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By NICHOLAS VINEN LED DAZZ LER Caution: these LEDs are so bright they will burn your eyeballs! W E’RE NOT JOKING about the warning. Even a brief glance at these white LEDs while they are operating at full power will leave spots before your eyes for quite a while afterwards. They are blinding and they do hurt your eyes. We definitely do not recommend looking at them for even the briefest glance. You wouldn’t look at a laser – well don’t look at these either! And don’t be fooled into thinking that the light output is in a narrow beam; the built-in optics do an excellent job of distributing it over a wide area, with a viewing angle of 130°. So 24  Silicon Chip even if you are well off-axis, they are painfully bright. Until now, you might have thought that a 50W halogen lamp was pretty bright but these LEDs are much brighter (at 900 lumens) and they use a fraction of the power – just 10W. It doesn’t take a mathematical genius to realise that this means big energy savings. At the time of publication, these are the brightest LEDs you can get (as far as we know). They are made by Seoul Semiconductor in Korea and they go by the utterly prosaic description of type W724C0-D1. Their rated brightness is 900 lumens, with a colour temperature of 6300K and a colour rendering index (CRI) of 70. Careful examination shows that they consist of four LED dies connected in parallel under a plastic lens which does a good job of focussing the light. However, this is all academic if you have no way of driving them. LEDs are quite difficult to drive correctly, especially when they need 2.8A at 3.6V. They require an efficient current source, otherwise the high efficiency of the LEDs can be spoiled by wasteful driving circuitry. This project will drive up to six of siliconchip.com.au Fig.1: the typical buck step-down regulator configuration (top) compared to the inverted configuration used in this circuit. In each case, the current flow is indicated during the two phases as I1 and I2. these dazzling LEDs (depending on supply voltage) and it also provides dimming. The efficiency of the circuit is up to 94.5% (see Figs.2-4). The operating supply voltage range is from 12-30V. This design will power virtually any high-brightness LED (1W, 3W, 5W etc) from a low-voltage DC supply, including both white and coloured types. It incorporates a low battery cut-out for 12V or 24V batteries to prevent overdischarge, a standby switch and an integrated fuse. The challenge Driving high-power LEDs is tricky. If driven just below their nominal forward voltage, little current will flow and not much light will be produced. Conversely, if driven just above their nominal forward voltage, they can overheat and burn out. The traditional approach is to use current-limiting resistors and a voltage source such as a 12V battery. This works but it wastes power in the current-limiting resistors and also has the disadvantage that the brightness of the LEDs varies quite markedly with relatively small changes in the supply voltage. As a result, it is much better to drive these high power LEDs from a regulated current source. This new design is a switchmode step-down regulator that uses a single high-current Mosfet. Each 10W LED siliconchip.com.au requires 2.8A at 3.6V and so with a 12V supply, you can drive three 10W LEDs in series. Or with a 24V supply, you can drive up to six LEDs. Unlike some other LED driver circuits, this one needs no adjustment to suit different LED types, except to change one resistor to set the amount of current they require. Hence, this driver circuit is suitable for driving virtually any high-power LED, including those from Cree and Luxeon. RMS parts (www.rmsparts.com.au) as Item Code W724C0-D1. At the time of writing they cost $26 each plus GST (less for bulk purchases). Also available from RMS Parts (but not listed on their website) are the small aluminium PC boards which are used to mount them. These have Item Code STAR-P7 and are available at additional cost (contact RMS Parts for more details). Where to get the LEDs The biggest problem with high-power LEDs is heat. Without an adequate These 10W LEDs are available from Heatsinking Specifications Input voltage ............................................................................................................12-30V Output current .............................................................................................................. 0-3A Input current ..........................................................................................................Up to 3A LED power ........................................................................................................ 1-10W each Number of LEDs .........................................................1-3 (12V supply), 1-6 (24V supply) Efficiency ....................................................................................Up to 94.5% (see graphs) Drop-out voltage ...........................................................................................................0.5V Features ..................................................................................... Dimming, standby switch Line regulation .............................................................<4% output variation over 12-30V Temperature regulation ............................<3% output variation under typical conditions Low battery cut-out settings ............................................................. 12V, 24V or disabled Low battery cut-out, 12V setting ............ 