What Is an Inductor? Core Types, Markings, and Selection Guide

What Is an Inductor? The 100-Word Direct Answer

An inductor is a passive two-terminal component that stores energy in a magnetic field when electric current flows through it. Measured in Henries (H), it fundamentally resists changes in current (AC) while passing direct current (DC) freely. The governing equation is V = L(di/dt), meaning the voltage across it is proportional to the rate of change of current. In practical electronics, you use inductors to filter high-frequency noise, store and transfer energy in switching regulators (buck/boost converters), and tune resonant RF circuits. Think of it as the electrical equivalent of a heavy water wheel: it takes effort to get it spinning (current building), but once spinning, it resists being stopped (current collapsing).

Inductor Core Types: Which One for Which Job?

The core material dictates an inductor's permeability, saturation current, and frequency response. Picking the wrong core is the most common reason a switching power supply whines or an RF filter fails. Here is the definitive comparison for bench and jobsite selection.

Decoding the Markings: Through-Hole Color Codes and SMD Text

When you are scavenging parts or verifying a BOM, you need to read the physical markings. Inductors use two primary coding systems depending on their package.

Through-Hole Axial Color Codes

Axial inductors (like the classic Bourns 78F series) use a 4-band color code similar to resistors, but the base unit is always microhenries (µH).

• Band 1: First significant digit
• Band 2: Second significant digit
• Band 3: Multiplier (number of zeros)
• Band 4: Tolerance (Silver = ±10%, Gold = ±5%)

Example: An inductor with Brown-Black-Brown-Silver bands translates to 1 - 0 - x10 = 100 µH ±10%. If the third band is Gold, it acts as a decimal multiplier (x0.1), so Brown-Black-Gold-Silver is 1.0 µH ±10%.

SMD Inductor Markings

Surface-mount inductors use a 3-digit or 4-character alphanumeric code. The base unit is again microhenries (µH) for standard power/RF parts, though nanohenry (nH) RF parts will explicitly state it in the datasheet.

• 3-Digit Code: First two digits are significant, the third is the multiplier (power of 10). Example: 101 = 10 × 10¹ = 100 µH. 470 = 47 × 10⁰ = 47 µH.
• Alphanumeric Code: The letter 'R' or 'N' acts as a decimal point. Example: 4R7 = 4.7 µH. R47 = 0.47 µH.

Failure Modes: How Inductors Die and What It Looks Like

Inductors are generally robust, but they do fail when pushed past their thermal or magnetic limits. Here is how to diagnose them on the bench.

1. Open Circuit (Wire Break)

• Visual Symptom: Often none. Sometimes you will see a microscopic break in the copper wire at the termination pad, or burnt enamel near the leads.
• Electrical Symptom: DCR (DC Resistance) reads infinite (OL) on a multimeter. The circuit output is dead.
• Cause: Mechanical shock, thermal cycling cracking the solder joint, or a massive current spike fusing the wire.

2. Shorted Turns (Insulation Breakdown)

• Visual Symptom: Bulging epoxy coating, scorched wrapping, or a distinct burnt smell. The core may look cracked.
• Electrical Symptom: DCR reads significantly lower than the datasheet spec. Inductance (measured with an LCR meter) drops drastically.
• Cause: Overheating melts the thin enamel insulation between wire windings, causing adjacent turns to short. This reduces the effective number of turns, killing the inductance.

3. Core Saturation (Design Flaw, Not Part Failure)

• Visual Symptom: None. The part looks pristine.
• Electrical Symptom: The inductor "whines" audibly. The switching MOSFET in your power supply gets violently hot and eventually fails. Current waveforms on an oscilloscope show sharp, unnatural spikes at the peaks.
• Cause: The DC bias current exceeded the inductor's I_sat (saturation current) rating. The core cannot hold any more magnetic flux, so the inductor effectively becomes a low-value resistor, causing massive current spikes.

The Substitution Rulebook: Swapping Parts Safely

When the exact BOM part is out of stock, you can substitute an inductor, but you must verify four parameters in this exact order. If any of these fail, the substitution will break your circuit.

1. Inductance (L): Must match within ±10%. A 10µH part can be replaced with a 9µH or 11µH part in most switching regulators, but RF matching networks require exact values (±2%).
2. Saturation Current (I_sat): The substitute's I_sat must be greater than or equal to the original. This is the current at which inductance drops by 20% to 30%. Never downsize I_sat.
3. RMS Current (I_rms): The substitute's I_rms (the current that causes a 40°C temperature rise) must be greater than or equal to the original. This prevents thermal failure.
4. Shielding: If the original was a shielded inductor (magnetic flux contained within the package), the substitute must be shielded. I once swapped a shielded ferrite for an unshielded powdered iron on a 2.4GHz WiFi board and killed the receiver sensitivity by 15dB due to radiated magnetic noise.

For reliable cross-referencing, use the Coilcraft Inductor Finder or the TDK Inductor Product Guide to filter by these exact parameters rather than just matching the microhenry value.

Decision Tree: Pick Your Exact Inductor in 60 Seconds

Stop guessing. Follow this decision path to land on the correct core type and a concrete, off-the-shelf part number for your next build.