The fundamental inductors unit is the Henry (H), defined as the inductance that produces one volt of electromotive force when the current changes at one ampere per second ($V = L \frac{di}{dt}$). In practical electronics, a 1 Henry inductor is physically massive and rarely used outside of heavy industrial filtering or audio crossover networks. On the bench, you will almost exclusively work in millihenries (mH), microhenries (µH), and nanohenries (nH). Selecting the right component requires translating this theoretical unit into physical constraints: DC resistance (DCR), saturation current ($I_{sat}$), and core material.

The Inductors Unit Explained: From Henrys to Nanohenries

Understanding the scale of the Henry is critical for troubleshooting and design. The energy stored in an inductor is calculated as $E = \frac{1}{2}LI^2$. Because current is squared, a small inductance handling high current stores significantly more energy than a large inductance handling low current.

  • Millihenries (mH, $10^{-3}$ H): Used in low-frequency applications like audio crossovers, 50/60Hz mains filtering, and low-speed motor drives. A typical 5mH toroidal inductor might be the size of a hockey puck.
  • Microhenries (µH, $10^{-6}$ H): The standard unit for switch-mode power supplies (SMPS). A 500kHz buck converter typically requires a 4.7µH to 22µH inductor. These are usually surface-mount (SMD) shielded ferrite or composite parts.
  • Nanohenries (nH, $10^{-9}$ H): Used in RF matching networks, high-speed digital signal integrity, and GHz-frequency filters. A 10nH inductor might just be a short trace on a PCB or a tiny 0402 ceramic chip.
Bench Tip: When measuring an unknown inductor with an LCR meter, match the test frequency to the application. Use 1kHz or 100Hz for mH-range power/audio inductors, and 100kHz or 1MHz for µH and nH-range switching/RF inductors. Core permeability shifts with frequency; a 10µH part measured at 1kHz might read 7µH at 1MHz.

Decoding Inductor Markings and Value Codes

Unlike resistors, inductor markings are not universally standardized across all form factors, but the EIA 3-digit and 4-digit codes dominate SMD components. The base unit for these codes is typically the microhenry (µH) for power parts and nanohenry (nH) for RF parts.

SMD Power Inductors (Base Unit: µH)

For SMD power inductors, the first two digits are the significant figures, and the third digit is the multiplier (number of zeros).

  • 100: 10 × $10^0$ = 10 µH
  • 101: 10 × $10^1$ = 100 µH
  • 472: 47 × $10^2$ = 4700 µH (4.7 mH)

When the value includes a decimal, the letter R replaces the decimal point.

  • 4R7: 4.7 µH
  • R10: 0.10 µH (100 nH)

SMD RF Inductors (Base Unit: nH)

RF chip inductors use the same logic, but the base unit is nanohenries, and the letter N replaces the decimal point.

  • 10N: 10 nH
  • 4N7: 4.7 nH
  • R10: 0.10 nH (rare, usually expressed as 100 pH)

Axial Leaded Inductors (Color Bands)

Axial inductors often use a 4-band color code similar to resistors, but read according to MIL-I-15156 standards. The first two bands are significant digits, the third is the multiplier, and the fourth is tolerance. The base unit is always microhenries (µH). A brown-black-brown-silver band sequence translates to 1-0-10¹ µH at 10% tolerance, yielding a 100µH inductor.

Inductor Construction Types and Selection Matrix

The physical construction of the core dictates the inductor's behavior under load, its temperature stability, and its electromagnetic interference (EMI) profile. Below is a decision matrix for the four primary construction types you will encounter.

Construction Type Core Material Typical Tolerance Tempco (ppm/°C) EMI / Shielding Typical Use Case
Air Core None (Air/Plastic) ±2% to ±5% ~0 (Highly stable) Unshielded (High flux leakage) RF tuning, high-frequency resonant tanks, audio crossovers.
Ferrite Drum (Unshielded) Manganese-Zinc or Nickel-Zinc Ferrite ±10% to ±20% +100 to +500 Unshielded (Moderate leakage) Low-cost buck converters, non-critical filtering. (e.g., Würth WE-LQ)
Shielded Composite / Powdered Iron Iron powder in epoxy/resin matrix ±20% to ±30% +50 to +150 Fully Shielded (Low leakage) High-density SMPS, automotive, noise-sensitive environments. (e.g., Coilcraft XEL)
Toroidal Tape-wound silicon steel or ferrite ring ±10% to ±15% Variable based on core Self-shielding (Closed magnetic path) High-current DC-DC, mains EMI chokes, audio output filters.