11.5V (negative-going), 12.0V (positive-going) Low battery cut-out, 24V setting ............ 23.0V (negative-going), 24.0V (positive-going) Low battery cut-out current ..............................................<1.5mA for 12V, <2mA for 24V February 2011  25 heatsink they get stinking hot and can be destroyed in a very short time. Even if you don’t run the LEDs hot enough to melt them, if they are operated at a high junction temperature, they will have a short life. So an adequate heatsink is very important. Our prototype set-up used three LEDs running from 12V. We also used a single, large heatsink (Altronics H0550), with the LEDs mounted 50mm apart via the above-mentioned STAR-P7 boards. They are secured using M3 x 6mm machine screws into tapped holes, with Nylon washers to prevent the screw heads from shorting the mounting boards to the heatsink. We will give more details on this later. Switching circuit Fig.2: efficiency curves for the LED driver for 1-3 10W LEDs. The efficiency is higher with more powerful LEDs and with more of them connected in series. Fig.3: the corresponding efficiency curves when using the driver with 5W LEDs. Note that for supply voltages above 16V, more than three LEDs can be driven in series and this will further increase the efficiency. Fig.4: this final graph shows the efficiency curves for 3W LEDs. The efficiency exceeds 90% for three 3W, 5W or 10W LEDs for any supply voltage below 15V. 26  Silicon Chip As noted above, the particular virtue of this LED driver circuit is its exceptional efficiency (up to 94.5%). This is achieved by an unusual switchmode configuration which regulates the LED current (to gain a good understanding of how current regulators work, see the separate article in this issue). Our circuit (see Fig.5) involves an N-channel Mosfet (Q4) driving a string of LEDs connected to the positive rail. We control the LED current with sensing resistor R1 which is between the LEDs and the positive rail. R1 is monitored by comparator IC5 and in conjunction with latch IC4b, controls the duty cycle of the switching pulses applied to the Mosfet. Refer now to Fig.1(a) which shows the traditional “buck” step-down configuration. This uses a switch (or switches) to alternately connect one end of an inductor (L1) to the positive supply rail and ground. It works like this: when the inductor is connected to the positive rail (phase 1), current flows through the inductor to the load, charging up the output capacitor (C1) and storing energy in the inductor’s magnetic field. The rate at which the current increases is limited by the inductor. When the switch changeover occurs (phase 2), current flow from the positive rail is interrupted and so the magnetic field in the inductor begins to collapse and the stored energy is then fed to the capacitor and the load. Again, the rate at which the current through the inductor decreases (and how the magnetic field collapses) is limited by its inductance. The proportion of time that current flows from the positive rail is the duty cycle and this controls the output voltage. This approach is efficient because the energy stored in the inductor’s magnetic field when S1 is connected to the positive rail is returned later, rather than just being converted to heat (as with a linear regulator). The circuit on the righthand side of Fig.1(a) shows a typical arrangement using a Mosfet (Q1) and a diode (D1) as the switching elements. Now take a look at Fig.1(b). These circuits are similar to those shown in Fig.1(a) but the polarity is reversed. The output voltage is now relative to the positive rail instead of to ground and this is how we have arranged the “LED Dazzler” driver circuit, because we wanted to use an N-channel Mosfet (as they are superior to P-channel Mosfets). With the traditional arrangement shown in Fig.1(a), siliconchip.com.au when Mosfet Q1 is on, its drain and source are at the positive supply potential. As a result, its gate must be driven at a higher voltage for it to stay on and this usually involves a charge-pump voltage booster circuit. The inverted arrangement gets around this problem since the Mosfet’s source is tied to ground and no boost circuit is necessary. Efficiency As can be seen from Figs.2-4, the efficiency is excellent for three LEDs driven from a 12-16V supply. It is highest for the 10W LEDs and drops off with increasing supply voltage. For high supply voltages, the efficiency can be improved by adding more LEDs in series. As a general rule, the number of LEDs that can be used is equal to the supply voltage divided by four and rounded down. The efficiency depends largely on the regulator duty cycle. At lower duty cycles, the switch-off time is longer. During this time, the “flywheel” diode (D1) is forward biased and its forward voltage (around 0.55V) represents a significant loss. Inductor core eddy current losses are also higher because the inductor’s ripple current increases. For a single LED with a forward voltage of 3.6V, the voltage loss across the flywheel diode is about 15% of the output voltage (note: energy is also lost in current sense resistor R1 used in the final circuit but this depends only on the output current). Circuit details Now take a look at the complete circuit shown in Fig.5. As mentioned earlier, the driver is a current regulator. We’ll start by describing the switching portion of the circuit, at right. Its main components are current sense resistor R1, inductor L1, Mosfet Q4, Schottky diode D1 and capacitor C1. When Mosfet Q4 is switched on, current flows through a path equivalent to phase 1 in Fig.1(b), ie, from