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12V 20-120W Solar Panel Simulator By JOHN CLARKE How do you test or develop a solar charge controller such as the unit described in SILICON CHIP last month? You could use a solar panel but then you are at the whim of the weather and time of day. Also you would need several panels of different sizes to test it properly. This device solves all those problems. T HIS SOLAR PANEL Simulator allows charge controllers to be tested without a solar panel. A simulator is handy because a solar cell panel will not always provide power and will certainly not deliver its full power output at all times. It is only around noon on a sunny day that the solar panel will deliver its rated power. In other conditions (eg, cloudy days), the panel delivers less than full power, while at night it will not deliver any power at all and may even draw power from the battery (unless precautions are taken). So when the Sun is not shining, an alternative source of power is necessary if you wish to test a charge con74  Silicon Chip troller such as the SILICON CHIP unit described last month. This is where this Solar Panel Simulator comes in handy. It can not only deliver power when required but can also deliver full power for as long as is necessary, regardless of the amount of sunlight. Typical system Solar panels are becoming increasingly popular for charging batteries and supplying power to equipment where mains power is not available. A typical system comprises the solar panel, a solar charge controller and a battery. The charge controller ensures that the battery is correctly charged and is a necessary part of the system. Without it, the battery may be overcharged by the panel, resulting in shortened battery life. Basically, this device can be set up to simulate a 12V solar panel rated anywhere from 20-120W. It can be used to ensure that the charge controller’s MPPT (maximum power point tracking) circuit is operating correctly and features adjustable open-circuit output voltage, adjustable voltage drop with current slope, an adjustable current limit threshold and an adjustable current limit slope to set the short-circuit current. Maximum power transfer For a solar panel simulator to be siliconchip.com.au SOLAR PANEL POWER CURVE SIMULATION (120W PANEL) 24V OPEN CIRCUIT VOLTAGE (Voc = 21.8) 22V VOLTAGE DROP WITH CURRENT SLOPE (VR3) 20V 17.8V 16V CURRENT LIMIT THRESHOLD (VR2) 14V 12V CURRENT LIMIT SLOPE (VR4) 10V 8V 6V 18V 17.2V 16V 14V 12V 10V 8V 6V 4V SHORT CIRCUIT CURRENT (Isc = 7.14A) 6.74A 2V 0V MAXIMUM POWER POINT 20V MAXIMUM POWER POINT 0 0.8 1.6 2.4 3.2 4.0 4.8 5.6 6.4 7.2 8.0 SHORT CIRCUIT CURRENT (Isc = 2.56A) 4V 2V 0V 2.32A OUTPUT VOLTAGE 18V OPEN CIRCUIT VOLTAGE (Voc = 21.4V) 22V OUTPUT VOLTAGE 24V SOLAR PANEL POWER CURVE SIMULATION (40W PANEL) 0 0.4 OUTPUT CURRENT (AMPS) Fig.1: the current/voltage curve for a typical 120W solar panel. VR1 in the simulator is used to set the open circuit voltage while VR2, VR3 and VR4 adjust the other parameters as shown. of any use, it must duplicate a solar panel’s characteristics. This is of particular importance when testing MPPT (maximum power point tracking) devices. MPPT charge controllers are designed to control power delivery to the battery so that the solar panel is always delivering the maximum possible power. To explain, standard charge controllers (ie, those without MPPT) incorporate a relay or solid-state switch to directly connect the solar panel to the battery. However, this does not fully utilise the power available from the solar panel when charging. To illustrate this, take a look at Fig.1 which shows the voltage/current curve for a typical 120W solar panel. As can be seen, its output follows a curve that ranges from maximum voltage when the output is open circuit (Voc) to maximum current when the output is shorted (Isc). For a 120W panel, Voc is typically 21.8V, while Isc is typically 7.14A. The maximum power delivered by the panel occurs at 17.8V for a current of 6.74A (ie, 120W). However, a charge controller that connects the solar panel directly to the battery will deliver 7.1A at 12V, siliconchip.com.au 0.8 1.2 1.6 2.0 2.4 OUTPUT CURRENT (AMPS) Fig.2: the current/voltage curve for a typical 40W solar panel. The simulator can also be adjusted to match this curve (or the curve for any other panel rated from 20120W using trimpots VR1-VR4. 