The reactance of an inductor (XL) is its opposition to alternating current (AC), measured in ohms (Ω). Unlike DC resistance, which dissipates energy as heat, inductive reactance temporarily stores energy in a magnetic field and returns it to the circuit. The direct formula is XL = 2πfL, where f is frequency in Hertz and L is inductance in Henrys. As frequency increases, the inductor's opposition to current flow increases proportionally.
Calculating the Reactance of an Inductor in Practice
On the workbench, treating an inductor as a single static value is a common mistake. An inductor's impedance is highly dependent on the frequency of the signal passing through it. A 10mH choke that acts as a near-short circuit at DC will present a massive 37.7Ω of reactance at 60Hz, and an effective open circuit at RF frequencies.
Let's look at a concrete numeric example. Suppose you are designing an LC low-pass filter for a 100kHz switch-mode power supply (SMPS) and you select a 10μH inductor. Using the formula:
- XL = 2 × 3.14159 × 100,000Hz × 0.00001H
- XL = 6.28Ω
At the 100kHz switching frequency, the inductor provides 6.28Ω of reactance to the AC ripple, while passing the DC load current with minimal loss (dictated only by its wire resistance). However, every physical inductor has a Self-Resonant Frequency (SRF). Above the SRF, parasitic winding capacitance dominates, and the component stops acting like an inductor and starts acting like a capacitor. For high-frequency filtering, you must select a part whose SRF is well above your target noise frequency. As detailed in Analog Devices' inductor selection guidelines, ignoring the SRF is the leading cause of EMI failures in custom DC-DC converters.
| Inductance (L) | XL @ 60 Hz (Mains) | XL @ 1 kHz (Audio) | XL @ 100 kHz (SMPS) | XL @ 1 MHz (RF) |
|---|---|---|---|---|
| 10 μH | 0.0037 Ω | 0.062 Ω | 6.28 Ω | 62.8 Ω |
| 100 μH | 0.037 Ω | 0.62 Ω | 62.8 Ω | 628 Ω |
| 1 mH | 0.37 Ω | 6.28 Ω | 628 Ω | 6.28 kΩ |
| 10 mH | 3.77 Ω | 62.8 Ω | 6.28 kΩ | 62.8 kΩ |
| 100 mH | 37.7 Ω | 628 Ω | 62.8 kΩ | 628 kΩ |
Inductor Core Types and Selection Criteria
Which type of inductor should you use for a specific job? The answer depends entirely on the core material, which dictates the component's permeability, saturation current, and temperature stability. Air cores handle infinite DC bias without saturating but require hundreds of turns to achieve usable inductance. Ferrite cores offer high inductance in tiny packages but will saturate and fail if the DC current exceeds their rating.
According to the Coilcraft Inductor Selection Guide, matching the core material to your circuit's DC bias and AC ripple profile is mandatory for reliable operation. Below is a breakdown of the most common physical constructions you will encounter.
| Core Type | Construction | Typical Tolerance | Tempco (ppm/°C) | Typical Use Case |
|---|---|---|---|---|
| Air Core | Copper wire wound on non-magnetic ceramic or plastic form | ±5% to ±10% | +50 to +100 | High-frequency RF, VHF/UHF tuning, high-current crossovers |
| Ferrite Rod | Wire wound on NiZn or MnZn cylindrical rod | ±10% to ±20% | +100 to +1000 | EMI suppression beads, AM radio loopstick antennas |
| Iron Powder | Toroidal or molded drum (carbonyl iron particles in binder) | ±10% to ±15% | +50 to +300 | Switch-mode power supplies (SMPS), Power Factor Correction (PFC) |
| Shielded Ferrite | Enclosed drum core with outer ferrite sleeve or epoxy mold | ±20% to ±30% | -200 to +500 | High-density DC-DC converters, SMD power chokes |
Selection Rule of Thumb: Use shielded ferrite or iron powder for power conversion where high DC bias is present and magnetic coupling to nearby traces must be minimized. Use air core or unshielded NiZn ferrite for RF signal paths where core saturation is irrelevant, but Q-factor (quality factor) and low parasitic capacitance are critical.