the positive supply input, through fuse F1 and resistor R1 and then through the LED string, inductor L1 and Mosfet Q4 to ground. Conversely, when Q4 is switched off, current circulates in a loop (phase 2) through R1, the LED string, inductor L1 and diode D1. Voltage drops are minimised by using a low-value current sense resistor (R1), a low-value inductor (with low siliconchip.com.au Parts List For LED Dazzler 1 PC board, code 16102111, 118 x 74mm 1 flange-mount plastic case (Altronics H0121) 4 No.4 x 9mm self-tapping screws (supplied with case) 1 PC-mount SPDT right-angle toggle switch (Altronics S1320) 1 knob to suit 9mm potentiometer (eg, Jaycar HK7734) 1 spring washer for VR1 or two flat washers 3 M3 x 10mm machine screws 3 M3 shakeproof washers 3 M3 nuts 1 47µH or 100µH 3A inductor (Altronics L6517 or Jaycar LF1272) 2 PC-mount M205 fuse clips 1 4A M205 fuse 2 2-way terminal blocks (Altronics P2034A) 1 Micro-U TO-220 heatsink (Jaycar HH8502, Altronics H0630) 1 6-pin 2.54mm pitch header (snap into 2 x 3-pin lengths) 2 jumper shunts 2 small cable glands for 3-6.5mm cables (Jaycar HP0720, Altronics H4305) 1 small cable tie 10cm 0.71mm diameter tinned copper wire 1 length of twin-core high-current cable (eg, Jaycar WB1754) to suit installation 1 1kΩ linear 9mm potentiometer (VR1) 1 5kΩ horizontal trimpot (VR2) Semiconductors 1 LM358 dual low power op amp (IC1) DC resistance), a high-current Mosfet and a Schottky diode. This improves efficiency and also means that we can run three 10W white LEDs with an input voltage of just 12.1V. With 2.8A flowing, the input-output voltage differential (drop-out voltage) is just 0.5V. The 1nF capacitor and 22Ω resistor between Q4’s drain and ground form a “snubber”. Basically, when Q4 switches off, it “shocks” the resonant circuit consisting of inductor L1 and any stray capacitance, creating a highfrequency burst which can produce 1 LM285Z-2.5 or LM385Z-2.5 voltage reference (IC2) 1 NE555/LM555 timer (IC3) 1 CD4013 dual CMOS D-type latch (IC4) 1 LM311 high-speed comparator (IC5) 1 78L12 linear regulator (REG1) 2 BC327 transistors (Q1, Q3) 1 BC337 transistor (Q2) 1 IRF540N Mosfet (Q4) 1 IRF1405 Mosfet (Q5) 1 1.5KE36CA or similar 33V AC TVS (TVS1, Jaycar ZR1177) 1 STPS1545F Schottky diode (D1) (Altronics Z0065) 1 1N4148 signal diode (D2) 2 15V zener diodes (ZD1, ZD2) 1 5mm red LED (LED1) Capacitors 2 1000µF 35V electrolytic (Altronics R5185) or 2 x 470µF 35V low-ESR electrolytic (Jaycar RE6338) 1 100µF 16V 1 47µF 35V 4 100nF MKT 3 10nF MKT 2 1nF MKT 1 100pF ceramic 1 6.8pF ceramic Resistors (0.25W, 1% unless stated) 1 270kΩ 1 10kΩ 1 150kΩ 3 8.2kΩ 1 120kΩ 2 5.6kΩ 4 100kΩ 2 2.2kΩ 2 47kΩ 1 1kΩ 3 33kΩ 2 10Ω 1 1Ω (for 1W LEDs) 1 0.33Ω 0.5W (for 3W LEDs) 1 0.22Ω 1W (for 5W LEDs) 1 0.1Ω 5W (for 10W LEDs) electromagnetic interference (EMI). The snubber damps the resulting oscillations, without having much effect on switching (see Fig.8). Control circuity Our first prototype used a switchmode controller IC (a TL3843) to control Q4. However, while this is a logical approach, converting the current flow to a feedback voltage for the IC introduces a delay and we could not get it to operate smoothly under all conditions. February 2011  27 28  Silicon Chip siliconchip.com.au CON1 + S D 2 120k ZD2 15V G 100k 10nF STANDB Y 3 10nF D2 270k LED DAZZLER S1 2 A K E 5.6k A 47k 4 IC2 LM385Z -2.5 2.2k 6 5 8 IN K A 2 3 100k IC1a IC1: LM358 1 VR1 1k 1k K A 2 6 7 100pF 100k 8.2k 1nF 8.2k SET OUTPU T CURREN T Vcc - 5.32V 8.2k VR2 5k 5.6k DIMMING 100 F 16V +12V (nominal) 100k LED1 47k 7 150k GND OUT REG1 78L12 IC1b 47 F 35V 100nF K +2.5V K A B C Q1 BC327 LED 1 1 IC 3 555 8 4 E 10nF 5 3 100nF 4 1 8 7 B C 100nF 9 6 3 5 4 IC4b CL K D Q Q S IN 2 1 12 13 OUT 78L12 Q Q GND 7 Vss IC4a CL K D R IC4: 4013B 8 S 11 R 14 Vdd 100nF B B C Q2 BC337 Q3 BC327 E E C K A K A K A S D K K G A D K A K A S D IRF1405, IRF540N K 10 Q4 IRF540N ZD1, ZD2 A K K A A 1nF L1 100 H 3A+ 1 D2: 1N4148 ZD1 15V G 2 OUTPUT TO LEDS CON2 R1 : 0. 1 55W 0.1 W (10W LEDS) 0.22 1W (5W LEDS) 0.33 0.5W (3W LEDS) 1.0 0.25W (1W LEDS) EXTERNAL LEDS * C1 1000 F 35V STPS1545F 10k 10 D1 STPS1545F LM385Z -2.5 A K +12V * USE UP TO 3 X 10W LEDS FOR 12V OPERA TION OR 6 X 10W LEDS FOR 24V OPERA TION 10 BC327,BC337 6 IC 5 LM311 6.8pF 3 2 5 2.2k 1000 F 35V Fig.5: the complete circuit diagram. The LEDs are driven by a switchmode circuit comprising inductor L1, Mosfet Q4, diode D1 and capacitor C1. This is controlled by IC4, a CMOS latch, which is in turn controlled by IC3, a 555 timer and IC5, a high-speed comparator. IC1, a dual low power op amp, provides a reference voltage for the comparator and also switches the circuit off if the battery voltage is low. 2011 1 3 LOW BA T CUT OUT Vcc TVS1 1.5KA36CA F1 4A SWIT CH ORIEN TATIO N JP2 1 2 24V 12V 1 SC JP1 3x 33k Q5 IRF1405 12-30V INPUT In the end, we solved the problem by designing a switchmode controller using several common ICs. As well as solving the delay problem, this controller also has a maximum duty cycle of 100% which reduces the drop-out voltage. By contrast, common switchmode controller ICs have a typical maximum duty cycle limit of 90-95%. The switching frequency is determined by 555 timer IC3 which is configured in astable mode and runs at 68kHz with a 99% duty cycle. The duty cycle is set by the associated 100kΩ and 1kΩ resistors, while the frequency is set by these two resistors and the 100pF capacitor on pin 2. The reason the duty cycle is so high is described later. IC3’s pin 3 output is connected to the CLK input (pin 11) of IC4b, which is half of a CMOS dual latch IC. This latch controls the regulator’s