7.05A at 13V and about 7A at 14.4V, equivalent to 85.2W, 91.7W and 101W respectively. As a result, utilisation of the available power from the solar panel is only 84% or less, depending on the battery voltage. The charge controller subsequently disconnects the solar panel when the battery is charged to prevent overcharging. By contrast, when an MPPT charge controller is used, the solar cell is loaded so that it delivers 6.74A at 17.8V, to obtain the full power from Main Features • • • • • • • Simulates 12V solar panels, 20W to 120W Can be run from a 24V battery or supply Adjustable open circuit voltage (Voc) Adjustable voltage drop with current Adjustable current limit threshold Adjustable current limit slope sets short circuit current Additional over-current protection the solar panel (120W). An efficient switchmode converter reduces this voltage so that it is suitable for charging the battery. If the battery voltage is 13V, then the charging current would be close to 9.3A, assuming a very efficient converter. Note that this 9.3A charging current is significantly higher than the panel delivers at 17.8V (the maximum power point) and is also higher than the 7.05A that the panel could deliver if connected directly to the battery. The SILICON CHIP Solar Panel Simulator duplicates the power curve of the solar panel. This allows you to check that the MPPT feature in the Solar Charge Controller is in fact drawing maximum power from the panel. A simple alternative In order to duplicate a solar panel power curve, the Solar Panel Simulator must allow adjustment of several of the curve’s parameters. These are the open circuit voltage (Voc), short circuit current (Isc), voltage drop with current, current limit threshold and the current limit slope. A simple solar panel simulator could be made using a variable voltMarch 2011  75 age is Vmp, then the series resistance would be calculated as (Voc – Vmp)/ Isc. For a 120W panel, the result is (21.8V - 17.8V)/7.14A = 0.56Ω. The power dissipation in this resistor at full power would be 7.14A2 x 0.56Ω = 28.56W (ie, I2R). Of course, if the output is shorted, this resistor needs to be able to dissipate power from the 21.8V power supply source at 7.14A, which is 156W. Normally, the output would not be shorted but if connected directly to a battery under charge, the output could be as low as 12V. In this case, the dissipation would be (21.8V – 12V) x 7.14A = 70W. POWER RESISTANCE POWER SUPPLY WITH CONSTANT CURRENT (ADJUSTABLE LIMIT) 12V BATTERY NON-IDEAL SOLAR PANEL SIMULATOR VOLTAGE DROP DUE TO RESISTANCE OPEN CIRCUIT VOLTAGE MAXIMUM POWER POINT Simulating a 40W panel CURRENT POWER SUPPLY LIMIT CURRENT Fig.3: the basics for a simple solar panel simulator. All that is required is a power supply with adjustable voltage and current limit (constant, not foldback) and a power resistor. However, as shown, such a simulator does not emulate the current/voltage curve of a solar panel very accurately. Q1, Q2, Q3 +24V F1 R1 + R2 SERIES ELEMENT + R5 Q4 OVERCURRENT LIMIT ERROR AMP ZD1, VR1, IC1a REFERENCE IC1b MINIMUM LOAD R6 CURRENT CONTROL 0V R3 + R4 SOLAR PANEL SIMULATOR OUTPUT IC1c, IC1d, VR2, VR3, VR4 – Fig.4: the block diagram for the Solar Panel Simulator circuit. The output voltage is controlled by up to three Mosfets (Q1-Q3) which are driven by error amplifier IC1b. The current control block provides feedback to the error amplifier and the reference block allows the open circuit output voltage to be adjusted. Q4 provides short circuit protection. age power supply with an adjustable current limit, in combination with a suitable series resistance. Fig.3 shows the details. In this case, the current limiting must be constant. Foldback current limiting can not be used, as this reduces the current as the output voltage drops. In operation, the power supply would be set for the solar panel’s Voc 76  Silicon Chip (open circuit voltage) and the current limit would be set for the appropriate Isc (short circuit current). The series resistance in the positive supply provides