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Solar Panel Protector and Optimiser by Ian Ashford This simple design offers two useful functions for solar installations, whether it be for the home, the shed or even the caravan. It reduces the chance of damage from lightning while also providing an ‘ideal’ blocking diode function so you can still get power from the panels when some Image source: https://unsplash.com/photos/a-group-of-buildings-with-red-roofs-VgF9kogcU1U are shaded. T he first function of this board is to arrest a lightning-induced surge before damage can occur to the downstream electronics. It also provides a blocking diode function for up to three solar panel strings. A blocking diode allows for the maximum power to be extracted from parallel strings when one or more panels are shaded. The board can be built to provide either or both functions; the choice is yours. Lightning surge protection Lightning is destructive and difficult to defend against. We use lightning rods for tall buildings, Earth conductors above high voltage transmission lines and there are even rockets specially developed for launchpad protection, which will launch themselves into storms trailing an Earthed wire. Many of these protection schemes perform as single-shot devices, but are still a small cost to pay for the protection provided. A single bolt of lightning may release between 200MJ and 7GJ of energy. For comparison, 5kW of solar panels on your roof would take around 11 hours to collect just 200MJ and 16 days to collect 7GJ, yet a storm can 28 Silicon Chip deliver this in a single, instantaneous pulse. And it can do it again, and again, and again in a short period. Lightning can cause significant damage to electrical goods, even if they are not directly in the path of the impact. An induced voltage or current wave can travel in a cable to all your most valued goods from a nearby lightning strike. Complex Earthing routes, including conductive items like train tracks and steel framed buildings, all affect the extent and magnitude of any induced pulses. Any conductive material within close proximity to the strike will likely carry significant currents as the charge dissipates. This design offers a solution for induced pulses. Unfortunately, it can’t do much to help if 7GJ lands in your backyard (or worse, on your panels!). Ultimately, whatever protection you put in place, there can always be a bigger event or a direct hit to thwart your efforts. This surge arrestor is a good start, but it isn’t guaranteed to provide protection for all events and for all causes. Australian and international design standards provide guidance for solar installations and the impact of a lightning induced surge. For example, IEC Australia's electronics magazine 61643 parts 31 and 32 contain relevant information. The standards provide guidance for these effects, and define a typical waveform so the design team can then build circuits and simulations to test their designs against. One of these ‘design’ spikes is a waveform that rises from 0V to 90% of the peak within 8µs (microseconds) and then decays to 50% of the peak within 20µs. This is known as an 8/20 waveform. It is very fast and short lived; this latter part is the key for the success of this design. The actual magnitude of the peak depends on many factors, including the proximity of the protection device to the source and obviously the intensity of the lightning bolt. Measurements conducted by people (who may or may not have been flying kites in a storm) indicate that a surge induced into a roof mounted solar array would require the device to curtail a peak current of up to 20,000 amperes. Even in a low-­impedance wire, this will raise a very high voltage. This circuit is designed to provide an alternative path for the surge, instead of via your panels and connected electronics. It is triggered when the surge voltage exceeds a design threshold. siliconchip.com.au Features & Specifications My installation has three sets of solar panels, all operating around 100Voc. From left to right, 3 strings to catch the westerly sunlight, (2.7kW), 2 strings to catch the sunrise, (1.8kW), and a main bank of 9 pairs of panels facing north, (3.5kW). Maximum protection occurs if the threshold is very close to, but just above, the open-circuit voltage of the connected solar panels. For this design, the fault current that can be absorbed is limited to 20kA with an 8/20 profile. The circuit does not activate under normal operating conditions, and will not affect the normal operation of the solar panels and collection system. For numerous reasons, rooftop solar panels are often installed electrically floating, with neither power conductor referenced to Earth. This design maintains that condition at all times, except during a surge event, when one or more conductors could be shorted to Earth as the device activates. A surge can manifest in one of several ways: it can form between the supply cables or on both conductors, raising a voltage spike between both conductors relative to Earth. The surge can be a positive or negative trending spike and would be superimposed onto the normal operating voltages within the circuit. The surge protection in this design is based on varistors. Until triggered, they exhibit properties similar to a back-to-back pair of zener diodes. Current will flow in either direction once the voltage threshold has been reached. Although small, they can conduct many thousands of amps for very short periods. To operate as a surge arrestor, the siliconchip.com.au varistor is placed between the source