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By John Clarke LED Lighting & Driver Kits from Oatley Electronics Oatley has four LED kits that can be driven from the one generalpurpose LED Driver, using a 12V DC source such as a battery. Battery-powered LED lighting is ideal for outdoor use, such as camping, in sheds or on small boats, where mains power is not available. Different lighting options suit various purposes ranging from wide coverage to more concentrated floodlighting. T he Oatley Electronics K491 LED Driver runs from a 12V supply and is included in one of four kits: K491PK1, K491PK2, K491PK3 or K491PK4. All four kits include various combinations of white LEDs. The K491 LED Driver is supplied as a kit in all four cases. It needs to be assembled by mounting the supplied components onto the PCB. There are not many parts to install, and the inductor is prewound, so it all goes together pretty quickly. Then it’s just a matter of wiring the Driver up to the supplied white LED lamps. Lighting options The four kits are as follows: 1) K491PK1 This kit includes the Driver plus four LED lamps in conical aluminium housings with reflectors to concentrate the light, as shown above. The four lamps are connected in parallel and driven at 35W total (or 8.75W per lamp). Because they are rated at 60W each, they are significantly under-driven, which means that they run cool and the lamp life should be very long. 2) K491PK2 This kit includes the Driver plus two 1.2m-long 18W tubes, similar in appearance to fluorescent tubes but containing strings of white LEDs instead. Again, they are driven in Table 1 – kit LED lighting options Kit Driver LEDs supplied LED connection Driven power Driving voltage Inductor tap R1 value K491PK1 K491 Four 60W LED lamps parallel 35W 20V DC 16 turns 0.05W K491PK2 K491 Two 1.2m-long 18W LED tubes parallel 28W 33V DC 12 turns 0.05W K491PK3 K491 Two 0.6m-long 8W LED tubes parallel 14W 50V DC 16 turns 0.1W K491PK4 K491 Two 12W LED floodlights parallel 20W 50V DC 16 turns 0.05W 92 Silicon Chip Australia's electronics magazine siliconchip.com.au Fig.1 (above): this circuit shows the basic operating principle of a DC-DC converter. Fig.2 (right): the block diagram of the Motorola MC34063 DC-DC converter, which is used in the LED driver. parallel but a bit closer to their ratings at 28W total (14W per tube). Still, they are under-driven, so they run relatively cool and should last a while. 3) K491PK3 This is similar to K491PK2, but you get the Driver plus two shorter 0.6m-long tubes rated at 8W each. They are driven in parallel at 14W, so 7W per lamp, just a bit under their rated power. 4) K491PK4 This version has the Driver plus two 12W LED floodlights. These are IP65 rated, so they can be left out in the weather. They include substantial heatsinking and adjustable mounting brackets. They are rated at 12W each and are driven at 20W total or 10W per lamp, so again, they are not being run at full power, extending their lifespans, while still providing a decent amount of light. Table 1 summarises these four configurations and has a few extra details that are needed to customise the Driver for each set of lamps. K491 LED Driver The K491 LED Driver is used in all four kits. The Driver can be set up for each lamp type by setting the tapping on the inductor with a wire bridge, and by changing the value of resistor R1 on the PCB. This LED Driver is designed to drive 10-40W of LED lighting from a 12V supply. It is a DC-DC boost converter based around an MC34063 controller IC. The LED lamps can comprise between three and 15 LEDs connected in series. The LEDs may be combined into a cluster, with a combination of series and parallel connections. White LEDs light up with around 3.0-3.3V across their terminals. When siliconchip.com.au connected in series, the overall voltage to drive them increases accordingly, with between 9V and 9.9V required to drive three LEDs in series. This rises to between 45V and 49.5V for 15 LEDs in series. Fig.1 shows the basic operating principle of the DC-DC converter. It incorporates an inductor, a diode, a switch and a capacitor. When switch S1 is closed, current flows through the inductor L1 and S1. L1 stores energy in its magnetic flux. When S1 opens, that energy is transferred, via diode D2, to the output filter capacitor and the load. In practice, the switch is a transistor or Mosfet, and the on and off times of the transistor’s conduction are varied to maintain the desired load voltage or current. The internal details of the Motorola MC34063 DC-DC converter controller IC are shown in Fig.2. It contains all the necessary circuitry to produce a step-up, step-down or inverting DC-to-DC converter. Its internal components comprise a 1.25V reference, a comparator, an oscillator, an RS flipflop and output transistors Q1 and Q2. The switching frequency is set by the capacitor connected to pin 3 of this IC. A 330pF capacitor sets it at about 90kHz (measured as 96kHz on our prototype). The oscillator