the necessary voltage drop. In practice, this resistor is chosen to drop the voltage to the maximum power point voltage for the panel at the maximum power point current. If the maximum power point volt- The corresponding figures are much lower if simulating a 40W solar panel. A typical 40W panel has a Voc of 21.4V and an Isc of 2.56A, while its maximum power point is at 17.2V and 2.32A. In this case, a 1.64Ω series resistor would be required and this would dissipate 10.75W at full power. If the output is short circuited, the dissipation in the resistor would be 54.8W. And when directly charging a battery at 12V, the dissipation would be (21.4 - 12) x 2.56A = 24W. Unfortunately, the simulator depicted in Fig.3 is not ideal because the current slope is not easily adjustable and its maximum power point is not correct. The current limit could be reduced to bring the maximum power point to the correct position but this also reduces the short-circuit current. In addition, making a resistor that will effectively dissipate the power required over a long period of time is not an easy task. A better simulator Instead of using a resistor, a better approach is to use a regulated linear supply designed with deliberately poor regulation. That’s because to simulate a solar panel, the voltage must drop under load – normally an undesirable characteristic for such a supply. The simulator is then completed by adding current limiting with an adjustable slope. Fig.4 shows the block diagram of the Solar Panel Simulator. Its input voltage is 24V so it can run from a 24V battery if necessary. The series element provides a voltage drop and is controlled to maintain the correct output voltage by error amplifier IC1b. siliconchip.com.au This op amp compares the output voltage to a reference voltage and controls the series element. When no current is drawn from the output, no voltage appears across resistors R3 and R4 and the current control output is at 0V. At the same time, resistors R5 and R6 divide the output voltage and drive the non-inverting input of the error amplifier (IC1b). As a result, the output voltage from the simulator is maintained so that the voltage on IC1b’s non-inverting input equals the reference voltage. When the simulator’s output supplies current, a voltage drop appears across R3 and R4. The current control block senses this and, in response, increases the voltage at the lower end of R6. As a result, the voltage at the non-inverting input of IC1b increases and so IC1b adjusts the series element resistance to reduce the output voltage (ie, to bring the non-inverting input voltage back to the reference voltage). As a result, the output voltage drops as the load current increases. This same current control block also has a second section which monitors the current through R3 and R4 but this only has an effect at higher current levels. This is configured to reduce the output voltage more dramatically and provides the steep reduction in voltage that occurs at currents above the maximum power point. Fuse F1 and the over-current limit circuit (based on transistor Q4) protect against excessive current flow should the output become shorted. If there is more than 0.7V across resistors R1 and R2, transistor Q4 conducts, in turn reducing the drive to the series element and thus preventing a higher current flow. Circuit details Refer now to Fig.5 for the full circuit details of the Solar Panel Simulator. Note that there are two different ground symbols used, one for the input power supply ground and one for the output ground. In order to simulate a 120W solar panel, three P-channel Mosfets (Q1Q3) are connected in parallel as the series control element. These Mosfets share the power dissipation, which can total more than 171W, ie, Vin x Isc where Vin is the input voltage (24V) and Isc (for a 120W panel) is 7.14A. A single IRF9540 Mosfet can dissipate 140W at a case temperature of siliconchip.com.au 25°C but must be derated at 0.91W/°C above 25°C. Under normal conditions, when providing the maximum power from the Solar Panel Simulator, the total dissipation in the Mosfets is (24V - 17.8V) x 6.74A = 42W, which is shared evenly. Note that either one or two of these power