Inductors and ferrite beads are among the least dramatic parts on a laptop board. They rarely fail on their own, they have no polarity, and to a multimeter both look like a piece of wire. Yet they are some of the most useful parts in diagnosis: they mark where one section of a circuit ends and another begins, they give you large pads to probe, and they are the natural points at which to split a shorted rail. This guide covers how to test them and how to use them as diagnostic tools.
What they are and where they sit
| Part | How it looks | Designator | Job |
|---|---|---|---|
| Power inductor (choke) | Square grey or black block, often several millimetres wide | PL, L | Stores energy in switching regulators; one per phase |
| Small inductor | Small rectangle, sometimes with a value code | L | Filtering, in some converters and RF circuits |
| Ferrite bead | Small black rectangle like a resistor | FB, L, PL | Blocks high-frequency noise on supply branches and signal lines |
| Common-mode choke | Small four-pad part on data lines | L, CM | Filters noise on USB, HDMI or other differential pairs |
Power inductors sit between a regulator's switching MOSFETs and its output capacitors. Ferrite beads sit where a rail branches off to feed a particular chip or connector, often next to a group of decoupling capacitors.
Background on both is in the glossary entries for power inductor and ferrite bead.
What a multimeter can measure
A multimeter cannot measure inductance, and it cannot tell you whether a ferrite bead filters properly at high frequency. What it can tell you:
- Continuity end to end — a good inductor or bead reads close to zero ohms. Power inductors read a few milliohms, far below what a normal meter resolves; ferrite beads read somewhere below an ohm.
- Open — a reading of OL end to end means the part, its joint or its winding is broken.
- Short to ground — measuring either end to ground tells you about the nets they connect, not about the part itself.
Out of circuit, an inductor's inductance can be checked with an LCR meter or a meter with an inductance mode, but on laptop boards this is rarely necessary.
Testing an inductor or bead in circuit
With the board unpowered:
- Set the meter to resistance or continuity.
- Measure from one end of the part to the other.
- Expect a reading near zero.
Because the part has such a low resistance, other parts in parallel do not affect the reading in any useful way. An open inductor or bead shows clearly as a high reading or OL, while every other reading is near zero.
When to suspect an open part
- A rail is present on one side of a ferrite bead and missing on the other.
- A regulator's output rail is missing while its switch node is active.
- A bead near a connector looks dark, cracked or corroded.
An open bead is often a symptom: a short downstream drew enough current to burn it. Before replacing it, measure the downstream side to ground.
The switch node of a buck regulator
In a step-down (buck) regulator, the inductor connects the switch node — where the high-side and low-side MOSFETs meet — to the output rail. The inductor's two pads therefore give you access to two different measurements:
| Pad | What it connects to | What to measure (unpowered) |
|---|---|---|
| Switch-node side | MOSFETs and the controller's phase pin | Resistance to ground and to the input rail |
| Output side | Output capacitors and the load | Resistance to ground — the rail's state |
A switch node shorted to ground points at the low-side MOSFET. A switch node shorted to the input rail points at the high-side MOSFET. An output shorted to ground points at the load or an output capacitor. Because the inductor reads nearly zero, both pads will show the same short — lift the inductor to tell which side has it. The structure of multi-phase core regulators, with one inductor per phase, is covered in VCore Phases, Inductors and Output Capacitors.
On a powered board, the switch node carries a fast square wave. A multimeter in DC mode shows its average, which is close to the output voltage, so the reading looks normal even though the node is switching. An oscilloscope shows the actual waveform.
Using inductors and beads to split a rail
When a rail is shorted and many chips hang off it, the fastest way to narrow the search is to divide the rail at its branch points. Ferrite beads, inductors and zero-ohm jumpers are those branch points.
- Find the shorted rail in the schematic and list the beads and jumpers that feed its branches.
- Lift one end of a bead.
- Measure each side to ground.
- The side that still reads shorted contains the fault.
- Repeat on that side until the short is in a small enough area to find with injection or a thermal camera.
This method is part of the wider short-hunting routine in How to Find a Short Circuit on a Laptop Motherboard.
Lifting a large power inductor needs more heat than a small bead, and its pads are connected to large copper areas. Use hot air with preheating, and work carefully to avoid lifting pads.
Bridging an open bead for diagnosis
Some technicians bridge an open ferrite bead with a thin wire to check whether the rest of a circuit works. As a short diagnostic step, with the downstream side confirmed free of shorts, this can tell you whether the bead was the only problem. It is not a repair: the bead's filtering role is lost, and a thin wire is not rated for the current a power bead carries. Fit a correct replacement afterwards.
How inductors and beads fail
- Open bead — the most common failure, almost always after a short downstream.
- Cracked inductor body — from impact or board flexing; the inductor may still conduct but can be noisy or intermittent.
- Corroded joints — after liquid damage.
- Overheated inductor — discoloured, sometimes with a buzzing noise when powered; usually a sign that the load or the regulator has a fault.
Inductors almost never fail short in a way a multimeter could see, because they already read close to zero.
Example: a missing USB rail
A laptop's left USB port provides no power. The schematic shows the port's 5 V supply passing through a load switch and then a small ferrite bead before reaching the connector. On a powered board, the 5 V rail is present before the bead and missing after it. With power removed, the bead reads OL end to end, and the connector side reads near zero ohms to ground. Under the microscope, the USB connector has a bent pin touching the shield. Straightening the pin removes the short; a new bead restores the rail. Replacing the bead without finding the bent pin would have burned the new one immediately. The broader method for inspecting components on a board is summarised in Inductors on Laptop Boards: How to Test Them.
Replacing an inductor or bead
For power inductors, match inductance, current rating, saturation current and package size; a wrong inductor can make the regulator unstable or overheat. For ferrite beads, match the impedance rating, the current rating and the package. A bead rated for too little current will fail again under normal load.
FAQ
Can a multimeter measure inductance?
Only if it has a dedicated inductance mode, and then only out of circuit. Most board repair does not require it — continuity and visual inspection are enough.
Why do both ends of an inductor read the same to ground?
Because the inductor itself is nearly a short. Any reading to ground on one end appears on the other end too. Lift the inductor to separate them.
Is it safe to replace a ferrite bead with a zero-ohm resistor?
Electrically it conducts, but it removes the noise filtering the designer intended. It may work in practice, but a proper bead is the correct repair.
Why does my inductor buzz?
Coil whine comes from the inductor's windings vibrating at the switching frequency under changing load. A small amount is common; a loud or new buzz can point at an abnormal load.
Are inductors and ferrite beads the same thing?
Both are coils of a sort, but an inductor stores energy in a regulator, while a ferrite bead turns high-frequency noise into heat. They look and test similarly with a multimeter.
Bottom line
Inductors and ferrite beads should read close to zero ohms; an open part shows as OL and almost always points at a short downstream. Use the inductor's pads to read the switch node and the output separately, use beads and jumpers to split rails when hunting shorts, and fix the cause of any burned bead before replacing it.