duty cycle. With the data input (pin 9) held high, when the CLK pin goes high, the latch is “set” and the output (pin 13) also goes high. This drives an emitter-follower buffer stage formed by transistors Q2 & Q3 which in turn drive Mosfet Q4. This buffer stage ensures that Q4 switches quickly despite its gate capacitance (2nF) and is necessary for the Mosfet to operate efficiently at 68kHz. The 10kΩ resistor to ground ensures that Q4 switches off when not actively driven, while the 22Ω resistor forms an RC filter with Q4’s gate capacitance to prevent gate voltage overshoot. Zener diode ZD1 protects Q4 against excessive gate voltage. Q4 switches off when IC4b’s reset input (pin 10) is pulled high, causing its output (pin 13) to go low and turn Q2 off and Q3 on. Q4 remains off until the next timing pulse from IC3, provided the reset pin is not still high (as it could be). If that pin is high when IC3’s output goes high, the latch is not set and that pulse is skipped entirely. Current comparator As noted previously, the sensing resistor R1 is connected between the positive rail and the LEDs. The current through this resistor (and thus the LEDs) is monitored by IC5, an LM311 high-speed comparator. It controls the reset input of latch IC4b, which is pulled up to 12V by a 2.2kΩ resistor. While ever the voltage on IC5’s inverting input (pin 3) is higher than siliconchip.com.au at its non-inverting input (pin 2), its pin 7 output is low and so the latch is not reset. However, when the voltage at pin 2 is higher than at pin 3, IC5’s output goes high, resetting the latch (IC4b) and thus switching off Mosfet Q4. The latch provides hysteresis, so the comparator circuit needs none. The inverting input (pin 3) of comparator IC5 is connected to the lower end of current sense resistor R1 via a divider network made up of two 8.2kΩ resistors. This gives the divider a ratio of 1:1. If powered from 12V, IC5’s valid input voltage range is 0.5-10V. The divider keeps the inputs within this range. The lower end of the divider is connected to a reference voltage which is at Vcc - 5.32V, where Vcc is the supply voltage. Since one end of R1 is connected to Vcc, in order to keep the division ratio constant, the reference voltage must be relative to Vcc. The comparator’s non-inverting input (pin 2) is also connected to a voltage divider, one end of which is at the same reference voltage as before, (Vcc - 5.32V). Its upper end is connected to Vcc via trimpot VR2, while brightness adjustment potentiometer VR1 is in the lower section of the divider. When the LEDs are not lit, there is no voltage across R1, so pin 3 of IC5 is at Vcc - 5.32/2 = 2.66V below Vcc. With VR1 at its minimum setting (ie, maximum resistance), the divider at pin 2 also has a ratio of 1:1 (assuming VR2 is trimmed correctly) and so IC5’s non-inverting input will also sit at about Vcc - 2.66V. As VR1 is turned clockwise, its resistance drops and the voltage at pin 2 of IC5 is reduced. As a result, IC5’s output switches low and releases the latch reset on IC4b. Mosfet Q4 then switches on at the next clock pulse from IC3 (ie, when pin 13 of IC4b goes high and turns on Q2). When it does, current through the LEDs increases and so does the voltage across R1, in turn reducing the voltage at pin 3 of IC5. When the current through the LEDs is high enough, the voltage at pin 3 of IC5 will be lower than at pin 2, causing the comparator’s output to go high. This then resets the latch and turns off the Mosfet. As a result, the current through the LEDs is regulated to a level controlled by the settings of VR1 and VR2. Fig.6: a simplified differential amplifier composed of an op amp and four resistors, two each of two different values. Its output voltage is calculated as Vout = VG + (Vin+ - Vin-) x (Ra/Rb). Note that if VR1 goes open circuit, Q4 will switch off. Worn pots can sometimes go open circuit so it’s important for the circuit to “fail safe” in this condition. Frequency compensation The 1nF frequency compensation capacitor between IC5’s inputs is critical, as it rolls off the comparator’s frequency response. It forms an RC filter with the resistors in the two voltage dividers and limits the rate at which the two input voltages can vary. Without it, the regulator’s duty cycle can swing between extremes on a pulse-by-pulse basis. For example, consider a scenario where we want a duty cycle of 50%. Without the capacitor, the average current through the LEDs may be correct but with the duty cycle oscillating between 25% and 75% at every other pulse. This is undesirable because it can generate sub-harmonics at a fraction of the 68kHz switching frequency, some of which are at audible frequencies. The resulting magnetostriction can result in an annoying high-pitched whine from the inductor. With a stable duty cycle, this does not occur. The 6.8pF capacitor connected between pin 2 of the 555 timer (IC3) and pin 2 of the comparator (IC5) also helps stabilise the regulator. The timing ramp of the 555 timer is a sawtooth pattern and the capacitor AC-couples this signal into the feedback, thereby providing “slope compensation”. This is why IC3 has a high duty February 2011  29 cycle; it results in an appropriate waveform for compensation. The result is that the switch-off current threshold is slightly lower at the end of each pulse than at the beginning and this eliminates duty-cycle “hunting”. Reference voltage circuit Fig.7: the yellow trace is the sawtooth waveform at pin 2 of timer IC3, the green trace the comparator output at pin 7 of IC5, the blue trace Mosfet Q4’s gate drive and