Mosfets can be omitted to simulate smaller panels. A quad op amp (IC1a-IC1d) controls the Mosfets. This device is powered from the 24V supply rail via diode D1 and a 100Ω resistor (on pin 4). Zener diode ZD5 (30V) protects the IC from over-voltage transients, while a 10µF capacitor filters the supply. Diode D1 provides reverse polarity protection. Zener diode ZD4 and its associated 1.2kΩ resistor generates a 9.1V rail. This is then fed to trimpot VR1 and buffered by voltage-follower stage IC1a to provide a variable 0-9.1V reference for IC1b. IC1b is the error amplifier and it monitors the simulator’s output voltage via a 100kΩ resistor to its pin 5 (non-inverting) input. The applied voltage is divided using a 47kΩ resistor which is connected to IC1d’s pin 14 output. IC1d’s output is at 0V when there is no current flowing through the Mosfets. IC1b’s pin 7 output drives the gates of the paralleled power Mosfets via separate 2.2kΩ resistors. These resistors isolate the gate capacitances from the op amp’s output to avoid oscillation. Zener diodes ZD1-ZD3 (18V) protect the Mosfets from excessive gate-source voltages. IC1b ensures a constant set output voltage from the simulator. For example, if VR1 is set so that the output voltage is 21.8V, the voltage at pin 5 (with no current flow in the output) will be 21.8V x 47kΩ/(47kΩ + 100kΩ) = 6.97V. As a result, IC1b controls its output so that its pin 6 inverting input is also 6.79V. It functions as an “error amplifier” because it amplifiers the error, or difference, between the target voltage (as set by VR1, via IC1a) and the actual output voltage (after division). Its gain is set to 100 by the 100kΩ and 1kΩ feedback resistors. Because its gain is so high, when IC1b’s pins 5 & 6 are at 6.97V, the output of IC1a is close to 6.97V (actually, about 7.11V). IC1b’s output will be about 3V below the input supply voltage. This is just low enough to Parts List 1 PC board, code 04103111, 99 x 76mm 1 diecast aluminium box, 119 x 94 x 57mm 2 IP65 cable glands for 4-8mm diameter cable 1 heatsink (see Table 1) 2 2-way PC mount screw terminals with 5.08mm pin spacing 2 M205 PC mount fuse clips 1 M205 fuse (F1) (see Table 1) 3 TO-220 Insulating bushes and Silicone insulating washers 4 15mm M3 tapped Nylon spacers 4 M3 x 12mm countersunk Nylon screws 4 M3 x 6mm machine screws 3 M3 x 10mm machine screws 3 M3 nuts 1 100mm length of 0.7mm enamelled copper wire 1 4m length of 0.315mm Nichrome resistance wire 2 10kΩ horizontal mount trimpot (VR1, VR3) 1 100kΩ horizontal mount trimpot (VR2) 1 2kΩ horizontal mount trimpot (VR4) Semiconductors 1 LM324 quad op amp (IC1) 3 IRF9540 P-channel 100V 23A Mosfets (Q1-Q3) (see Table 1) 1 BC557 PNP transistor (Q4) 1 1N4004 1A diode (D1) 1 1N4148 switching diode (D2) 3 18V 1W zener diodes (ZD1ZD3) (see Table 1) 1 9.1V 1W zener diode (ZD4) 1 30V 1W zener diode (ZD5) Capacitors 1 10µF 35V PC electrolytic 3 10nF MKT polyester (code 10n or 103) Resistors (0.25W, 1%) 3 100kΩ 1 47kΩ 4 10kΩ 3 2.2kΩ (see text & Fig.5) 1 1.2kΩ 2 1kΩ 2 100Ω 3 10Ω (see text & Fig.5) Selected 5W resistors (see Table 1) March 2011  77 R1* 24V INPUT Q1 IRF9540 F1 + S + ZD1 R2* – OUTPUT D 18V 1W A K CON1 10k G B A 10Ω C E D1 1N4004 Q4 BC557 – 2.2k CON2 Q2 IRF9540 K S D ZD2 100Ω 18V 1W K A V+ K 1.2k A ZD5 30V 1W G 10Ω 10 µF 35V 2.2k R5 100k Q3 IRF9540 S D ZD3 18V 1W K A +9.1V ZD4 9.1V 1W O/C VOLTAGE K 3 VR1 10k A 2 10Ω 4 IC1a G 2.2k 1 10nF 11 100k 1k 6 5 D2 1N4148 10 CURRENT LIMIT THRESHOLD VR3 10k 9 1k IC1c 8 A K 7 IC1b 10k 12 R6 47k 14 IC1d 100k IC1: LM324 10nF VR2 100k 100Ω VR4 2k 13 10k 10nF * SEE TEXT VOLTAGE DROP WITH CURRENT CURRENT LIMIT SLOPE 10k R3* R4* D1 A SC 2011 SOLAR PANEL SIMULATOR D2 A K B ZD1–5 A IRF9540 BC557 K K E G C D D S Fig.5: the complete circuit for the solar panel simulator. IC1b forms an error amplifier which controls Mosfets Q1-Q3 to set the output voltage. IC1d monitors the current through the output using resistors R3 & R4 and, together with IC1c, controls IC1b so that the output voltage behaves like a solar panel. The Mosfets and zener diodes highlighted in yellow (and their associated 10Ω and 2.2kΩ resistors) are necessary to simulate higher-power panels – see Table 1. turn the Mosfets on and so a