of the surge and a safe return path, short-circuiting the surge, while avoiding the downstream hardware. In this design, varistors are installed across the string outputs, to address a surge on one or the other supply line, and also between Earth and each of the two conductors, to provide a path for a surge that raises the potential of both conductors relative to Earth. It is likely that multiple varistors will conduct if a surge propagates through the circuit. Varistors, like zener diodes, are available in a range of voltage and wattage ratings. Ideally, the selection of the varistor should be specific to a particular installation to maximise the protection provided. Commercial surge arrestors are designed for a generic installation, allowing solar string voltages up to 1000V. In this case, the varistor would only provide protection against surges of around 1200V, which for most ● PV panel protection for lightning-induced voltage spikes for up to three strings ● Surge peak capacity of 20kA ● Maximum total throughput of 60A (20A per string) ● 120V maximum open-circuit string voltage ● Maximum protection via selectable surge activation level to suit the installation ● Up to three blocking diodes to prevent energy loss into shaded strings ● Additional units can be connected in parallel if required ● Blocking diodes utilise ‘ideal diodes’ to reduce power losses ● Small footprint ● Low cost installations is already causing damage to your inverters and charge controllers. We want to keep the activation just above the maximum, normal voltage of the system. This may be as low as 25V for a nominal 12V panel, as commonly used in caravans and campers. The varistors must be chosen to prevent activation under normal conditions. As a guide, the voltage rating for the varistor should be above the Voc rating of one panel, multiplied by the number of panels in series, plus an additional 10% to allow for extremely cold weather or minor variances within the components. In this design, there is provision for three solar strings to be connected on one circuit board. Each string has its own surge arrestor components. The positive conductors connect to a common rail, so only one varistor is required to provide a path to Earth, allowing for a reduced parts count. How much energy is in an 8/20 surge? The datasheet states that a surge protector which uses V25S115P varistors will clamp the surge at 295V at 100A. For a peak current of 20kA, and with the varistor clamped at 295V, the peak power would be 5.9MW (295V × 20kA). The duration of the wave, making some assumptions for the decay beyond 20µs, would be around 30µs. So the energy from the lightning surge would equate to the average power level, multiplied by the duration: 5.9MW × 30µs ÷ 2 = 88J. This is the equivalent energy of a 5kg weight suspended 1.8m above the ground. The V25S115P is rated for a pulse of 230J, comfortably over the 88J of the 8/20 surge. Not bad for a device that retails for around $2.50 in batches of 10. Australia's electronics magazine March 2026  29 Multiple boards can be used if required by a particular setup. For maximum protection of downstream appliances, the varistor should have the lowest trigger voltage rating available while staying above the applied solar panel string voltage. For our first example, three identical solar strings are to be connected to the surge arrestor. Each string is comprised of two series-connected 440W solar panels with an open-circuit voltage (Voc) of 52.2V. The string Voc is therefore 104.4V (2 × 52.2V). To ensure against a higher than expected Voc due to cold weather and for component variance, make it 114.8V (10% higher). Thus, we need to select a varistor with a DC voltage rating in excess of 115V. The V25S115P is suitable for an 8/20 22kA peak surge waveform. The datasheet states that the device will commence conducting between 162V (minimum) and 198V (maximum), which is above the calculated value. In our second example, two low-voltage panels are to be connected to the surge arrestor, one per string. Each panel has a Voc of 22V. Allowing an extra 10%, requires a minimum varistor DC voltage rating of 24.2V. In this case, no low-voltage 20kA devices were available for selection. For example, the V20H20P is suitable for an 8/20 5kA peak surge waveform. This device will commence conducting between 30V (minimum) and 36V (maximum). The lower commencement of conduction will offer better protection for voltage-sensitive appliances, even with a lower energy surge capacity. Ultimately, the decision on which varistor to select is something that will need to be addressed for each installation. The PCB has extra holes to cover several different varistor footprints, to account for the different design selections. Datasheets and searchable datasets for these and other varistors are available from major suppliers, including Mouser (https://au.mouser.com/c/circuit-protection/varistors/?mounting%20 style=Through%20Hole&instock=y). Due to cost constraints or other reasons, this is not always the way. Without blocking diodes, the output voltages of the two strings would both be dragged down by the lower illuminated string, resulting in less power being collected. Some of the energy harvested by the illuminated string would also be conducted into the less illuminated string. The only time this system could work optimally is around midday, when the sun is overhead and the strings are equally illuminated. Placing a blocking diode in each string will improve the situation, preventing any losses into the less illuminated panel. For those who enjoy taking