What Are Inductors? Core Types, Markings, and Selection Guide

The Short Answer: What Are Inductors and How Do They Behave?

An inductor is a passive electronic component that stores energy in a magnetic field when electrical current flows through it. Its fundamental job in a circuit is to oppose changes in current. If current tries to increase, the inductor pushes back; if current tries to drop to zero, the inductor collapses its magnetic field to keep the current flowing, often generating a massive voltage spike in the process.

The governing equation is V = L(di/dt). The voltage across the inductor (V) is proportional to its inductance (L, measured in Henries) multiplied by the rate of change of current over time (di/dt).

The Flywheel Analogy: Think of an inductor as a heavy mechanical flywheel in a water pipe. When you open the valve (apply voltage), the water (current) doesn't instantly reach full speed; it takes time to spin the heavy flywheel up. Once it is spinning, if you suddenly slam the valve shut (open the circuit), the flywheel's inertia wants to keep the water moving. Since the water has nowhere to go, it creates a massive pressure spike (inductive kickback) that can burst the pipe (blow your switching MOSFET). This is exactly why we use flyback diodes across relay coils and motor windings.

In DC circuits, once the magnetic field is fully established, an ideal inductor acts as a short circuit (zero resistance). In AC circuits, it exhibits inductive reactance ($X_L = 2\pi fL$), meaning its impedance increases as the frequency goes up. This makes them indispensable for filtering high-frequency noise and shaping power supply outputs. For a deeper mathematical breakdown of reactance, the Electronics Tutorials guide on inductors provides excellent AC phasor diagrams.

Inductor Core Types and Selection Matrix

Not all inductors are created equal. The material wrapped inside the copper windings—the core—dictates the component's saturation current, frequency limits, and thermal behavior. Selecting the wrong core material is the most common reason a DIY switching power supply fails under load.

Which type for which job? If you are building an RF antenna matching network at 433 MHz, you must use an air-core or ceramic-core inductor; ferrite will introduce massive core losses and detune the circuit. Conversely, if you are designing a 5V-to-3.3V buck converter pulling 10 amps, an air-core inductor would require thousands of turns of wire, resulting in unusable DC resistance (DCR). You need a metal alloy or ferrite core to multiply the magnetic flux density, allowing for a physically small part with low DCR.

Decoding Physical Markings and Safe Substitution Rules

Unlike resistors with their standardized 4-band color codes, inductor markings can be notoriously cryptic, varying between SMD chip manufacturers and through-hole suppliers. Here is how to read the most common EIA-style 3-digit and 4-digit codes printed on SMD and molded radial inductors.

• The 3-Digit Code (Microhenries): The first two digits are the significant figures, and the third digit is the multiplier (number of zeros). The base unit is almost always microhenries (µH).
  • 101 = 10 × 10¹ µH = 100 µH
  • 472 = 47 × 10² µH = 4,700 µH (or 4.7 mH)
  • 4R7 = The 'R' acts as a decimal point = 4.7 µH
• Tolerance Letters: Usually found at the end of the numeric code.
  • M = ±20% (Most common for power inductors)
  • K = ±10%
  • J = ±5%

How to Substitute Safely When the Exact Part is Missing
You are repairing a board or building a prototype, and the BOM calls for a specific Würth WE-PD or Coilcraft part that is out of stock. Substituting an inductor is not as simple as matching the microhenry value. You must verify three critical parameters using the manufacturer's datasheet (resources like the Coilcraft Inductor Finder are invaluable for cross-referencing):

1. Inductance (L): Must be within ±20% of the original. Going too high will slow down the transient response of a power supply; going too low will increase output ripple.
2. Saturation Current ($I_{sat}$): The substitute's $I_{sat}$ must be greater than or equal to the original. Never substitute a lower $I_{sat}$ part, or you risk catastrophic switch failure during load transients.
3. Thermal Current ($I_{rms}$): The substitute's $I_{rms}$ rating must handle the continuous DC load without the part exceeding a 40°C temperature rise.

The DCR Trap in Current-Mode Control: Here is a non-obvious bench gotcha. If you are substituting an inductor in a peak current-mode controlled buck converter, do not blindly choose a part with ultra-low DC Resistance (DCR). Many controllers rely on the voltage ramp generated across the inductor's inherent DCR to stabilize the inner current loop. If you swap in a modern metal-alloy inductor with a DCR of 0.5mΩ to replace an older ferrite part with a DCR of 15mΩ, the controller won't see a sufficient sense ramp, leading to subharmonic oscillation and erratic switching. If you must use a lower DCR part, you will need to add an external RC ramp injection network or increase the slope compensation.

Real-World Failure Modes and Visual Diagnostics

Inductors are generally robust, but when pushed past their physical limits, they fail in highly specific ways. When troubleshooting a dead power supply or a blown motor driver, pull out your 10x loupe and inspect the inductor for these visual symptoms.

1. Thermal Overload (Exceeded $I_{rms}$)

Visual Symptom: The epoxy coating or plastic overmold is discolored (yellowed or browned), the surrounding PCB solder mask is blistered, and the part smells distinctly of burnt phenolic resin. The copper windings inside may have melted, creating an open circuit.

The Fix: Your continuous load current is exceeding the part's thermal rating. You must either increase the physical size of the inductor to get more thermal mass, or select a part with thicker copper wire (lower DCR) to reduce $I^2R$ heating.

2. Core Saturation and Inductive Kickback

Visual Symptom: The inductor itself looks perfectly fine—no burns, no cracks. However, the driving MOSFET, diode, or switching IC nearby has a physical crack in its silicon package, or measures as a dead short on your multimeter.

The Fix: The inductor core saturated, turning it into a piece of straight wire, which allowed current to ramp out of control and blow the switch. Alternatively, if the circuit was opened without a freewheeling path, the $V = L(di/dt)$ kickback arc'd across the switch. Check your freewheeling diode orientation and verify the inductor's $I_{sat}$ is rated for the peak fault current, not just the nominal operating current. All About Circuits offers a great primer on calculating these transient voltage spikes.

3. Mechanical Cracking (Thermal Shock / Board Flex)

Visual Symptom: A hairline crack running through the ferrite core or the ceramic body of an SMD inductor. On shielded SMD parts, you might see a microscopic split in the metal housing or the termination pads lifting slightly from the component body.

The Fix: Ferrite is essentially ceramic glass; it is incredibly brittle. If the PCB flexes during depaneling or if the inductor undergoes rapid thermal cycling (like in a high-power PFC circuit), the core can snap. This changes the magnetic air gap, drastically altering the inductance value and causing high-frequency ringing. Replace the part and consider adding a fillet of silicone RTV to the base of large through-hole toroids to absorb mechanical vibration.

4. Moisture Ingress and Corrosion

Visual Symptom: Green or white crusty oxidation forming where the copper wire meets the termination pin, particularly on unshielded radial drum-core inductors used in outdoor or high-humidity environments.

The Fix: The thin enamel wire coating was compromised, allowing humidity to attack the copper. This increases the DCR over time, leading to gradual voltage droop and eventual thermal failure. For harsh environments, always specify fully shielded, epoxy-encapsulated, or conformal-coated inductor assemblies.