How to protect IoT boards against power surges and spikes

Why do IoT boards fail so often due to electrical surges?

Anyone who designs embedded devices and internet of things (IoT) hardware knows that the lab is a forgiving place. On the workbench, power is clean, voltage is stable, and tests run without major scares. However, when the hardware goes out into the field, whether in street lighting poles, industrial sheds, or residential rooftops, reality is unforgiving. Distant lightning strikes, switching of large electric motors on the neighboring grid, and even power drops create devastating voltage surges. If your board lacks proper defenses, the main microcontroller will burn instantly, turning a promising project into electronic waste. To avoid this nightmare, we need to understand how to harden the hardware by combining specific protection components.

In practice, this means it is not enough to throw any random component at the board input; you need a defense hierarchy. Think of it like castle security: first there is the moat to contain the bulk of the attack, then the inner walls, and finally elite guards protecting the king (which, in our case, is the microcontroller or the Wi-Fi/LoRa radio chip). When a power spike occurs, if the initial defenses fail, the most sensitive components will absorb the energy and be destroyed. Before building your next CAD version, it is worth reviewing basic bench concepts, such as how to diagnose voltage drops in switched-mode power supplies, ensuring your circuit operates within safe margins even before experiencing a surge.

The role of the varistor in cutting high-energy peaks

The first major element in our defense line is usually the metal oxide varistor, commonly known as MOV. Simply put, a varistor is a resistor that changes its value depending on the applied voltage: when voltage is normal, it behaves like an open circuit, almost as if it were not there. But as soon as a high-voltage spike appears—for example, a thousand-volt surge coming from the power grid—the varistor's resistance drops drastically within nanoseconds. It acts as a diversion, pulling this flood of energy to the ground wire and preventing it from advancing into the rest of the board.

The big secret when sizing a varistor for an IoT board is correctly calculating the maximum continuous operating voltage. If you choose a varistor with a nominal voltage that is too low for your 12V or 24V power line, it will constantly trigger with any normal fluctuation, heating up and eventually catching fire. On the other hand, if the nominal voltage is too high, the spike will pass freely before the varistor begins to conduct, destroying the downstream regulator components. Furthermore, just like projects requiring how to implement PID control on ESP32 to maintain industrial process stability, thermal and electrical protection sizing must be tested under real load and noise conditions.

TVS diodes: the ultra-fast barrier against rapid transients

Although the varistor is excellent for draining a lot of energy at once, it is relatively slow to react to extremely fast surges or induced electromagnetic pulses. This is where TVS diodes come in, standing for Transient Voltage Suppressor. In practice, the TVS diode acts as a bidirectional or unidirectional electronic safety valve placed right on data tracks and sensitive communication inputs. When the voltage exceeds a safe threshold, the diode conducts almost instantly, clamping the voltage level to a harmless level for the microcontroller pins.

The main advantage of the TVS diode over the varistor is its response speed and precision at the cutoff voltage. While a varistor can degrade after absorbing multiple large surges, modern TVS diodes withstand multiple events as long as the dissipated energy does not exceed the thermal limit of the silicon. When designing exposed communication buses, such as RS-485 ports or external sensor lines, placing a TVS diode very close to the physical connector is a mandatory engineering practice. This same attention to signal integrity and network detail is also seen when studying how to configure QoS in switches to prioritize PROFINET, where data flow stability prevents catastrophic losses in noisy environments.

Resettable fuses: smart protection against short circuits

When a varistor or TVS diode acts to drain a high-energy surge, it essentially creates a low-resistance path to ground or between power lines. If this surge is prolonged, electric current will spike, generating immense heat that can carbonize the board or break traces. To prevent the entire board from turning to ashes, we use a fuse at the beginning of the input line. However, in IoT devices installed in hard-to-reach locations, using a traditional glass fuse that blows and requires manual replacement is unfeasible. The modern solution is the use of resettable fuses, technically known as PTCs.

The resettable fuse works with a special polymer that reacts to heat generated by excess current. When a short circuit or prolonged surge occurs, the polymer temperature rises rapidly, causing it to change from a conductive state to a highly resistive state, blocking almost all current flow. As soon as the fault is cleared and the circuit cools down, the component returns to its normal low-resistance state, automatically reconnecting the system. This saves absurd field maintenance costs and ensures the device starts operating by itself after a severe grid transient.

Final considerations on hardware hardening

Designing robust IoT boards requires a mindset shift: the experienced designer doesn't just ask if the circuit works on the bench, but how it survives the worst possible scenario in the field. The synergistic combination of varistors at the main input, TVS diodes protecting data lines, and resettable fuses containing overcurrents forms an unbeatable hardware-hardening triad. Each component has a specific response time and energy capacity, working together to exhaust destructive energy before it reaches expensive semiconductors. By adopting these design guidelines from the initial schematic, you guarantee longevity, reduce warranty costs, and build a solid reputation for your connected products.