the pink trace is the current through the LEDs. The frequency has been lowered because of probe capacitance. Current through the LEDs builds while Q4’s gate is high and decays while it is low. The positive edge of the comparator output (green) corresponds with Q4 switching off (blue) and the beginning of the timer ramp (yellow) corresponds with it switching back on. The Vcc - 5.32V reference in this circuit is derived from a 2.5V reference voltage by op amp IC1a, which is configured as a differential amplifier. Fig.6 shows a simplified version of IC1a’s circuit. Its output is the difference between its two inputs multiplied by its gain and that output can be shifted by a predetermined offset voltage which we will refer to as “VG” (virtual ground). In our case, the differential amplifier’s inputs are connected to two voltage dividers, each consisting of resistors Ra & Rb. These dividers set the gain of the amplifier and since Ra is 100kΩ and Rb is 47kΩ, the resulting gain is about 2.13. So let’s plug in some values. VG is in fact Vcc, Vin- is the 2.5V reference (provided by IC2) and Vin+ is tied to ground (ie, 0V). So the output voltage is: Vout = VG + (Vin+ - Vin-) x Ra/Rb = Vcc + (0 - 2.5V) x 2.13 Simplifying this gives: Vout = Vcc + (-2.5V) x 2.13        = Vcc - 5.32V Low battery cut-out Fig.8: the green trace at top is the voltage across flywheel diode D1, while the yellow trace at the bottom is Mosfet Q4’s gate waveform. When Q4 switches off, the diode becomes forward biased and quickly clamps the rising voltage from the inductor. A small amount of ringing can be seen when this occurs, which is quickly damped by the snubber. After a short period, the voltage across D1 drops to below 500mV despite carrying a few amps. 30  Silicon Chip IC2 is an LM358Z-2.5 (or LM258Z -2.5) shunt regulator and this provides the 2.5V reference for IC1a. It is also used by IC1b for the low battery cut-out detector. This “micropower” voltage reference diode has 1-3% accuracy (depending on the part used) and operates with a current as low as 10µA. The 2.5V reference is fed to pin 5 of IC1b, its non-inverting input, via a 2.2kΩ resistor. The 2.2kΩ and 150kΩ feedback resistors provide hysteresis (0.5V for a 12V supply and 1.0V for a 24V supply). This prevents the circuit from rapidly switching when the supply voltage is marginal, due to feedback caused by the voltage drop along the supply leads. The supply voltage (Vcc) is divided and applied to pin 6 of IC1b (ie, to its non-inverting input). The division ratio is set by jumper JP1. For 12V batteries, the ratio is 120kΩ:33kΩ and the low-battery cut-out voltage 11.5V. For 24V batteries, the ratio is siliconchip.com.au 120kΩ:16.5kΩ and the cut-out voltage is 23V. When Vcc is above the threshold, the voltage at pin 6 of IC1b is higher than at pin 5, so its pin 7 output goes low. As a result, current flows through LED1 and its 5.6kΩ current-limiting resistor. This turns on PNP transistor Q1, supplying current to IC5 and regulator REG1. If Vcc drops below the negativegoing threshold, the output of IC1b goes high, switching Q1 off and powering down most of the circuit. LED1’s forward voltage drop (about 2V) allows Q1 to be turned off despite IC1b’s output only being able to swing up to about Vcc - 2V. LED1 isn’t intended as a power indicator; the high brightness LEDs do a fine job of that. If jumper JP1 is left open, the low battery cutout is disabled since pin 6 of IC1b is pulled up to Vcc by the 120kΩ resistor. In this case, D2 prevents pin 6 from exceeding IC1b’s maximum input voltage. REG1 has a dropout voltage of about 1.7V so when Vcc is below 13.7V, the nominal 12V rail at its output is not regulated. All the components it powers run down to at least 9.8V, below which the low battery cutout normally activates. With a 12V supply voltage, because of Q2’s base-emitter voltage, Q4’s maximum gate voltage is around 9V. That’s still enough to turn it on fully (see the IRF540N datasheet). Standby switch When the standby switch (S1) is in the off position, it forces the low battery cut-out to operate by connecting pin 6 of IC1b to ground. In this condition, 1.5-2mA is drawn from the supply. The advantage over switching the input supply directly is improved reliability. When the supply connection is made, there is a current inrush which can stress the switch and the power supply. Using the standby switch avoids this. The standby switch has a 3-pin header to select which position (up or down) is off, depending on how the unit is mounted. Circuit protection The PC board is fitted with a 4A fuse (F1) to protect against circuit faults, while Mosfet Q5 provides protection against reverse supply polarity. This Mosfet acts like a diode but has a siliconchip.com.au Choosing Alternative Parts The flywheel diode (D1) is specified as an STPS1545F. Other Schottky diodes in the TO-220AC (two lead) package can be used if they have a current rating of 7A or more and a reverse breakdown voltage of at least 30V. Diodes with a lower reverse breakdown voltage are better because generally, the higher the reverse breakdown voltage, the higher the forward voltage. It’s also possible to use two Mosfets of the same type for Q4 & Q5 (either IRF540N or IRF1405) but doing so will reduce efficiency. If they are both IRF540Ns, the dropout voltage will increase. The inductor can be either a 47µH 3A high-frequency toroid from Altronics (Cat. No L6517) or a 100µH 3A ferrite choke from Jaycar (Cat. LF1272). The 100µH inductor provides smoother current regulation but this makes no real difference when driving LEDs. Altronics