small amount of current flows through the 10kΩ resistor across the output. This system provides negative feedback, so that the correct output voltage (as set by VR1) is maintained, even though the characteristics of the Mosfets can vary with temperature and other factors. If the output voltage drops, IC1b’s output goes lower and 78  Silicon Chip increases the drive to Mosfets Q1-Q3 to maintain the target output voltage. So this part of the circuit behaves like a linear regulator. Resistors R3 and R4, in combination with amplifier IC1d, monitor the current through the load. This feeds back into the output voltage since IC1d’s output is connected to the lower end of the voltage divider made up of R5 & R6. The higher the output current, the greater the voltage across R3 & R4 and thus the greater the voltage at the pin 14 output of op amp IC1d. This in turn increases the voltage at the non-inverting input of error amplifier IC1b. As a result, the error amplifier’s output increases and this throttles back the Mosfets to reduce the Solar Panel Simulator’s output siliconchip.com.au voltage. Trimpot VR2 sets the gain for IC1d and thus controls the rate at which the output voltage drops with increasing current. Op amp IC1c sets the current limit and also controls the rate at which the output voltage drops off when it is reached. At low currents, IC1c’s output is lower than the voltage at the junction of resistors R3 & R4 and so diode D2 is reverse biased. As a result, it does not affect the error amplifier’s input voltage. IC1c is configured with a much higher gain than IC1d (about 100). The current limit threshold, as set by trimpot VR3, holds the output of IC1c low until a preset current is reached. Above that point, IC1c takes over from IC1d due to its high gain. Basically, VR3’s setting determines the current at which the output voltage begins to steeply decline. When the set level is exceeded, IC1c controls the error amplifier via diode D2 and a 10kΩ resistor, dramatically reducing the simulator’s output voltage due to its high gain. The actual rate of voltage drop with current is set by adjusting IC1c’s gain using VR4. Construction The circuit is easy to build, with all parts mounted on a PC board coded 04103111 and measuring 99 x 76mm. This board is mounted on 15mm tapped spacers inside a diecast aluminium box measuring 119 x 94 x 57mm. Note, however, that additional heatsinking for the Mosfets is necessary – see photos & Table 1. Note also that, depending on the solar panel being simulated, some parts may not be required. Table 1 summarises the parts needed to simulate various solar panels and their corresponding heatsink requirements. Before mounting any parts, check the PC board for broken tracks and Fig.6: follow this overlay diagram to build the PC board. Mosfets Q1-Q3 are lined up along the edge of the board as they require a large heatsink. Some of the components (R2, R4, Q2-Q3, ZD2-ZD3 and some 10Ω and 2.2kΩ resistors) are only required for simulating larger solar panels – see Table 1 below. Table 1: Mosfets & Current Sensing Resistors Solar Panel Short Circuit Current & Fuse Rating (F1) Resistors R1 & R2 Resistors R3 & R4 Mosfets & Zener Diodes Required Heatsink <2A 0.47Ω 5W (R1) 0.22Ω 5W (R3) Q1 & ZD1 2.1°C per watt 2-4A 0.22Ω 5W (R1) 0.1Ω 5W (R3) 1.4°C per watt 4-8A 0.22Ω 5W (R1) 0.22Ω 5W (R2) 0.1Ω 5W (R3) 0.1Ω 5W (R4) Q1& Q2 ZD1 & ZD2 Q1, Q2 & Q3 ZD1, ZD2 & ZD3 for shorts between tracks and pads. Check also that the hole sizes are correct for each component to fit neatly. The screw terminal holes are 1.25mm in diameter compared to the 0.9mm holes for the ICs, resistors and diodes. Larger holes again are required for the fuse clips – test fit these clips to ensure that the holes are correct. Begin the assembly by installing the wire links, then install the resis- 0.7°C per watt tors. Table 2 shows the resistor colour codes but you should also check each one using a DMM before it is installed. The values for resistors R1-R4 must be selected according to the panel to be simulated – see Table 1. Note that R2 & R4 are not needed to simulate panel current ratings of less than 4A. Resistors R1 & R2 are chosen so that the current limit is greater than the