Decoding Physical Markings and SMD Codes
When scavenging parts or verifying a BOM, you need to know what the markings on the physical part mean. Inductor coding is less standardized than resistor coding, but two dominant systems exist for through-hole and surface-mount components.
Surface Mount (SMD) Inductor Codes
SMD power inductors typically use a 3-digit numeric code or an alphanumeric 'R' notation, with the base unit being microhenries (μH).
- 3-Digit Code: The first two digits are the significant figures, and the third digit is the multiplier (number of zeros).
- 100 = 10 × 100 = 10μH
- 101 = 10 × 101 = 100μH
- 472 = 47 × 102 = 4700μH (or 4.7mH)
- R-Notation: The letter 'R' acts as a decimal point for values under 10μH.
- 4R7 = 4.7μH
- R22 = 0.22μH
Through-Hole Radial Color Bands
Leaded radial inductors often use a 4-band color code identical to the MIL-spec resistor color code, but the base unit is microhenries (μH) instead of ohms.
Example: A part with Brown - Black - Brown - Silver bands translates to:
Brown (1) | Black (0) | Brown (×10) | Silver (±10% tolerance) = 100μH at 10% tolerance.
Failure Modes, Visual Symptoms, and Safe Substitution
Inductors rarely fail open without a preceding catastrophic event. Understanding how they fail is critical for troubleshooting blown power supplies and designing reliable replacements. For a deeper dive into the AC theory behind inductive kickback and flyback voltages, refer to the All About Circuits AC theory chapter.
Common Failure Modes and Visual Symptoms
- Core Saturation (Thermal Runaway): If the DC current exceeds the inductor's saturation current (Isat), the magnetic core saturates. Inductance drops to near zero, reactance vanishes, and the component acts like a low-resistance wire. The resulting massive current spike melts the winding enamel. Visual Symptom: Discolored or blistered epoxy coating, melted copper windings visible under a magnifying glass, and a distinct 'burnt electronics' smell.
- Dielectric Breakdown (Winding Short): High voltage spikes (inductive kickback) can arc between adjacent windings, breaking down the thin polyurethane or polyimide enamel insulation. This creates a shorted turn, drastically lowering the inductance and Q-factor. Visual Symptom: Often invisible externally; requires an LCR meter to detect a drop in inductance and a spike in Equivalent Series Resistance (ESR).
- Mechanical Shock: Dropping a PCB or subjecting it to high vibration can crack the brittle ferrite core. Visual Symptom: A hairline fracture on the ferrite sleeve or toroid. This introduces an unintended air gap, causing the inductance value to plummet unpredictably.
How to Substitute Safely When the Exact Part is Missing
When you are out of stock on a specific BOM inductor, you cannot simply swap in another part with the same microhenry rating. To substitute safely, you must match or exceed five critical parameters:
- Inductance (L): Must be within the original tolerance (usually ±20% for power chokes).
- Saturation Current (Isat): The substitute's Isat must be ≥ the original. If you use a part with a lower Isat, the core will saturate during peak load transients, destroying your switching MOSFET.
- Thermal Current (Irms): The substitute's Irms must be ≥ the original to prevent the wire from overheating under continuous DC load.
- DC Resistance (DCR): The substitute's DCR should be ≤ the original. Higher DCR increases I²R losses, reducing overall power supply efficiency and causing localized heating.
- Self-Resonant Frequency (SRF): The substitute's SRF must be higher than the circuit's switching frequency. If the SRF is too low, the inductor becomes capacitive at the switching node, causing massive ringing and EMI failures.
Shielding Caveat: Never substitute an unshielded inductor (like a bare bobbin or rod core) for a shielded one in a noise-sensitive or high-density layout. The stray magnetic flux from an unshielded part will couple into nearby feedback traces or hall-effect sensors, causing erratic regulation or audible whining. If you must use an unshielded substitute temporarily, orient the part 90 degrees to sensitive traces and verify the switching node ringing with an oscilloscope before committing to the swap.
