How to test open inductors on board with digital multimeter

How inductors work and why they fail

The inductor, popularly known as a coil, is a fundamental passive component in virtually any modern printed circuit board. It basically consists of a conductive wire wound around a core that can be made of air, ferrite, or iron. In practice, this means it temporarily stores energy in the form of a magnetic field when electric current flows through it, helping filter out noise and stabilize voltages. True to their name, inductors work by creating electromagnetic induction to oppose sudden variations of electric current in the circuit.

Despite their apparent constructive simplicity based on enameled copper wires, inductors are subject to severe failures resulting from voltage spikes, overheating, or manufacturing defects. When a coil suffers excessive thermal stress, the insulation of the enameled wire can melt and cause shorted turns or, in the worst scenario, the wire can break completely on the inside. When this happens, we say the inductor is open, completely interrupting the flow of electric current and paralyzing the operation of the stage where it resides. This is why accurate bench diagnosis becomes essential for any technician repairing various electronic equipment.

Identifying SMD and PTH inductors on the circuit board

Before grabbing the multimeter probes, the first practical step is to visually locate and identify the type of inductor present on the board under analysis. PTH components, standing for Plated Through-Hole, feature leaded terminals that pass through the board from one side to the other, very common in linear power supplies, input filters, and older boards. On the other hand, SMD components, meaning Surface-Mount Device, are designed for surface mounting, soldered directly onto the copper pads of the top face of the board, frequently found in smartphones, laptops, and modern motherboards.

On the board's silkscreen, meaning those white markings printed by the manufacturer, inductors are usually identified by the letter L followed by a number, such as L1, L2, or L101. Differentiating an inductor from a similarly shaped resistor or capacitor might seem challenging for beginners, but the physical shape usually gives away its nature. While resistors are typically rectangular and black with numerical markings and ceramic capacitors are usually brown or gray without ohmic continuity, inductors often reveal the wound wire inside or feature dark gray ferrite shielded bodies. Understanding this visual identification saves time and ensures you apply the correct measurement technique for each installed component.

Preparing the multimeter for bench diagnostics

The digital multimeter is the primary and most accessible tool for testing the integrity of an inductor directly on the workbench. To start the procedure, you must set the instrument's rotary selector to the continuity scale, commonly represented by a diode symbol accompanied by an audible beep signal. In some more advanced models, this exact scale shares the low ohmic resistance measurement mode, which greatly facilitates the rapid sweep process of traces and passive components on unpowered electronic boards.

It is mandatory to emphasize that any continuity or resistance test on the board must be performed with the equipment completely disconnected from the power grid and with the main capacitors properly discharged. Measuring semiconductors, inductors, or resistors with the circuit energized not only destroys the multimeter instantly due to overvoltage, but also poses severe electric shock risks to the operator. With the multimeter configured and the board unpowered, touch the probes together to confirm that the internal circuitry of the instrument is working and emitting the audible continuity beep without noticeable delays.

How to test an open inductor using continuity

Position the multimeter probes on the two outer terminals of the inductor you want to test on the board. If the inductor is intact and free of internal breaks, the electric current generated by the multimeter will flow freely throughout the copper wire winding, causing the device to emit a continuous beep and display a resistance value very close to zero ohms. This behavior indicates that the coil exhibits perfect electrical continuity, ruling out the hypothesis of a total physical break in its constructive body.

On the other hand, if the multimeter remains silent and the display shows an open circuit sign, usually indicated by an isolated number '1' or the letter OL meaning Over Limit, you are facing an open inductor. This definitively proves that the internal conductive wire of the coil has completely broken, preventing the passage of electricity and requiring component replacement. It is worth noting that small-value inductors or those built with extremely thin wires may exhibit a small noticeable ohmic resistance, but the total absence of continuity combined with the display locked in an open circuit seals the diagnosis of an irreversible defect in the part.

In-circuit measurement pitfalls and alternative paths

Although performing the test directly on the board is fast and convenient, every technician needs to know the limitations of this method to avoid false diagnoses. The phenomenon of in-circuit measurement, meaning testing the component soldered to the circuit, occurs because traces, very low-value resistors, diodes, or other parallel windings can create alternative paths for the multimeter current. Thus, even if an inductor shows continuity, the measured value may reflect other elements connected to the same node on the printed circuit board.

When readings are suspected to be masked by adjacent components, the safest practical recommendation is to desolder at least one of the inductor terminals from the board using a proper soldering iron and a solder sucker or desoldering wick. With one side free, the inductor becomes completely isolated from the rest of the circuit, allowing a clean, reliable measurement free from external interference. This extra care separates amateur diagnosis from professional technical analysis, ensuring you replace only the defective component. If you wish to deepen your knowledge in precise diagnostics, check out our guide on how to test optocouplers on board with a multimeter to master even more bench techniques. To explore these architectural concepts further in production, see our related technical analysis on How to unblock a port in the Windows firewall.

Final considerations on inductor diagnostics

Mastering the technique of testing open inductors using a digital multimeter on the workbench represents a turning point in the efficiency of any electronics repair laboratory. Understanding the relationship between the physical principle of induction and practical manifestations of wire breakage failures allows you to solve complex defects in switched-mode power supplies, industrial control boards, and audio equipment much faster and more assertively. Constant practice, combined with the correct use of continuity and resistance scales, turns the multimeter into a natural extension of the operator's technical reasoning.

Maintaining heightened attention to the constructive details of SMD and PTH components, together with the essential care of completely unpowered circuits before starting any inspection, ensures personal safety and the integrity of work tools. By correctly identifying and isolating defective inductors, the technician avoids unnecessary replacements and raises the quality standard of performed repairs. To further expand your range of practical tests on discrete components, we recommend supplementary reading on how to test a Zener diode on board with a multimeter to enhance your diagnostic skills.