How to test open inductors on board with digital multimeter

The critical role of inductors in power supplies and boards

When opening a power supply, a television board, or a frequency inverter, we encounter various robust components. Among them, power inductors stand out, which are basically coils of copper wire wrapped around a ferrite or iron magnetic core. In practice, the role of this component is to temporarily store energy in the form of a magnetic field and filter out unwanted electrical noise. In modern fast-switching circuits, they help maintain the stability of the electrical current feeding the rest of the board.

However, due to the high currents flowing through these circuits, these components suffer greatly from voltage spikes and excessive thermal dissipation. Over time, the extremely thin enameled wire that makes up the internal coil can break completely. When this happens, we say the inductor is open, completely interrupting the flow of electricity and causing the equipment to stop working altogether. To deepen your workbench diagnostic knowledge, it is worth checking out our guide on how to test power MOSFETs on board with a digital multimeter, another component that often burns out alongside damaged power supplies.

Before touching any multimeter probe to an electronic board, there is a golden rule that should never be ignored: safety. Never perform measurements on live circuits connected to the grid unless the procedure specifically requires active voltage testing. In addition, the large electrolytic capacitors neighboring the power inductors store dangerous electrical charges for several minutes even after the device is unplugged. In practice, this means a careless move can blow your multimeter or cause a severe short circuit on the board.

Preparing the workbench and safely discharging capacitors

The correct procedure requires you to unplug the equipment and wait a few minutes for the natural drainage of filter capacitors. If necessary, use an appropriate discharge resistor to drain remaining energy before starting physical inspection. Visually inspect the body of the inductor for clear signs of overheating, such as darkened ferrite, cracks in the plastic housing, or the characteristic odor of burnt varnish. Many serious failures leave obvious visual traces that facilitate immediate diagnosis at first glance.

The digital multimeter is the most accessible and fast tool to perform an initial triage on electromagnetic components on the electronics workbench. To find a broken coil, turn the central selector of your measuring instrument to the continuity symbol, which usually resembles a stylized loudspeaker or a diode. In practice, this function injects a small direct current through the test probes and measures the resulting electrical resistance, emitting a sound signal (beep) if the circuit offers low resistance to electron flow.

How to configure the multimeter for the continuity test

Before touching the board, touch the red probe to the black probe to test if the instrument is beeping correctly and if the internal battery has adequate charge. If the display shows a very high fixed number or the open circuit symbol (usually represented by the letters 'OL' for open loop), the multimeter is indicating that continuity is broken. If you also need to check surge protections on the same board, consult the detailed guidelines in how to test a varistor on board with a digital multimeter to complement your failure analysis.

Correct execution of the test requires attention to copper trace paths and parallel connected components that could interfere with the reading. Follow the steps below carefully to check the component directly on the printed circuit board:

1. Completely disconnect the equipment from the electrical grid and ensure large capacitors are discharged.
2. Turn the digital multimeter selector to the continuity test scale with a buzzer or to the lowest ohmic resistance scale (200 ohms).
3. Firmly place the black probe on one of the inductor's solder terminals on the bottom of the board and the red probe on the opposite terminal.

The step-by-step procedure for testing the inductor on board

If the inductor is intact, the continuous copper wire will present very low electrical resistance, causing the multimeter to emit a continuous sound beep and display values very close to zero ohms on the digital screen. On the other hand, if the internal coil is broken due to overheating, the instrument will remain silent and the display will show the open circuit indicator. Remember that parallel tracks or diodes and transformers connected to the same circuit can create alternative current paths, generating false readings that require lifting at least one terminal of the component.

A very common mistake among electronics beginners is believing that the continuity test with a beep solves 100% of problems related to power inductors. In practice, the continuity function only tests whether the circuit is closed from end to end, meaning the wire is not broken. However, an inductor can suffer from a subtler problem known as a shorted turn. This occurs when the varnish insulation of two adjacent turns of copper wire melts, causing them to touch and reduce the effective inductance of the component without opening completely.

Identifying traps and turns-to-turns shorts in the component

When this happens, the standard multimeter will still emit the continuity beep because the wire remains whole from one side to the other, but the board will still fail in operation due to losing the ability to filter energy properly. If you suspect this type of defect in switched-mode power supplies or high-frequency circuits, the ideal is to rely on a bench meter or a specific inductance and Q-factor meter. If your circuit uses adjacent voltage regulators that may also have been affected by the short, it is worth reviewing the procedure in how to test AMS1117 voltage regulator on board with meter to ensure a complete repair.

Final considerations on the maintenance of magnetic circuits

Accurate diagnosis of magnetic components on printed circuit boards is an essential skill that separates an amateur technician from a qualified workbench professional. Knowing how to use the multimeter's continuity function to detect open inductors saves precious time and avoids unnecessary replacement of expensive parts in power supplies and inverters. However, it is fundamental to always maintain caution regarding parallel components that can mask measurements, requiring partial removal of the component from the board when persistent doubts remain. With practice, patience, and respect for electrical safety standards, magnetic fault diagnosis becomes a fast and highly reliable process in your daily work.