How to test open inductors on board with digital multimeter

Why inductors fail and how to spot an open circuit

In modern electronics, the inductor is a core component that temporarily stores energy in a magnetic field when electric current flows through it. In practice, this means it acts as a noise filter or blocker in power supplies, motherboards, and radio frequency circuits. When an inductor suffers a current overload or overheating, the microscopic internal copper wire can melt, interrupting the electrical flow and creating what we call an open inductor. Identifying this fault requires patience and the proper use of bench tools to avoid misdiagnoses in complex circuitry.

Before grabbing the soldering iron, the technician must understand the basic anatomy of the component on the printed circuit board. Inductors can appear in axial formats, looking like a regular resistor with green or blue coating, or in SMD formats, which are tiny gray or black blocks soldered directly to the board surface. For diagnosing issues in neighboring components, such as the diodes explained in the guide on how to test zener diodes on board with a digital multimeter, the bench approach shares the same necessity of isolating the circuit from mains power to prevent false readings caused by charged capacitors.

How to prepare the digital multimeter for on-board testing

The digital multimeter is the technician's main ally on the electronics and automation workbench. To hunt down an open inductor, the first adjustment needed on the meter is selecting the continuity scale, also known as the beep mode. In practice, this function injects a small current through the circuit and measures electrical resistance, sounding a characteristic beep if the path is closed and uninterrupted. In addition, familiarity with the ohmic resistance scale helps when the coil possesses an intrinsic resistance that alters the standard behavior of the sound buzzer.

Before touching the probes to the board, make sure the equipment under test is completely powered off and main capacitors are discharged. Measuring semiconductors and coils with the circuit powered can fry the multimeter or yield misleading readings induced by residual voltages. For anyone dealing with switch-mode power supplies and DC-DC converters, it is also worth checking complementary diagnostic techniques, such as those described in the article about how to test SMD voltage regulators with a multimeter on bench, thereby ensuring a complete check of the power supply stage on the board.

Step-by-step procedure to test SMD or axial inductors

Executing the test procedure requires precision to avoid damaging the small solder pads on the board. Follow this practical bench sequence to verify coil integrity:

1. Unplug the device from the wall outlet and wait a few seconds for electrolytic capacitors to drain.
2. Turn the rotary dial of the digital multimeter to the continuity position identified by the diode or sound wave symbol.
3. Touch the red and black test probes to the two extreme terminals of the inductor soldered on the board.
4. Observe the display and listen for the sound: if the meter beeps continuously and shows near-zero resistance, the inductor is closed and intact.
5. If the display shows an open circuit signal (usually represented by OL or 1) and there is no beep, the internal coil is broken.

In many practical situations, testing directly on the board can trick the technician due to parallel components, such as low-value resistors or other windings closing the circuit through alternate paths. If the multimeter indicates continuity but the device still fails to operate, lifting one side of the inductor off the board with a soldering iron and flux may be necessary to repeat the measurement in isolation.

Interpreting results and evaluating component isolation

When the inductor exhibits infinite resistance (open circuit), replacement is unavoidable. However, one must investigate the root cause before putting a new component in place, since an inductor does not burn out by chance. Current overloads caused by short circuits in downstream stages of the board are usually the main culprits. Validating associated semiconductors, following methods akin to those discussed when examining power transistors per the material on how to test power MOSFETs on board with a digital multimeter, ensures the new inductor will not be destroyed upon the very first power-up of the repaired circuit.

Another important factor is the pure ohmic resistance of the coil. Line filter inductors or high-power supply chokes feature thick wires with near-zero resistance, while RF inductors or coils with many turns of fine wire can present dozens of ohms of internal resistance. Knowing the design and checking the schematic helps differentiate a normal winding from a partially shorted turn or one damaged by excess heat over years of continuous operation.

Final considerations on inductor maintenance

Correctly diagnosing an open inductor on the board saves time and prevents unnecessary replacement of expensive integrated circuits. Although visual inspection helps spot burn marks, ferrite cracks, or bulging resin, electrical confirmation with the digital multimeter is irreplaceable in workbench routines. Mastering the use of resistance and continuity scales turns the technician into a safer professional, capable of resolving complex faults with agility and surgical precision in any type of electronic equipment.