.2V - 3.8V range. Power off, disconnect the resistor and then re-apply power. Measure the voltage between pins 1 & 8 of the 555 (IC1). These are the power supply pins, so your meter should read 5.0V or thereabouts. All done! Assuming your board passed the tests, hook up the LED leads to the output terminals. Be particularly careful that the anode (+) terminal of the LED connects to the positive (+) output, as the LED module will be destroyed if reverse voltage is applied. Hold your breath and power up. Don’t stare directly into the LED beam at close range, as it is (according to Luxeon) bright enough to damage your eyesight! Brightness control To enable brightness control, install a jumper shunt on JP1. Now by rotating VR1, you should be able to vary LED intensity from dim to almost full brightness. LED carrier board mounting To make a neat “one-piece” module, the LED carrier board can be mounted www.siliconchip.com.au Parts List 1 PC board, code 11112031, 80mm x 66mm 1 2.5mm PC-mount DC socket 1 2-way 2.54mm terminal block 1 2-way 2.54mm SIL header 1 jumper shunt 1 Universal ‘U’ heatsink 4 M3 x 10mm tapped spacers 1 M3 x 10mm pan head screw 6 M3 x 6mm pan head screws 6 M3 flat washers 3 M3 nuts Red & black light-duty hook-up wire Heatsink compound 1 9V DC 500mA (min.) plugpack (see text) 1 100kΩ miniature horizontal trimpot Take care to ensure that all polarised parts are correctly oriented when building the power supply PC board. Note that this prototype PC board differs slightly from the final version shown in Fig.6. Semiconductors 1 LM317T adjustable voltage regulator (REG1) 1 78L05 +5V regulator (REG2) 1 555 timer (IC1) 2 PN100 transistors (Q1, Q2) 2 1N4004 diodes (D1, D2) 2 1N4148 diodes (D3, D4) 1 1W Luxeon Star LED w/optics (see text) Capacitors 1 100µF 35V PC electrolytic 1 10µF 16V PC electrolytic 1 220nF 63V MKT polyester 1 100nF 63V MKT polyester 2 10nF 63V MKT polyester 1 1nF 63V MKT polyester The completed LED carrier board provides a convenient method for mounting the 1W Star LED module and also provides heatsinking. piggyback style on the power supply board. To do this, insert an M3 x 25mm screw in one corner hole and slide on a 15mm spacer from the bottom. Wind up an M3 nut to hold the spacer in place, then repeat for the other corner. The completed assembly can now be slipped into place in the two corner holes of the power supply board, replacing the existing M3 x 6mm screws (see photos). With the carrier board installed, you’ll note that the brightness trimpot (VR1) is no longer easily accessible. If you need to continually vary the brightness with the board in-situ, then you can either reposition the trimpot to the opposite (copper) side of the board or install an external potentiometer. www.siliconchip.com.au When installing an external pot, keep the wire length as short as possible (say, no more than about 50mm) and twist the three connecting wires tightly together. Where to get the Stars The 1W Luxeon Star LEDs are currently available from the Alternative Technology Association at www.ata. org.au You can check out their on-line shop at http://www.bizarsoftware. com.au/index.html Lumileds also manufacture higher output (5W) white and blue Stars. Naturally, these devices are considerably more expensive that the 1W versions and require more elaborate heatsinking. Their higher current requirements (up to 700mA) make them unsuitable Resistors (0.25W, 1%) 2 3.3kΩ 1 120Ω 2 1kΩ 2 47Ω 1 10Ω 5W 5% (for testing) 1 3.9Ω 5W 5% Parts for optional LED carrier 1 PC board, code 11112032, 80mm x 26mm 2 M3 x 15mm untapped brass or nylon spacers 2 M3 x 25mm pan head screws 4 M3 x 6mm pan head screws 6 M3 nuts 4 M3 flat washers 1 small cable tie for use with this supply. Detailed technical information on Luxeon Star LEDs can be obtained from the Lumileds web site at www. SC lumileds.com December 2003  39