is used to drive the flip-flop which, in turn, drives the output transistors. The inductor current is sensed at pin 7. When this reaches its peak, the flip-flop and the output transistors are switched off. The time for which the output transistors are switched on is determined by the comparator, which monitors the output voltage. When the pin 5 comparator input exceeds the 1.25V Australia's electronics magazine reference, indicating that the output voltage exceeds the required level, the comparator goes low. This resets the flip-flop, holding the transistors off. Conversely, if the output voltage is too low, the inverting input of the comparator will be below the 1.25V reference, so the output transistors can be toggled on by the RS flip-flop at the rate set by the oscillator. In voltage-regulation mode, the target output voltage is set using a voltage divider that applies a fixed fraction of the output voltage to feedback pin 5. However, if the circuit is configured so that the target output voltage is never reached and voltage to pin 5 is always below the reference, the circuit then operates in current-limited mode. In this case, the peak current sets the duty cycle, and this plus the inductance of L1 sets the average current delivered to the load. Circuit details The complete circuit of the Driver is shown in Fig.3. The internal transistors of IC1 are connected as a 93 The assembled Driver has just four wires connected: two for power in (at right) and two going to the LEDs (at left). Darlington to drive the gate of Mosfet Q2 high via diode D1, to switch it on. Q2 acts as the switch (S1) shown in Fig.1. When pin 2 of IC1 goes low to turn off Mosfet Q2, PNP transistor Q1 switches on to discharge Q2’s gate capacitance, giving a rapid turn-off. When Q2 is on, current begins to flow in inductor L1. Resistor R1 (0.1W or 0.05W) between pins 6 & 7 of IC1 sets the peak current delivered to the inductor. IC1 does this by switching off Q2 when the voltage across R1 reaches 0.33V. So the peak current is limited to 3.3A when R1 is 0.1W or 6.6A when it is 0.05W. Each time Q2 is switched off, the voltage at its drain rises because of the energy stored in inductor L1. As the current can no longer flow in Q2, it is diverted through diode D2 instead, flowing into the two 100μF 63V electrolytic capacitors, the 47nF ceramic capacitor and the load. Diode D2 is a schottky type with a fast response to cope with the high switching frequency of about 96kHz. It also has a low forward voltage, reducing power dissipation and improving efficiency. Voltage regulation is provided by the feedback network from the output to pin 5, mainly the 43kW resistor from the output and the 1kW resistor to ground. The output voltage is maintained when the voltage at pin 5 equals the internal reference of 1.25V. The 43kW and 1kW resistors reduce the voltage by a factor of 44 ([1kW + 43kW] ÷ 1kW). So the output voltage is limited to 1.25V × 44 = 55V. This voltage regulation protects the Mosfet (Q2) and the output capacitors from excessive voltage should the LED lamp load become disconnected or if the circuit is run without a load. Power for the circuit is from a 12V DC supply, with supply filtering provided by another two 100μF 63V electrolytic capacitors plus a second 47nF ceramic capacitor. Power delivery Remember that the average current delivered to the load via diode D2 is less than the peak current in L1, and power to the load depends on the value of the inductor and the peak current. For this circuit, the inductor is tapped to select an inductance that provides a suitable power output for the particular LED lighting load that’s connected. Fig.3: the circuit diagram for the LED driver kit from Oatley Electronics. 94 Silicon Chip Australia's electronics magazine siliconchip.com.au Fig.5: the overlay diagram of the LED Driver. Note that one of the resistors near IC1 is marked as 39kW on the PCB silkscreen but should be 43kW as shown here. L1 has taps at four, eight, 12 and 16 turns. The power versus LED voltage graph (Fig.4) shows the typical power levels for various configurations. Note that the power shown in this graph is the power drawn from the battery and not that delivered to the load. The efficiency of the circuit is high, so the graph gives a reasonable idea of the power delivered to the load. Construction Construction involves inserting and soldering the parts onto the PCB. Follow the overlay diagram, Fig.5, for the correct placement of each component. Begin with the 1/4W resistors, including the 0W resistor (used as a wire link). The colour coding for these is shown in the parts list, but you should check each value with a multimeter to ensure each is placed in the correct position. Note that R4 on the PCB screen print is marked as a 39kW resistor, but it should be 43kW. Fit diode D1 next, noting that the cathode (striped end) is to the left. IC1 can also be mounted now, taking care to orientate it as shown. Fig.4: the typical power level for various configurations