a caravan off grid, charging the battery should be easy using the panel on the van roof and an additional plug-in panel, which is shifted around during the day to catch those fleeting rays. Unfortunately, the additional panel rarely doubles the solar collection since the default wiring in most caravans has any additional panel wired in parallel, and they share a single charge controller. Power from the higher voltage panel is wasted, flowing into the other panel instead of charging the batteries. With a blocking diode inserted after each panel, the maximum energy available is sent to the battery, eliminating any waste. To prevent losses, the blocking diodes in this design are provided using ‘ideal diodes’. A standard diode would dissipate around 10W when conducting 10A. The ideal diodes have a voltage drop of around 0.1V and therefore only dissipate around 1W for the same function. For the 200W panel used on a caravan, that saving represents an appreciable portion of the energy available for collection. This part of the circuit is similar to our Ideal Diode circuits published in the December 2023 (siliconchip.au/ Article/16043) and September 2024 issue (siliconchip.au/Article/16580). This version can be simpler because it’s used in a very specific configuration. The design includes a small heatsink for each Mosfet to allow for measuring of the short-circuit current rating of the attached array, a measurement that is required to be performed before completing the commissioning of a solar installation. Australia's electronics magazine siliconchip.com.au Blocking diodes The second part of the design is thankfully less energetic and much simpler. When two or more solar panels or strings are operated in parallel, even if electrically identical, they will have minor differences in performance. All other things being otherwise equal, the hotter panel will have a marginally lower peak operating voltage than the cooler panel and will produce a little less power. For minor differences, the parallel strings will both provide power at an average voltage and deliver only slightly below the peak power levels expected. If one panel is heavily shaded, though, the output from the shaded string is well below the other. The higher voltage string will push current into the other string, wasting power that could have been delivered to your appliances. If three or more strings are connected together, the shaded string could be damaged by the current from the unshaded strings, fusing internal conductors or even the cables and conductors between the panels. Installation guidelines were recently amended by regulators and now all new installations, where more than two strings are connected, must be fitted with a blocking diode to prevent this reverse current. This was not mandated as a correction for existing installations. A blocking diode will also allow the less productive string to be excluded should its output fall too low. A high-quality maximum power point tracker (MPPT) will still perform faultlessly with the diodes in circuit, and will continue to find the maximum power point whether it be from one string in full sun or with two strings operating at the lower, shaded panel voltage. The blocking diodes will ensure that power always goes to your appliances and never flows from one string into another, avoiding losses. For some households, a north-facing rooftop is not readily available, and the panels may be split into two halves. One string will be on an east-facing roof, the other west-facing. These installations should really be using two solar charge controllers, one to handle each orientation, to ensure that the maximum power is collected throughout the day. Selecting the varistors 30 Silicon Chip The surge protection devices must be installed in a suitable enclosure to prevent inadvertent contact. Choose a location electrically close to the panels, to allow for some additional protection to the downsteam devices due to any additional cable length and resistance offered by any conductors or isolation devices further along the circuit. During the test, the two outputs, labelled Common Positive and Common Negative on this design, may be shorted together, resulting in 0V between the two. Current will continue to flow through the Mosfet, but its driver chip will be unpowered, providing no gate voltage to the Mosfet. Under these unique conditions, the Mosfet’s dissipation will be similar to that of a silicon diode, typically in the order of 1W per amp of current. To prevent damage to the Mosfet, this test should be undertaken only for short durations, monitoring the temperature of the Mosfet. The preferred solution is to measure the short-circuit current by shorting the inputs to the PCB instead, excluding the Mosfets from the short-circuit path. Circuit details The circuit is shown in Fig.1; it contains three nearly identical circuits, duplicated to provide the surge arrest function and blocking diode function for three strings. The circuit can be built for one, two or three strings by omitting the appropriate parts. siliconchip.com.au If surge protection is not required, omit the varistors. If blocking diodes are not required, the Mosfets and associated parts can be omitted and wire links added, shorting the source and drain at each Mosfet location. Two varistors are installed per solar panel string, plus one additional varistor from the common positive rail to Earth. All varistors are identical and should be selected to provide maximum protection to your solar array, as per the accompanying panel. The ideal diodes are based on the ZXGD3111 