How to protect industrial power supplies against surges with varistors

Why do industrial power supplies suffer from electrical surges?

In industrial plants, the power supply is rarely a perfectly stable line. Powerful electric motors turning on and off abruptly, large transformers being switched in the substation, and even nearby lightning strikes inject devastating voltage spikes into the grid. In practice, this means that a switched-mode power supply, responsible for converting wall outlet energy into stable DC currents to feed sensitive circuits, must handle surges that can reach thousands of volts in fractions of a second.

When these invisible spikes travel along the cables, they arrive directly at the input of the electronic board. Without any defense mechanism, internal semiconductors suffer immediate dielectric breakdown, turning expensive components into unusable silicon. It is precisely in this critical overvoltage scenario that the varistor steps in as the equipment's first shield of protection, as detailed in our guide on how to protect electronic components against voltage surges.

What is a metal oxide varistor and how does it work?

The metal oxide varistor, popularly known as an MOV, is a semiconductor component built essentially from zinc oxide grains compacted with other metal oxides. Its electrical behavior is fascinating and resembles two back-to-back zener diodes. Under normal operating conditions, when grid voltage is within the expected range, the varistor exhibits an absurdly high electrical resistance, behaving practically as an open circuit that consumes no power.

However, as soon as an aggressive overvoltage hits its terminals and exceeds its clamping voltage, the crystal structure undergoes an instantaneous molecular turnaround. Resistance plummets vertically in nanoseconds, transforming the component into a very low-resistance path to ground or to the other leg of the grid. At this moment, it diverts the destructive surge away from the noble components of the switched-mode power supply, sacrificing part of its own structure to save the rest of the circuit.

How to calculate operating voltage and safety margins

Incorrectly sizing a varistor is a design error that can cause fires or destroy the power supply needlessly. The first critical step is determining the maximum continuous operating voltage the component will handle day in and day out. If the factory's nominal AC grid voltage is 220 volts, the actual peak voltage of that sine wave reaches approximately 311 volts. Additionally, industrial grids frequently swing positively up to 10% or 15% above the nominal value.

Therefore, the chosen varistor must never have a working voltage lower than the expected peak line voltage plus this tolerance margin. In practice, for a 220V grid, we specify a varistor whose clamping voltage and continuous AC operation allow it to tolerate at least 250V to 275V effective, ensuring it does not false-trigger during mild normal utility fluctuations while maintaining operational stability without premature wear.

Calculating energy and absorption capacity in Joules

Beyond voltage, the amount of energy the surge carries is the deciding factor for whether the varistor will survive the impact or turn to ashes. This energy is measured in Joules and depends directly on the transient peak current and how long the surge lasts before being dissipated. In severe industrial environments, switched-mode power supplies require components with high energy robustness, capable of handling dozens or hundreds of Joules without suffering catastrophic thermal degradation.

To ensure selection is precise, engineers calculate the estimated energy of the worst-case expected surge on the input bus. If the component is undersized at this stage, the first lightning strike or heavy switching will cause a physical explosion of the varistor's ceramic casing. Therefore, we always opt for generous Joule margins, distributing multiple varistors or choosing physically larger models when printed circuit board space permits.

Where to place the varistor in the power supply input circuit

The physical location of the varistor on the switched-mode power supply's printed circuit board directly influences its protection efficiency. It must be positioned as close as possible to the power cable entry, right after the main fuses and before any electromagnetic interference filter or wave rectifier. This strategic position ensures the surge is intercepted before it even has a chance to spread across traces and induce noise or damage in other stages.

It is worth noting that the connecting wires between the input and the varistor act as tiny antennas and inductors, generating unwanted voltage drops during high-speed surges. The shorter and thicker the copper traces connecting the varistor to the grid, the lower the residual voltage that will pass to the internal circuit. Proper use of these spacings and connections is just as vital as the preventive maintenance we cover when we test switched-mode power supplies under load on the workbench.

Why does the varistor need a series thermal fuse?

Over the years and after absorbing dozens of small daily surges, the varistor's internal zinc oxide structure begins to degrade cumulatively. When this happens, leakage current through the component increases gradually, raising internal operating temperature until it causes a frank short-circuit. If there is only a standard fast-acting fuse, the amount of heat generated can start a fire before the fuse blows.

The established engineering solution to mitigate this critical risk is to associate a thermal fuse or a varistor with integrated thermal disconnection, placed in direct physical contact with the component's body. As soon as temperature exceeds safe limits due to aging, the thermal fuse opens the circuit instantly, isolating the faulty varistor safely before material damage or risks to machine operators occur. To explore these architectural concepts further in production, see our related technical analysis on Why the circuit board resets and how to test traces.

Final considerations on reliability and industrial protection

Properly sizing varistors in industrial switched-mode power supplies is not just a secondary design detail, but the thin line separating a robust machine from equipment prone to chronic maintenance. Understanding peak voltage subtleties, correctly calculating Joule energy, and ensuring a clean physical board layout ensure that electronics operate with high availability, even in hostile electrical grids full of impulsive noise.

Investing time in proper surge protection planning avoids absurd financial losses from field board replacements and unplanned production stoppages. By aligning electrotechnical theory and good assembly practices, designers and technicians deliver durable, safe systems truly prepared to face the severe challenges of the modern industrial environment.