Failure Modes: What a Dead Inductor Looks Like

Inductors are generally robust, but they fail catastrophically when pushed beyond their thermal or magnetic limits. Unlike capacitors that degrade slowly, inductor failures usually take down the surrounding switching MOSFETs or diodes.

Safety Warning: Inductors store energy ($E = \frac{1}{2}LI^2$). Never probe an inductor in a live, high-current circuit without proper differential probes. Interrupting the current path or shorting the terminals while energized will result in a high-voltage flyback spike that can destroy your multimeter or cause an arc flash.

1. Open Circuit (Wire Break)

  • Visual Symptom: Scorched epoxy coating, cracked ferrite core, or a visibly broken wire at the terminal pad.
  • Electrical Symptom: DCR reads 'OL' (Open Loop) on a multimeter. The circuit loses all output voltage.
  • Root Cause: Exceeded $I_{rms}$ (RMS current) rating, causing the copper wire to overheat and melt, or mechanical shock cracking a brittle ferrite drum.

2. Shorted Turns (Insulation Breakdown)

  • Visual Symptom: Bulging epoxy, melted plastic casing, or a distinct burnt-varnish smell. The core may show heat discoloration (brown/black scorch marks).
  • Electrical Symptom: DCR drops significantly below the datasheet spec. Inductance (L) drops sharply. The power supply will likely trip overcurrent protection (OCP) or blow the input fuse.
  • Root Cause: Voltage spikes exceeding the dielectric strength of the wire's enamel insulation, causing adjacent windings to short. This creates a low-resistance loop that rapidly heats up and destroys the part.

3. Core Saturation (Functional Failure)

  • Visual Symptom: None. The part looks perfectly fine.
  • Electrical Symptom: On an oscilloscope, the inductor current waveform changes from a smooth exponential curve to a sharp, linear ramp during the switch-on time. The switching MOSFET runs extremely hot.
  • Root Cause: The peak current exceeded the $I_{sat}$ (Saturation Current) rating. The magnetic core cannot store any more flux, causing the inductance to drop to near-zero (essentially becoming just a piece of wire with low DCR). Fix: Select a part with a higher $I_{sat}$ rating or increase the switching frequency to reduce peak ripple current.

The Substitution and Selection Decision Path

When the exact BOM part is out of stock, or you are designing a circuit from scratch, use this decision path to select a safe substitute. The golden rule of inductor substitution: Never substitute a part with a lower $I_{sat}$ or $I_{rms}$ rating. You can always use a higher current rating, provided the physical footprint and DCR are acceptable.

Design Constraint / Scenario Required Parameter Concrete Part Pick (Default)
High-Frequency Buck Converter (1MHz+) Low core loss, shielded, low DCR, small footprint. Coilcraft XEL4020 Series (e.g., XEL4020-100 for 10µH). Excellent for 1-3MHz switching.
General Purpose 5V/3.3V Buck (500kHz) High $I_{sat}$, moderate cost, shielded. Würth Elektronik WE-PD Series (e.g., 74477420 for 10µH). The industry workhorse.
Audio Crossover / Low-Freq Filter High inductance (mH range), high $I_{rms}$, low DCR. Jantzen Audio Air Core or Iron Core Coils. Avoid ferrite to prevent hysteresis distortion.
RF Matching / GHz Filters Tight tolerance (±2%), high SRF (Self-Resonant Frequency), nH range. Murata LQG15HS Series (e.g., LQG15HS4N7S for 4.7nH). Multilayer ceramic construction.
Substituting Unshielded with Shielded EMI reduction needed, footprint must match. Always Safe. Shielded parts can replace unshielded parts of the same value and current rating without circuit modification.
Substituting Shielded with Unshielded Cost reduction, exact part missing. Unsafe if EMI is critical. Only do this if the circuit is enclosed in a metal chassis or EMI testing is not required. Expect increased radiated noise.

For further reading on magnetics design and core selection, refer to the Coilcraft Magnetics Glossary for standardized terminology, and the Analog Devices guide on inductor selection for buck regulators for deep-dive math on ripple current calculations.

Default Bench Recommendation: If you are prototyping a standard DC-DC switching regulator and need a reliable, broadly available 10µH inductor that will not saturate under moderate loads and will not fail EMI pre-compliance testing, stock your lab with the Würth Elektronik 74477420 (10µH, 4.5A $I_{sat}$, shielded). It costs roughly $1.50 in single quantities, uses the standard 1210 (3225 metric) footprint, and provides a robust baseline for 90% of hobbyist and commercial 500kHz buck designs.