chip, an active ORing Mosfet controller with a 200V upper limit. This controller requires diodes in the negative leg, rather than the more traditional positive supply conductor. The controller will switch on the Mosfet once the voltage measured between the source and drain connections exceeds the internal threshold of around 3mV. A simple linear power supply based on transistor Q4 and zener diode D1 provides approximately 18V to each of the ideal diode drivers. For string voltages less than 20V, all components associated with the voltage regulator can be Australia's electronics magazine omitted, with power for the driver chip being provided directly from the common positive rail by shorting the emitter and collector pads for Q4. In this case, retain the three bypass capacitors close to the driver chips. For intermediate voltages, in the range of 20V to 60V, one of the 47kW resistors should be replaced with a short length of wire to ensure sufficient current flow to the zener diode and to maintain regulation for the base current flowing into Q4. The Mosfets are N-channel types, which outperform P-channel units in the magnitude of the internal resistance, current capacity and most importantly their cost. When selecting a Mosfet, choose a component that will ensure that the Vds and current rating comfortably exceeds the maximum Voc and Isc of the attached strings and choose a component with an RDSon of less than 10mW. This requirement is easily achieved at lower voltages and lower current levels. At the time of writing, the NTP011N15MC costs a little over $3 a piece. It has a 150V drain-to-source breakdown rating and can conduct March 2026  31 73A. In this design, it is safe to utilise these for a solar array up to 120V and 20A. PCB design The circuit board configuration is shown in Fig.2. Termination points are provided for the solar panel strings on the left-hand side of the board. For each string, the positive and negative terminals straddle a low-impedance Earth conductor, providing a very short path for any surge currents. All terminals to the board are rated well in excess of the 200V upper limit for the ideal diode driver chip, and can handle a continuous current of 80A. During a surge, it can be expected that all conductive surfaces on the left Fig.1: the circuit consists of three virtually identical blocks, with the power supply components (Q4, ZD1 etc) and VAR1 shared between them. Each block has one varistor between the inputs, one from the negative input to Earth and one from the shared positive output to Earth. The ICs make the Mosfets act like almost ideal diodes. 32 Silicon Chip Australia's electronics magazine hand side of the board, including the Earth connection, will be operating at their upper design limits and may even show signs of charring around component legs where the copper conductor areas are smallest. Absorbing or redirecting 20kA is not an easy task. Assembly The device is built on a double-sided PCB coded 17112251 that measures 74.5 × 150mm. A mix of throughhole and SMD parts are all mounted on the top side. All of the SMDs are large enough to be installed by a competent constructor using a decent soldering iron if a hot air rework station is not available. Construction is simple. Start by inspecting the board for any obvious defects; there are only a few finer tracks and these should be an easy task to confirm that they have continuity. Pay particular attention to the supply tracks that start from the bottom of the board, running up the middle, to the controller for Q1. Start by installing the controller chips first; with seven leads, they are difficult to get in the wrong orientation. Then fit the parts associated with the 18V supply along the base of the board, followed by the capacitors beside the driver chips. Clean up any solder bridges and retouch any connections that may be incomplete or lack fusion. Then press the terminals onto the board. They are a firm fit and should not fall out after installation. Turn over the board or solder from the top if you have room. Solder all four legs, ensuring a good conductive path for each. The Mosfets are next; each tab is tied to the drain. No isolation washer was used on the prototype boards as the heatsinks are well spaced and pose no greater touch risk than the adjacent lugged terminals. In each case, secure the heatsink to the Mosfet using a 3mm washers, nut and bolt. Press the heatsink onto the board, aligning the Mosfet leads. After seating the heatsink, solder the support legs to the board and then solder the Mosfet leads. If the blocking diode function is not required, don’t fit the Mosfets but remember to solder a shorting wire between the drain and source at each Mosfet location. Carefully unpack the varistors and place them on the board, as low as they siliconchip.com.au Fig.2: when assembling the PCB, fit the SMDs first and take care with the orientation of IC1-IC3 and ZD1. The ICs should have a dot, divot or beveled edge indicating the pin 1 side and they must be orientated as shown here. ZD1’s cathode stripe goes towards the regulator. Attach the Mosfets to the heatsinks before soldering the pins. will go without cracking any of their rigid coating. Solder the legs from the underside, trimming the excess away. Set-up and testing There are no adjustments to be made to the board. After completing the construction, check for any shorts or dry joints, rectifying as required. Testing is a two-step process. Step one is to confirm operation of the power supply. Connect a DC supply to the output terminals, paying attention to the polarity. Raise