The fundamental principle that inductors always oppose a change in AC current (as well as transient DC) is governed by Faraday’s and Lenz’s laws of electromagnetic induction. When alternating current flows through a coil, the continuously changing magnetic field induces a back-electromotive force (back-EMF) that directly fights the change in current. This opposition, known as inductive reactance ($X_L$), is not a static resistance but a dynamic, frequency-dependent impedance. Understanding this behavior is critical for designing power supplies, RF filters, and motor drives, where selecting the wrong core material or misreading a part marking can lead to core saturation, thermal failure, or catastrophic MOSFET destruction.
The Physics: Why Inductors Always Oppose a Change in AC
Unlike resistors, which oppose current flow uniformly regardless of frequency, inductors react to the rate of change of current ($di/dt$). The governing equation is $V = -L(di/dt)$. When AC current reverses direction, the magnetic field stored in the inductor's core must collapse and rebuild in the opposite polarity. The energy required to force this change manifests as a voltage spike opposing the source.
This opposition is quantified as inductive reactance ($X_L$), measured in ohms ($\Omega$):
$X_L = 2\pi f L$
Where $f$ is frequency in Hertz and $L$ is inductance in Henries. Consider a 10 mH (0.01 H) inductor:
- At 60 Hz (Mains AC): $X_L = 2 \times \pi \times 60 \times 0.01 = 3.77 \Omega$
- At 100 kHz (Switch-mode supply): $X_L = 2 \times \pi \times 100,000 \times 0.01 = 6,283 \Omega$
The Water Analogy: Imagine a heavy water wheel in a pipe. When water (DC) flows steadily, the wheel spins freely, offering little resistance. But if you rapidly try to reverse the water's direction (AC), the inertia of the heavy wheel fights the reversal. The faster you try to reverse the flow (higher frequency), the harder the wheel pushes back.
Inductor Types and Selection Criteria
Selecting the right inductor requires matching the core material and physical construction to your circuit's frequency, current, and EMI constraints. Here is how the primary types compare on the bench.
| Type / Construction | Core Material | Typical Tolerance | Tempco (ppm/°C) | Typical Use Case |
|---|---|---|---|---|
| Air Core (Wire wound on ceramic/plastic) | None (Air) | ±2% to ±5% | +50 to +200 | RF tuning, VHF/UHF filters, high-Q resonant tanks. |
| Shielded SMD (e.g., Bourns SRP series) | Carbonyl iron / Ferrite powder in epoxy | ±10% to ±20% | +100 to +300 | DC-DC buck/boost converters, high-current power rails. |
| Unshielded Drum (e.g., Coilcraft MSS series) | Ferrite drum core | ±10% to ±20% | +150 to +400 | Low-cost power filtering, non-critical EMI environments. |
| Toroidal (e.g., TDK B82791 series) | Ferrite or MPP (Molypermalloy powder) | ±15% to ±30% | +200 to +500 | Common-mode chokes, high-inductance AC line filtering. |
Decoding Physical Markings and Safe Substitution
Physical inductors rarely have the luxury of large printed values. Decoding their markings accurately is the first step in reverse-engineering or repairing a board.
How to Read SMD and Through-Hole Codes
- SMD 3-Digit Code: Read the first two digits as significant figures, and the third as the multiplier (number of zeros) in microhenries (µH).
Example:101= 10 × 10¹ µH = 100 µH.470= 47 × 10⁰ µH = 47 µH.2R2uses 'R' as a decimal point = 2.2 µH. - SMD 2-Digit Code (Nano-henries): Common in RF chip inductors (e.g., 0402 packages).
10= 10 nH.2N7= 2.7 nH. - Through-Hole Color Bands: Read identically to the 4-band resistor color code, but the base unit is microhenries (µH), not ohms. A band sequence of Brown-Black-Orange means 10 × 10³ µH = 10,000 µH (10 mH).
How to Substitute Safely When the Exact Part is Missing
When a specific part (e.g., a Wurth 744043100) is out of stock, you can substitute safely by following this strict hierarchy of parameters:
- Inductance (L): Must be within ±20% for power applications. For RF/timing, match within ±2%.
