Inductor Description and Selection: Decoding Markings, Types, and Substitutions

An accurate inductor description on a datasheet is never just a single Henry value. It is a multi-dimensional profile of magnetic, thermal, and physical limits. When you spec an inductor for a buck converter, an RF filter, or an audio crossover, looking only at the microhenry (µH) rating is the fastest way to fry a MOSFET or tank your efficiency. The real engineering happens in the margins: saturation current, DC resistance, and self-resonant frequency.

This guide strips away the abstract textbook theory and gives you the bench-level framework to read inductor markings, decode datasheet parameters, diagnose failures, and safely substitute parts when your exact BOM item is on a 40-week lead time.

The Core Inductor Description: What Actually Matters on the Spec Sheet

Before comparing types, you need to know which five parameters define a power inductor's real-world behavior. According to Coilcraft's inductor design guidelines, these are the non-negotiable metrics:

• Inductance (L): The nominal value, usually measured at a low AC voltage and a specific frequency (e.g., 100 kHz). It drops as DC bias current increases.
• DC Resistance (DCR): The resistance of the wire winding. Lower is better. High DCR causes I²R heating and kills converter efficiency.
• Saturation Current (Isat): The DC current at which the inductance drops by a specified percentage (usually 20% or 30%). Exceeding Isat turns your inductor into a low-value resistor, spiking current and destroying downstream switching FETs.
• RMS Current (Irms): The DC current that causes the component's temperature to rise by 40°C above ambient. This is a thermal limit, not a magnetic one.
• Self-Resonant Frequency (SRF): The frequency where the inductor's parasitic parallel capacitance resonates with its inductance. Above the SRF, the part acts like a capacitor. Your operating frequency must be well below the SRF.

Inductor Type Comparison: Construction, Tolerance, and Tempco

Not all inductors are built the same. The core material and winding method dictate where the part belongs in your circuit. Here is how the four main families stack up.

Decoding Inductor Markings and Color Codes

When you are scavenging parts from a donor board or trying to identify a reel that lost its label, reading the physical inductor description markings is critical. Unlike resistors, inductor codes can be highly manufacturer-specific, but industry standards do exist.

SMD Power Inductor Codes (3-Digit System)

Most shielded and unshielded SMD power inductors use a three-digit code where the first two digits are the significant figures and the third is the multiplier (number of zeros), expressed in microhenries (µH).

• 100 = 10 × 10⁰ = 10 µH
• 101 = 10 × 10¹ = 100 µH
• 472 = 47 × 10² = 4700 µH (4.7 mH)

The 'R' Exception: When the value is less than 10 µH, the letter 'R' acts as a decimal point. 4R7 means 4.7 µH. R10 means 0.10 µH.

Multilayer and RF Inductor Codes

Small RF inductors (like the Murata LQG series) often use a two-digit code plus a letter, expressed in nanohenries (nH).

• 10N = 10 nH
• 4N7 = 4.7 nH (The 'N' is the decimal point for nH)

Axial Color Bands

Through-hole axial inductors (like the Vishay IM series) use color bands similar to resistors, but the base unit is microhenries. The first two bands are digits, the third is the multiplier, and the fourth is tolerance (Gold = ±5%, Silver = ±10%). A band sequence of Brown-Black-Brown-Silver translates to 1-0-10¹ = 100 µH ±10%.

Real-World Failure Modes and Visual Symptoms

Inductors rarely fail silently. When they exceed their physical limits, they leave forensic evidence on the PCB. Here is how to diagnose a dead coil based on visual and electrical symptoms.

Safe Substitution: What to Do When the Exact Part is Missing

Supply chain shortages frequently force engineers to substitute inductors. You cannot just swap parts based on the µH value. Follow this strict substitution hierarchy to ensure your circuit survives the swap. For deeper application notes on magnetic substitutions, refer to Würth Elektronik's component selection guides.

1. Match Inductance (L): The substitute must be within ±10% of the original nominal value. A 4.7 µH part can usually replace a 4.2 µH part, but a 10 µH part will alter your control loop compensation and may cause subharmonic oscillation.
2. Verify Saturation Current (Isat): The substitute's Isat must be greater than or equal to the original. Never downgrade Isat. If the original was 5A (20% drop), a 6A substitute is safe. A 4A substitute will cause peak-current limit trips or FET destruction.
3. Check DC Resistance (DCR): The substitute's DCR should be less than or equal to the original. Higher DCR increases heat and drops output voltage under load.
4. Confirm Self-Resonant Frequency (SRF): The substitute's SRF must remain at least 20% higher than your circuit's maximum switching or operating frequency.
5. Physical Footprint and Height: Ensure the pad geometry matches (e.g., 1210 vs 1812) and that the Z-height clears your enclosure or shielding cans.

Example Substitution: Your BOM calls for a Coilcraft XEL4030-103ME (10 µH, 4.5 A Isat, 32 mΩ DCR). It is out of stock. You can safely substitute a TDK SPM4030T-100M (10 µH, 4.8 A Isat, 39 mΩ DCR). The Isat is higher (safe), and while the DCR is slightly higher (39 vs 32 mΩ), the thermal margin in a standard 2A buck converter is wide enough to absorb the extra 7 mΩ without exceeding the 40°C temperature rise limit.

The Decision Tree: Picking the Right Inductor for Your Circuit

Stop guessing which family to use. Follow this decision path to land on a concrete part type and a proven default series for your next build.