short circuit current for the solar panel Table 2: Resistor Colour Codes o o o o o o o o o siliconchip.com.au No.   3   1   4   3   1   1   1   3 Value 100kΩ 47kΩ 10kΩ 2.2kΩ 1.2kΩ 1kΩ 100Ω 10Ω 4-Band Code (1%) brown black yellow brown yellow violet orange brown brown black orange brown red red red brown brown red red brown brown black red brown brown black brown brown brown black black brown 5-Band Code (1%) brown black black orange brown yellow violet black red brown brown black black red brown red red black brown brown brown red black brown brown brown black black brown brown brown black black black brown brown black black gold brown March 2011  79 INSULATING WASHER INSULATING BUSH M3 x 15mm SCREW M3 NUT TO220 DEVICE (HEATSINK) PC BOARD BOX SIDE Fig.7: Mosfets Q1-Q3 must be electrically isolated from the case using silicone insulating washers and insulating bushes. After mounting each device, use your DMM (set to a high Ohms range) to check that its metal tab is indeed isolated from the case. A finned heatsink is necessary to keep the Mosfets cool. Table 1 shows the Mosfets required and the corresponding heatsink requirements for different output currents. Note that the Mosfets must be electrically isolated from the case – see Fig.7 above. under simulation. If both resistors are used, they should be stacked, one on top of the other – see photo below. Extra mounting holes are included for the second resistor. If both R3 and R4 are used, they are mounted side-byside on the PC board. Diodes D1 & D2 and the zener diodes are next on the list. These must be mounted with the orientations shown. Install zener diodes ZD1-ZD3 as required, as indicated in Table 1. ZD4 and ZD5 are required in all cases. IC1 can now be soldered into place (pin 1 at top right), followed by the capacitors. Make sure that the electrolytic type is orientated correctly. The trimpots can then go in, followed by the two 2-way screw terminal blocks. Be sure to mount latter with their entry This close-up view shows the PC board with all Mosfets, zener diodes and resistors installed. holes facing outwards. Follow these with the fuse clips for F1. Make sure that these are orientated correctly, with the end stop toward the outside of the fuse for each clip. If this is not done, you won’t be able to install the fuse later on. The PC board assembly can now be completed by installing Mosfets Q1Q3 – see Table 2. Each of these devices is installed with its metal tab facing outwards and with the mounting hole centre in each tab about 21mm above the PC board. Final assembly The PC board can now be mounted inside its box. Start by placing the board inside the case and marking out the positions of the four mounting holes. These should then be drilled using a 3mm drill. Countersink the holes on the outside of the case, then install the four 15mm x M3 tapped spacers and temporarily secure the board in place. Next, mark the mounting holes for the Mosfets, then remove the PC board and drill the holes to 3mm. That done, use an oversize drill to remove any metal swarf so that the area around each hole is perfectly smooth. This is necessary to prevent punch-through 80  Silicon Chip siliconchip.com.au of the insulating washers when the devices are secured to the case. Once the mounting holes have been drilled, you can use the case as a template to mark out the corresponding holes in the selected heatsink (refer Table 1 to select a suitable heatsink). Once that’s done, the Mosfets and the heatsink can be fastened to the side of the case as shown in Fig.7. Note that it’s necessary to isolate each device tab from the case using an insulating washer and insulating bush. Once they have been installed, use a DMM (set to Ohms) to confirm that the metal tabs are indeed isolated from the case. If a low-resistance reading is measured, loosen each device in turn until the fault clears and check for puncture marks or holes in the silicone washer for the faulty assembly. Make sure the Mosfets are securely attached to the heatsink (or side of the box) to ensure that the heat is efficiently transferred. Finally, you will need to drill holes in either side of the case, near the screw terminal block, to accept the external wiring