This article introduces a lightning protection design method that transforms the transient signals experienced by components during an instantaneous lightning strike test into reference transients as specified in component datasheets. This approach simplifies the selection of suitable components and eliminates the need for trial and error during the design phase, making the process more efficient and reliable. Additionally, we describe a technique to determine the minimum wire width required to withstand transient currents. A free graphical user interface (GUI) is introduced, which allows users to perform all calculations outlined in reference [1] and generate output results. These results can then be compared with component datasheets to ensure proper selection and ultimately lead to the design of a highly robust lightning protection circuit. To reduce aircraft weight and fuel consumption, manufacturers are increasingly using carbon composite materials instead of aluminum for fuselages. However, this shift has unintended consequences—increasing the indirect impact of lightning strikes on avionics systems. More intense transient signals now require stronger protection at the avionics interface. Yet, due to space constraints, manufacturers and suppliers aim to maintain the same equipment size, necessitating careful design of additional lightning protection circuits to use the smallest possible components. Component datasheets typically provide ratings based on standard reference transients, which may not match the actual lightning transients encountered in aircraft environments. As a result, engineers often rely on experience or select the largest available components that fit on a PCB. Wire widths were traditionally determined using IPC guidelines, but these were developed for continuous current rather than transient conditions, leading to unnecessarily wide traces. After initial design, prototypes are built and tested, followed by analysis to confirm if the selected components and wire widths are adequate. This “trial and error†process is time-consuming and resource-intensive. Fortunately, the GUI discussed in this article can streamline the process, helping designers avoid unnecessary delays and optimize their designs from the start. Chapter 22 of Reference [2] outlines the lightning impulse test transients required by the FAA for indirect lightning strike testing. Waveform 4 (WF4), shown in Figure 1, represents the transient signal used for metal aircraft. In contrast, Waveform 5A (WF5A), depicted in Figure 2, is designed for synthetic material aircraft. WF5A includes parameters such as open-circuit voltage (VOC) and short-circuit current (ISC), which help calibrate the source impedance of the test signal generator. Figure 1: Waveforms mentioned in Chapter 22 of Reference [2]. Figure 2: WF5A mentioned in Chapter 22 of Reference [2]. To determine the test level for avionics, the instantaneous lightning strike signal must be applied to the fuselage or a simulated version. Reference [3] outlines how transient signals are applied to different zones of the fuselage, as defined in literature [4]. Each zone produces specific voltage values, and for avionics connected via cables, the sum of voltages across multiple zones is calculated and doubled for each signal line. When applying the process described in [3], the lightning strike current for avionics testing has the same rise time and pulse width as WF4. However, synthetic material fuselages significantly distort this signal, while metal fuselages cause minimal distortion. As a result, WF5A has a longer duration than WF4. Additionally, composite fuselages transfer more energy from the lightning strike to the avionics. To simulate this, WF5A has a source impedance of 1 Ω, compared to 5 Ω for WF4. The distortion caused by the fuselage is due to the diffusion of the composite material and the structural voltage drop (current × resistance) coupling. Figure 3 provides a simplified visual representation of these couplings. Avionics systems, such as radios and antennas, are interconnected. The coupling effect between them can be modeled as resistors, inductors, and capacitors. The resistor represents the structured voltage drop, while the reactance accounts for the diffusion coupling. Together, these effects extend the waveform’s duration. Because WF5A has a longer duration and lower source impedance, it delivers more energy to the avionics, increasing the challenge for protection circuits. 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