- Saturation Current ($I_{sat}$): The substitute's $I_{sat}$ must be equal to or greater than the original. If $I_{sat}$ is too low, the core saturates, inductance drops to near zero, and your switching MOSFET will instantly short and fail.
- RMS Current ($I_{rms}$): Dictates thermal limits. The substitute must handle the continuous load without exceeding its temperature rise rating (usually +40°C over ambient).
- DC Resistance (DCR): Should be equal to or lower than the original to prevent excess $I^2R$ heating and voltage drop.
Failure Modes and Visual Diagnostics
Inductors are generally reliable, but they operate under severe thermal and magnetic stress in power circuits. When they fail, they rarely just 'open' like a fuse. Here is what to look for on the bench.
- Enamel Breakdown (Shorted Turns):
Symptom: The inductor measures a much lower DC resistance (DCR) than the datasheet specifies, and inductance drops significantly. Visually, you may see dark, charred spots on the copper winding or a melted epoxy coating.
Cause: Voltage spikes exceeding the dielectric strength of the wire's thin enamel insulation, causing adjacent windings to short. The effective number of turns drops, reducing inductance and causing massive localized heating. - Core Saturation & Thermal Runaway:
Symptom: No obvious visual damage initially, but the component runs too hot to touch (>100°C) and the circuit exhibits high ripple voltage. Over time, the PCB pads may discolor (brown/black FR4 scorching).
Cause: Operating above the $I_{sat}$ or $I_{rms}$ limits. The core loses its permeability, forcing the inductor to act like a low-value resistor. - Mechanical Cracking (SMD Flexure):
Symptom: Intermittent circuit operation or a complete open circuit. Under a 10x loupe, you will see a hairline crack running through the ferrite core body or the solder joint lifting from the PCB pad.
Cause: PCB bending during depaneling or connector insertion, combined with the magnetostrictive vibrations of the ferrite core over thousands of thermal cycles.
Diagnostic Tip: A standard multimeter (like a Fluke 87V) can only check for dead shorts or opens via DCR. To verify actual inductance and detect shorted turns, you must use an LCR meter (e.g., Keysight U1733C) set to the circuit's operating frequency (1 kHz for power, 100 kHz+ for RF).
Frequently Asked Questions
Do inductors block AC completely at high frequencies?
No, they do not block it completely, but they severely attenuate it. Because inductive reactance ($X_L$) scales linearly with frequency ($X_L = 2\pi f L$), a high-frequency AC signal sees a massive impedance. In practical EMI filtering, a ferrite bead (a specialized lossy inductor) is used to convert high-frequency AC noise into trace amounts of heat rather than just reflecting it back into the circuit.
Why do inductors oppose a change in current but not steady DC?
The opposition relies entirely on a changing magnetic field ($di/dt$). When steady DC flows through an inductor, the magnetic field is fully established and static. Because the field is no longer expanding or collapsing, no back-EMF is induced. The inductor then behaves exactly like a standard wire, offering only its parasitic DC resistance (DCR) to the current flow.
What happens if I use an unshielded inductor in a sensitive audio circuit?
Unshielded inductors project a significant alternating magnetic field into the surrounding space. If placed near high-impedance audio traces, op-amp inputs, or microphone preamps, this stray flux will induce a 50/60 Hz mains hum or high-frequency switching noise directly into the audio path. Always specify magnetically shielded SMD inductors or encapsulated toroids for mixed-signal and audio PCB layouts.
How do I test if an inductor has failed shorted internally?
Measure the DC resistance (DCR) with a high-precision multimeter and compare it to the manufacturer's datasheet. If the measured DCR is significantly lower than the specified value (e.g., measuring 0.02 Ω when the datasheet calls for 0.15 Ω), internal winding insulation has failed, creating a shorted turn. Follow this up by measuring the inductance value with an LCR meter; a shorted turn will cause the measured inductance to drop drastically below the nominal rating.