connections. These can be secured using cable glands. Setting up The step-by-step setting-up procedure is as follows: (1) Set trimpots VR3 & VR4 fully clockwise and install fuse F1. (2) Apply 24V to CON1 and adjust VR1 for the correct solar panel open circuit voltage at the output (CON2). (3) Attach a variable resistive load made from Nichrome wire to the simulator’s output (see adjacent panel and Table 3 for the details on making this load). (4) Adjust this resistance to give the maximum power point. This resistance value can be calculated by dividing the maximum power point voltage by the maximum power point current. Table 3 shows some typical values for various panels. (5) Adjust VR2 for the correct output voltage at the maximum power point. (6) Adjust VR3 slowly anti-clockwise until the voltage suddenly drops, then Making An Adjustable Load From Nichrome Wire An adjustable load is necessary to test and calibrate the simulator and this can be made using Nichrome wire. Table 3 shows the load resistances required for maximum power from a number of typical solar panels ranging from 40W to 120W. Each resistance is used to load the Solar Panel Simulator at the maximum power point for a given panel size. A lower resistance is then required to check the current limit threshold and the current slope. Note that it is not necessary (nor desirable) to short circuit the output of the Solar Panel Simulator. The slope of the current limit can be checked against the graph for that panel by loading the simulator with resistances above 0Ω. Nichrome wire can dissipate about 50W per metre before it becomes red hot. Assuming a diameter of 0.315mm, it has a resistance of about 13.77Ω per metre. As a result, you may need to use several paralleled strands of Nichrome wire to share the current and reduce heating to an acceptable level. A variable resistance can be made by first connecting the 0V output of the Solar Panel Simulator to one end of the wire. A flying lead with a clip can then be used to connect the positive output to various points along the wire. The wire itself should be wrapped around insulating material such as a length of timber. Alternatively, it can be suspended on a board between two points. Note that the resistance wire will become hot in use and could scorch any timber that it comes in contact with if left on for long enough. For this reason, keep it clear of any combustible material, do not touch it during operation and do not wrap it around plastic pipe or conduit. Table 3: Test Load Resistance Required For Setting The Maximum Power Point Panel Rating Maximum Power Point Load Resistance Required For Maximum Power Nichrome Wire (0.315mm diameter at 13.77Ω/m) 40W 17.2V <at> 2.32A 7.41Ω 65W 17.2V <at> 3.78A 4.55Ω 80W 17.6V <at> 4.55A 3.87Ω 120W 17.8V <at> 6.74A 2.64Ω 2 x 1080mm-long parallel strands 2 x 991mm-long parallel strands 2 x 843mm-long parallel strands 2 x 767mm-long parallel strands back off slightly in the other direction. (7) Reduce the load resistance (ie, by sliding the clip along the Nichrome wire) until the output voltage falls to 10V (but don’t go lower as this greatly increases the dissipation). (8) Check the voltage/current graph for your panel to determine its output current at that voltage and adjust VR4 to match this current. This gives the correct current slope for the simulator. For example, for a 120W solar panel, the output current at 10V is typically about 7A – see Fig.1. For a 40W panel, the corresponding figure is about 2.45A. Note that this adjustment is not particularly critical. To measure this current, simply connect your DMM (set to Amps) in series with the load. Alternatively, you can calculate the required voltage drop across R1 (or R1 & R2) and adjust VR4 to give this voltage. Once the adjustments have been completed, the Solar Panel Simulator SC is ready for use. Issues Getting Dog-Eared? Keep your copies safe with these handy binders. REAL VALUE AT $14.95 PLUS P & P Available Aust, only. Price: $A14.95 plus $10.00 p&p per order (includes GST). Just fill in and mail the handy order form in this issue; or fax (02) 9939 2648; or call (02) 9939 3295 and quote your credit card number. siliconchip.com.au March 2011  81