also has a 3A 100µH inductor but it has a higher DC resistance than either of the specified parts, so it is not ideal. A 5A inductor could also possibly be used but will be a tight fit in the case. Finally, although we have specified a bidirectional TVS, a unidirectional TVS can be used instead (eg, Altronics Z0127). However, if this part is used, the fuse will blow if the supply polarity is reversed. much lower forward voltage, thereby improving efficiency. If the supply polarity is correct, Q5’s gate is pulled up via a 100kΩ resistor. This switches Q5 on and completes the circuit to ground. However, if the supply polarity is reversed, the gate is instead pulled low, switching Q5 off and preventing current flow. Zener diode ZD2 protects Q5 from damage by limiting its gate voltage to +15V. The unit can operate from supply voltages up to at least +30V (32V absolute maximum), while the reverse polarity protection circuit works for voltages down to -55V. Any voltage spikes higher than this (eg, due to load dumps) cause transient voltage suppressor TVS1 (1.5KE36CA) to conduct, shunting current away from the circuit. In extreme cases, the fuse may blow. Construction All the parts mount on a singlesided PC board coded 16102111 and measuring 118 x 74mm. Begin by examining the copper side for defects such as hairline cracks or under-etched areas. It’s also a good idea to test fit the larger components (eg, the switch, inductor, 5W resistor, terminal blocks, Mosfets etc) to check that the hole sizes are correct. The specified case has corner pillars so if your board does not already have corner cut-outs, now is the time to cut and file them to shape. Now refer to Fig.9 which shows the board assembly. Fit the three wire links first, followed by all the 0.25W resistors. Table 1 shows the resistor colour codes but you should also check each one on a digital multimeter before it is installed. The 1N4148 diode (D1) and the two 15V zener diodes (ZD1 & ZD2) are next. These devices are polarised so orientate them as shown on the layout diagram. That done, install the transient voltage suppressor (TVS1). If TVS1 has a stripe, then line it up as shown on the layout; otherwise it can go in either way. The specified part (1.5KE36CA) is quite large and must be mounted about 3mm above the PC board, so that it fits between the fuseholder and terminal block. Check that these parts will fit before soldering and trimming its leads. Next, install the four DIP ICs. These can either be soldered direct to the board or you can use sockets if you prefer. Don’t get the three 8-pin ICs mixed up; they are all different so check Fig.9 carefully when installing them. Make sure that each IC is correctly orientated and note that the 14-pin IC (IC4) faces in the opposite direction to IC3 & IC5. The MKT and ceramic capacitors are next on the list (they can go in either way around). After that, fit the three small-signal transistors in the plastic TO-92 packages followed by REG1 and IC2. If necessary, use small pliers to crank their leads out and then back down parallel again so that they fit their mounting holes. Check the markings on these devices February 2011  31 Fig.9: follow this overlay diagram when building the PC board. The holes on either side of L1 allow a cable tie to pass through the toroid and hold it to the board. Below is the completed PC board, mounted inside the case. carefully, to ensure they go in the correct locations. The red 5mm red LED can now go in. Push it all the way down, with its flat edge (indicating the cathode lead) orientated as shown, then solder its leads. Horizontal trimpot VR2 (5kΩ) 32  Silicon Chip can then be installed on the board. Now for the two Mosfets (Q4 & Q5). Once again, these are different types so don’t get them mixed up. To install them, first bend their leads down by 90° about 5mm from their bodies. That done, fit them to the PC board and secure them in place using M3 x 10mm machine screws, shakeproof washers and nuts. Install the screws from the copper side of the board and tighten them firmly before soldering the device leads. Note: do NOT solder the leads first, otherwise you could crack the PC board tracks as the screws are tightened. Next, install the Schottky diode (D1) using the same method but with a micro-U heatsink between it and the PC board. Make sure the heatsink does not touch any other components. Thermal paste is not required between the device tab and the heatsink but it won’t hurt. After that, solder in the two M205 fuse clips, making sure that the small retaining tabs go towards the outside and that they are pushed all the way down onto the board. Solder one pin on each side and then check that the fuse fits before soldering the other. The two 3-way pin header sections for JP1 and JP2 are next on the list, after which you can install the two small electrolytic capacitors on either side of the 78L12 regulator (REG1). Don’t get these capacitors mixed up (they have different values and different voltage ratings). Check to ensure that they are orientated correctly. Follow these with the two screw terminal blocks (CON1 & CON2). Be sure to install them with their wire entry holes facing away from the fuse clips. Now for the current-sense resistor. This must be chosen (with regards to both its value and power rating) to suit the type of LEDs you are using (see parts list & Fig.9). The selected resistor can be mounted flat against the board since it runs at a fraction of its specified rating. Once this resistor is in, install the 3A inductor and secure it using a small cable tie (see photo). This