of the LED cluster. Note that this graph shows the power drawn from the battery. siliconchip.com.au Australia's electronics magazine R1 is installed as either one or two 0.1W 1W resistors (R1a & R1b), with two resistors giving the 0.05W total resistance. There are four sets of holes for these resistors. For the K491PK3 kit requiring 0.1W, install either R1a or R1b but not both. You can use straight leads. For the other kits, fit both resistors and bend the leads, as shown in Fig.5. Install inductor L1 next. You can check that it is in the correct orientation by verifying that the lower five sets of pins on the right-hand side have wires attached to them on the former. If not, you need to rotate it by 180°. Solder all the pins of L1 and then fit transistor Q1, taking care to orientate it correctly. Follow with the four ceramic capacitors, which are not polarised, then the four 100μF electrolytic capacitors, which are polarised. Ensure that each electrolytic capacitor’s positive side (with the longer lead) goes in the top PCB hole in all cases. Next, install diode D2. If an SR1060 is supplied, this will come in a TO-220 package, and it must be fitted with the metal tab towards the top of the board. However, our sample kit came with an SR350 in an axial package. In this case, it is mounted vertically, with the cathode (striped end) to the left. The anode should be placed in the right PCB hole, with the diode body upright and the cathode lead bent over by 180° to insert into the left PCB hole. Leave the diode body about 5mm above the PCB for improved cooling. Mosfet Q2 comes in a TO-220 package, and it is mounted with the tab toward the edge of the PCB, and with the mounting hole 15mm above the May 2022  95 PCB. After soldering it, slip the heatsink over it; it is secured with spring pressure. You could add an M3 x 6mm screw and nut to further secure it if you want to. Inductor tap selection On the underside of the PCB are the tapping selections for inductor L1, shown at the right of Fig.5. You need to connect a wire link from pin 13 on the underside of L1 to the COM connection. Then, connect either the 12T tap or 16T tap (see Table 1) by soldering in one of the dashed wire links. Only one of these should be fitted. The power input is via wires or pins soldered to the +12V IN and GND terminals at the upper right and lower right of the PCB, respectively. It is crucial to connect the input supply with the correct polarity to the K419 Driver, as there is no reverse polarity protection. Also ensure that you connect the LED arrays with the right polarity, with all the common anodes to the V+ OUT terminal at upper left, and the common cathodes to the GND terminal at lower left. If soldering the input and output wires directly to the PCB (as we expect most constructors would), it’s good practice to add some form of strain relief to prevent the solder joints from fracturing. Parts List – Oatley LED Kits 1 set of LED lights (see Table 1 for kit options) 1 single-sided PCB coded K419, 92 x 64mm 1 prewound multi-tapped inductor (L1) 1 TO-220 clip-on heatsink Semiconductors 1 MC34063AP DC-DC converter, DIP-8 (IC1) 1 C8550 PNP transistor, TO-92 (Q1) 1 IRFZ44Z 55V 31A 13.9mW N-Channel Mosfet, TO-220 (Q2) 1 1N5817, 1N5818 or 1N5819 1A 20-40V schottky diode, DO-41 (D1) 1 SR350 50V 3A schottky diode, DO-41 (D2) OR 1 SR1060 60V 10A schottky diode, TO-220-2 (D2) Capacitors 4 100μF 63V electrolytic 3 47nF ceramic disc 1 330pF ceramic disc Resistors (all 1/4W, 1% unless otherwise noted) 1 43kW 2 1kW 1 22W 2 0.1W 1W 1 0W You could do this by adding a reasonable amount of neutral cure silicone sealant around each wire, holding them to the PCB while limiting the amount of flexing that can occur. Preparing the tubes The K491PK3 kit contains two 0.6m-long tubes. As supplied, they include an internally-installed LED driver that was designed for use with AC mains voltage. This needs to be removed by undoing the screws that hold the end caps in position and removing the end caps. Then, cut the white wires so that the installed driver module can be removed. Next, cut the red and black wires that connect between the LEDs and the driver module near to the Driver, and drill holes in the end cap so these red and black wires can pass through. Then replace the end caps. The red and black wires connect to the K419 driver output, red to V+ OUT and black to GND. Availability & pricing At the time of writing, the K491PK4 kit is $42, K491PK2 is $40, and the other two kits are $30 each. Postage is around $10 in most cases, although it might be a bit more depending on how many you order. You can order these kits and more details on the Oatley Electronics site: SC siliconchip.com.au/link/abd1 The K491PK4 version of the kit comes with these two 12V floodlights instead of the LED lamps. The K491PK2 version comes with the two 1.2m 18W tubes shown below, while the PK3 instead comes with the shorter 0.6m 8W tubes. 96 Silicon Chip Australia's electronics magazine siliconchip.com.au