the voltage from zero to approximately 30V; the 18V rail will begin to rise, then should be fixed around 18V as the connected supply continues to rise. Do not proceed past 20V if the rail is not performing as expected. The 18V rail can be measured on pin 3 of Q4, with ground being the negative output terminal. siliconchip.com.au Carefully confirm that the 18V rail is present on the top side of the bypass capacitors for IC1-IC3. If all is correct, disconnect the testing power supply and continue with installation. The board needs to be housed in a conductive metal enclosure that is well Earthed. Drill and/or punch the enclosure panels to allow for cable glands and/or MC4 style connectors. Drill a neat hole and remove any paint adjacent ready for bolting an Earth cable to the external face of the enclosure. Use star washers to ensure the bolt has a good electrical connection to the box, as shown in Fig.3. Use a similar connection internally for the PCB’s Earth connection and don’t forget to Earth the door if it is hinged. Fig.3: how to attach an Earthing bolt to the interior of the enclosure. Star washers should be used to ensure a good electrical connection. Australia's electronics magazine March 2026  33 Parts List – Solar Panel Protector (per board) 1 double-sided PCB coded 17112251, 74.5 × 150mm 5-9 4mm screw terminals (CON1-CON9) [Amphenol AMT0440008TH0000G] 5-9 M4 × 6mm panhead machine screws (for CON1-CON9) 3-7 varistors, type depending on PV array details (VAR1-VAR7) (see panel; V25S115P used in the prototype) 1-3 ZXGD3111N7TC N+1 ORing Controller ICs, SOIC-7 (IC1-IC3) 1-3 NTP011N15MC 150V 74A N-channel Mosfets, TO-220 (Q1-Q3) 1-3 PCB-mounting TO-220 heatsinks [Wakefield-Vette 657-10ABPE] 1 PZTA42 300V 500mA NPN transistor, SOT-223 (Q4) 1 SMAZ18-13-F 18V 1W or CMZ5931B 18V 1.5W zener diode, DO-214AC (ZD1) 4 4.7μF 50V X7R M3216/1206 SMD MLCC capacitors 2 47kW ±5% ¼W M3216/1206 SMD resistors 1-3 M3 × 10mm panhead machine screws 1-3 M3 hex nuts 8 M3 × 6mm panhead machine screws 4 12mm-long M3-tapped Nylon spacers * wiring is not included in the parts list Why no fuses? Would a fuse on the supply cables prevent damage downstream? In this application, any fuses must be able to interrupt the surge from arcing over and therefore need an interrupt rating of at least 20kA. If not adequately rated, the fuse will continue to conduct after the wire has evaporated, performing more like a 0W fluorescent tube than a protection device. For a typical solar panel, the short circuit current would be around 9A, so a 10A-rated fuse should be sufficient. During a lightning induced surge, the current will rise rapidly toward the peak at 20kA. Intuition and basic maths tells us that 20kA is much, much bigger than 10A and hence the fuse will blow. Right? Unfortunately fuses don’t operate instantaneously, they take a finite time to melt, even at 20kA. A typical 10A fuse with a rated interrupt value of 20kA will take approximately 50µs to break at 20kA, too long to be of any benefit when controlling an 8/20 surge. For the protection of electronics, very fast acting devices are required; fuses just aren’t fast enough. Another photo showing the internals of the Solar Panel Protector. If you were to use a fuse, it would need a 20kA rating, like this SPF001 1000V DC fuse by Littlefuse. 34 Silicon Chip Australia's electronics magazine If directly terminating cables to the PCB, measure twice and cut once, allowing a little extra length for bends and for any minor mistakes when crimping the lug to the cable. It is better to be looking at the cable rather than looking for it. If using MC4 panel sockets/plugs, use connecting cable of similar cross sectional area; multi-strand, if possible, to allow for tighter bends. Ensure all connecting cables are rated for the currents and voltages being applied. The current rating is specifically important because the solar panels will be delivering their rated current for many hours at a time, often on hot days. Ensure all cables are correctly run and secured using the correct torque for each terminal (1.1Nm/10lbf. in). Connections must be made by an appropriately skilled person for low-voltage applications and, where mandated due to higher voltages, you must use a qualified electrician. If in doubt, have an electrician skilled in solar installations perform the work. Connections to the solar array should only be undertaken with the panels isolated. Do not work on live cables. Once all terminals are connected, visually check for the correct polarity if using colour-coded cable. Close the cabinet and re-energise. If your charge controller is showing an input, assuming it is sunny, then all is going well. If not, double-check the polarity of any connections and rectify as required. The voltage drop across each ‘diode’ is difficult to measure. The best way to do this, is to measure the voltage from the common negative output back to the individual negative inputs, and be very careful around the supply cables. In normal operation, this should be around 10mV for each amp of current flowing. If all is OK, that’s it. Close the lid. Good practice dictates that the Earth conductor should be run with the output conductors in the same conduit and be terminated to the frame of the inverter or Earthed charge controller. Ensure that the downstream Earth connection is well grounded and securely attached. For isolated applications, like a caravan being used off grid, there will be no Earth connection tied to the soil outside. Connect the Earth terminal to the frame of the inverter or charge SC controller. siliconchip.com.au