cable tie passes up through one of the adjacent holes, then through the inductor core and finally back down through the opposite hole. Tighten the cable firmly before trimming away the excess. The two large (1000µF) electrolytic capacitors can now be installed. Be sure to use one of the specified types, as their ripple current rating must be over 1A. It’s also important to note that they are orientated differently, so take care here. They must be pushed fully down onto the board before besiliconchip.com.au ing soldered, otherwise they won’t fit in the case. The board assembly can now be completed by mounting the switch and potentiometer VR1. As before, make sure these parts are properly seated against the board before soldering their pins. Finally, if you are going to install the unit in a moving vehicle (car, boat, caravan, etc), it is a good idea to additionally secure some of the larger parts using neutral-cure silicone sealant. These parts include all the electrolytic capacitors, the inductor, the pot and the switch. If this is not done, vibration may cause the leads to eventually crack. Mounting the LEDs The method we used for heatsinking the LEDs is not very practical for a typical installation. If the LEDs are to be mounted on the underside of a horizontal surface (shelf, cupboards, etc), one possibility is to mount them on a large, rectangular aluminium sheet. This sheet will act as the heatsink while being slim enough so that it is not normally visible. The supply wiring could pass through holes drilled in the sheet, with plastic insulation preventing accidental shorting of the supply terminals. Alternatively, the LEDs could be mounted individually on separate heatsinks. Computer CPU heatsinks could be used, as the integrated fan will assist cooling and the fan can be run from the same 12V supply that’s This view shows the fully-assembled PC board. Take care with component placement and orientation and note that IC3 (555) & IC4 (4013B) face in opposite directions. used to power the LEDs. We chose to avoid fans as the LEDs will almost certainly outlive the fan bearings. Mounting the LEDs properly is important. The first job is to solder them to an aluminium substrate circuit board. To do this, spread some thermal transfer compound on the metal underside of the LED, then place it on top of the board and solder the four pins. The board does a good job of drawing heat away from the pads, so you’ll need a hot soldering iron to do this properly. Be sure to solder the LEDs onto their substrate boards with the correct orientation. If you look closely at Table 2: Capacitor Codes Value 100nF 10nF 1nF 100pF 6.8pF µF Value 0.1µF 0.01µF 0.001µF   NA   NA IEC Code EIA Code 100n 104   10n 103    1n 102 100p 101   6p8 6.8 Table 1: Resistor Colour Codes o o o o o o o o o o o o o o o o o siliconchip.com.au No.   1   1   1   5   2   3   1   3   2   2   1   2   1   1   1   1 Value 270kΩ 150kΩ 120kΩ 100kΩ 47kΩ 33kΩ 10kΩ 8.2kΩ 5.6kΩ 2.2kΩ 1kΩ 22Ω 1Ω 0.33Ω 0.22Ω 0.1Ω 5W 4-Band Code (1%) red violet yellow brown brown green yellow brown brown red yellow brown brown black yellow brown yellow violet orange brown orange orange orange brown brown black orange brown grey red red brown green blue red brown red red red brown brown black red brown red red black brown brown black gold brown orange orange silver brown red red silver brown not applicable 5-Band Code (1%) red violet black orange brown brown green black orange brown brown red black orange brown brown black black orange brown yellow violet black red brown orange orange black red brown brown black black red brown grey red black brown brown green blue black brown brown red red black brown brown brown black black brown brown red red black gold brown brown black black silver brown black orange orange silver brown black red red silver brown not applicable February 2011  33 The power LEDs are soldered to small circuit boards and attached to a large heatsink. NYLON WASHERS are electrically connected, so it doesn’t matter if they are bridged with solder when the LEDs are being mounted on the substrate boards. Next, drill and tap the heatsink to accept the mounting screws. That done, solder the power leads to the LEDs, then spread thermal grease on the underside of each aluminium circuit board and screw it down firmly onto the heatsink. Note that you must fit Nylon washers under the screw heads, to avoid shorts to the heatsink. Once they are all in position, their leads can be connected to the driver circuit. Make sure that these leads are securely anchored, so that they cannot come adrift and cause damage. DO NOT under any circumstances run the LEDs without a heatsink. If you do, they can quickly overheat and fail. Test & calibration Fig.10: these full-size panel labels can be copied and used as drilling templates for the front and rear panels of the case. Use whichever pair is appropriate for your installation, so that the labels are the right way up when the box is installed. the boards, you will see “+’ and “-” signs adjacent to the pads, signifying the anode and cathode connections respectively. The cathode side of each 34  Silicon Chip LED is indicated by a tiny black dot on one of the leads (you will need a magnifying glass to see this). Note that the two leads at each end The completed PC board can now be tested and calibrated. Here’s the step-by-step procedure: (1) Install the 4A fast-blow fuse and turn both VR1 and VR2 fully anticlockwise. (2) If you have an adjustable DC supply, then test the low battery cut-out feature first. To do this, leave the power LEDs