What Are Inductors? A Decision-Forward Guide to Types, Markings, and Selection

An inductor is a passive two-terminal component that stores energy in a magnetic field when electrical current flows through it, inherently resisting changes in current. Measured in Henries (H), its core function is to block high-frequency AC while passing DC. If you are asking what are inductors in a practical sense, think of them as the "flywheels" of your circuit: they smooth out current ripple in power supplies, tune frequencies in RF tanks, and choke electromagnetic interference (EMI) on data lines.

The Physics and Math: How Inductors Actually Work

Unlike capacitors that store energy in an electric field and resist changes in voltage, inductors store energy in a magnetic field and resist changes in current. The governing equation is Faraday’s law of induction, expressed as:

V = L × (di / dt)

Where V is the induced voltage, L is inductance in Henries, and di/dt is the rate of current change over time. This relationship explains why inductors are both incredibly useful and potentially destructive.

Numeric Worked Example: Suppose you have a 100mH relay coil carrying 10mA (0.01A) of steady DC current. A transistor switches off the current in 1 microsecond (1µs or 1×10⁻⁶s).
V = 0.1H × (0.01A / 0.000001s) = 1,000 Volts.
That 1kV spike will instantly avalanche a standard 2N2222 transistor or arc across a mechanical switch contact. This is why we clamp inductive loads with diodes or snubber networks.

Inductor Types and Selection Matrix

Not all inductors are created equal. The core material dictates the saturation current, temperature coefficient (tempco), and frequency response. Below is a decision matrix to help you select the right construction for your specific application.

Selection Rule of Thumb: If your circuit handles more than 500mA of continuous current, you must use a wire-wound inductor (Ferrite or Powdered Iron). Multilayer ceramic SMD inductors will overheat and fail due to high DC resistance (DCR) and low current ratings.

Decoding Inductor Markings and SMD Codes

Reading inductor markings is a frequent stumbling block, especially with surface-mount device (SMD) footprints where space is limited. The coding system borrows heavily from resistor color codes and SMD capacitor numbering, but the base unit is almost always microhenries (µH).

SMD 3-Digit Numeric Codes

The first two digits are the significant figures, and the third digit is the multiplier (number of zeros).
Example: A code reading 101 means 10 × 10¹ µH = 100 µH.
Example: A code reading 472 means 47 × 10² µH = 4,700 µH (or 4.7 mH).

The 'R' Decimal Indicator

For values under 10 µH, the letter 'R' replaces the decimal point.
Example: 4R7 translates to 4.7 µH.
Example: R22 translates to 0.22 µH (or 220 nH).

Axial Color Bands

Through-hole axial inductors use four color bands identical to the resistor color code, but the resulting value is in microhenries. A brown-black-brown-silver band sequence means 1-0-×10 µH with a 10% tolerance, yielding a 100 µH inductor. Always verify with an LCR meter, as some vintage military-spec parts use different base units.

Failure Modes: Visual Symptoms and Bench Testing

Inductors are generally robust, but they operate under severe thermal and magnetic stress in power applications. When they fail, they usually exhibit one of three distinct modes. Use a multimeter for DC resistance (DCR) checks and an LCR meter for inductance verification.

1. Open Circuit (Thermal Overload)

• Visual Symptom: Scorched epoxy coating, melted solder pads, or a visible break in the fine copper wire at the termination point.
• Bench Test: Multimeter reads infinite resistance (OL) across the terminals.
• Root Cause: RMS current exceeded the wire's thermal limit, melting the internal winding. Common in unshielded power inductors placed near heat sources without adequate airflow.

2. Shorted Turns (Insulation Breakdown)

• Visual Symptom: Bulging or blistered outer casing. The part may smell faintly of ozone or burnt plastic. Sometimes there are no visual clues.
• Bench Test: DCR is significantly lower than the datasheet spec, and measured inductance drops drastically (e.g., a 10µH part reads as 2µH).
• Root Cause: Voltage spikes or excessive heat melted the thin enamel insulation between adjacent wire turns. The current now takes a "shortcut" through the shorted turns, reducing the total number of active coils and thus the inductance.

3. Magnetic Saturation (Functional Failure)

• Visual Symptom: None. The part looks perfectly pristine.
• Bench Test: Inductance reads correctly at low signal levels on an LCR meter, but drops to near-zero under operating DC bias.
• Root Cause: The peak current exceeded the component's saturation current ($I_{sat}$). The magnetic core cannot hold any more flux, causing the inductor to temporarily behave like a piece of straight wire. This results in massive current spikes that destroy the driving MOSFET.

The Substitution Decision Tree and Default Picks

When the exact BOM inductor is out of stock or you are reverse-engineering a board with missing parts, you must substitute safely. Follow this decision path to select a replacement without blowing up your power stage or detuning your RF circuit.

Step 1: Match the Inductance Value

For power supply filtering (buck/boost converters), a ±20% deviation in inductance is usually acceptable and will only slightly alter the ripple voltage and transient response. For RF tuning, LC oscillators, or timing circuits, you must match the value within ±2% or your frequency target will drift.

Step 2: Check Current Ratings (The Critical Trap)

Inductors have two distinct current ratings. Your substitute must meet or exceed both compared to the original part:

• $I_{rms}$ (Thermal Current Rating): The maximum continuous DC current the wire can handle before overheating. Dictated by DCR.
• $I_{sat}$ (Saturation Current Rating): The peak current at which the inductance drops by a specified amount (usually 20% or 30%). Dictated by core material and geometry.

Rule: $I_{sat}$ must be higher than your circuit's peak switching current. $I_{rms}$ must be higher than your maximum continuous load current.

Step 3: Evaluate DCR and Footprint

Lower DC Resistance (DCR) is always better for power efficiency, but physically larger. Ensure the substitute footprint matches your PCB pads. If substituting a through-hole part for an SMD pad, you can use enameled magnet wire to bridge the gap, but keep the leads short to avoid adding parasitic inductance.

The Concrete Default Picks

Stop guessing and keep these proven workhorses in your bench inventory. When in doubt, default to these specific part families:

By understanding the physical construction, decoding the markings accurately, and respecting the dual current ratings ($I_{rms}$ and $I_{sat}$), you can confidently specify, test, and substitute inductors in any DC-DC, RF, or EMI filtering application. For deeper magnetics design and core loss calculations, utilize manufacturer tools like the Würth Elektronik REDEXPERT simulator or consult the Coilcraft Design Library for application-specific selection guides. For foundational theory on magnetic fields and inductance, the All About Circuits textbook chapter on inductors remains an excellent, mathematically rigorous reference.