disconnected and set the supply to 11V. If the supply has a current limit feature, set it to 100mA or less and apply power to the board. LED1 should remain off and the current consumption should be below 2mA. (3) Turn the voltage up to above 12V and check that LED1 turns on. The current consumption should increase to around 12mA. If either condition is not met, switch off and check for mistakes (eg, reversed or swapped components). (4) To calibrate the unit, first determine the rated current for the LEDs you are using. You will need a 0.1-0.47Ω 5W resistor. If you are building the unit from a kit, you should have a spare resistor that will do the job. (5) Connect the resistor in series with a digital multimeter (DMM) set to read amps (on its 5A or 10A range). Switch off and connect this arrangement between the power LED terminals on the PC board, ie, to CON2. (6) Set trimpot VR2 to its mid-point and turn potentiometer VR1 fully anticlockwise. Leave JP1 and JP2 open (ie, no shorting blocks installed). (7) Connect a high-current 12V-24V supply, switch on and check that the current reading on the DMM is close siliconchip.com.au to zero. The supply current should be around 12mA. (8) Slowly turn VR1 clockwise and check that the current through the test resistor eventually starts to rise. Turn VR1 fully clockwise and adjust VR2 for the correct current flow. Note that the LED driver may produce a whine during this step as it delivers a much lower voltage than normal. It should go away when the LEDs are attached. (9) Switch off and connect the LEDs in place of the test resistor. Make sure the LEDs are not pointing at your eyes, turn VR1 fully anticlockwise and re­ apply power. Slowly turn VR1 up and check that the LEDs light and that their brightness is adjustable. Trim VR2 for the correct maximum current. (10) Switch off and install a shorting block on JP1 to suit your installation (either 12V or 24V). If you are running the LED driver from a mains-powered supply you can leave it out but it is better to use the 12V setting, to reduce the inrush current when power is first applied. Preparing the case The PC board has been designed to fit inside an Altronics H0121 flangemount plastic case. All you have to do is drill the necessary holes in the front and rear panels, fit the labels, mount the PC board on the integral stand-offs and connect the cables. Fig.10 shows the front and rear panel labels which can be copied and used as drilling templates. Alternatively, you can download them as PDF files from the SILICON CHIP website and print them out. Use a small pilot drill to start each hole, then carefully enlarge it to size using a tapered reamer. You need to drill two holes in the front panel to accept to switch and pot shafts and another two in the rear panel to accept cable glands. Once these holes have been drilled, the labels can be laminated and affixed in position using a smear of silicone sealant. Alternatively, you can print the labels out back-to-front on clear film (make sure you printer can handle it) and silicone them into place. Printing them out back-to-front means that the labels are must be mounted with the ink towards the panel, so that this side is protected Once the labels are in place, wait for the silicone to cure, then cut out siliconchip.com.au The completed unit minus the leads. It can be mounted with the case flanges either up or down, while power can come from any 12-30V 3A DC supply. the holes using a sharp hobby knife. The PC board assembly can now be installed. First, slide a spring washer over the potentiometer shaft (or use two or three flat washers), then insert the board into the case, angled so that the pot and switch shafts go through their respective holes first. You may have to flex the box slightly to get the board in but if that fails, enlarge the pot and switch holes slightly. Once the board is in place, secure it to the integral case standoffs using the supplied self-tapping screws. That done, fit the potentiometer nut – there won’t be much exposed thread so use small pliers to push it down and turn it until it catches the thread. Do it up firmly, then check that the shaft is perpendicular to the edge of the case. If not, you will need to remove the board and add another washer. Because the cable gland nuts are large, there won’t be enough room for them between the PC board and the lid. To solve this, secure each nut in a vice between two scrap pieces of wood and file down the protruding ring on one side that so it is flush with the hexagonal surface. Do the same to the opposite side of each nut, then install the cable glands with the two filed edges against the PC board and facing up. You will also need to cut and file away two notches in the rim around the edges of the lid so that it clears the nuts. This can be done using sidecutters and a flat file. It’s now just a matter of passing the power supply and LED cables through the glands, stripping the ends and attaching them to the screw terminal blocks. The glands can then be tightened to secure the cables to the case. All that remains now is to install the switch orientation jumper JP2. To do this, fit the jumper shunt to one pair of pins on JP2 and apply power. Toggle the Standby switch and if its action is the opposite of what you require, move the shorting block to the other end of JP2. That’s it, the assembly is complete and you can now attach the lid and operate the unit. Just remember our warning about not looking at the LEDs when they are at full brightness, or even approaching full brightness for SC that matter. February 2011  35