The Short Answer: What Is an Inductor and How Does It Work?
An inductor is a passive electronic component that stores energy in a magnetic field when electrical current flows through it. Its fundamental property is inductance (measured in Henries, H), which dictates its ability to resist changes in current. If a resistor resists current flow, an inductor resists changes in current flow.
The governing equation is V = L(di/dt). This means the voltage across an inductor is proportional to its inductance (L) multiplied by the rate of change of current (di/dt) through it.
Worked Numeric Example: Imagine a 10µH inductor in a switching power supply. If the controlling MOSFET turns off and forces the current to drop from 3A to 0A in 1.5 microseconds (1.5µs), the rate of change (di/dt) is 2A/µs (or 2,000,000 A/s). The inductor will generate a voltage spike of:
V = 10 × 10⁻⁶ H × 2,000,000 A/s = 20 Volts.
This 20V flyback spike is exactly why we place freewheeling diodes or snubber circuits across inductive loads; without a path for that energy, the voltage will rise until it arcs across the switch or destroys the silicon.
Think of an inductor as a mechanical flywheel. It takes effort to get a heavy flywheel spinning (building a magnetic field), but once it is spinning, it takes significant force to stop it (collapsing the magnetic field). In DC circuits, an inductor acts like a short circuit (a plain wire) once the magnetic field is fully established. In AC circuits, it presents impedance (X_L = 2πfL) that increases with frequency.
Inductor Core Types: Which One for Which Job?
The core material inside the copper windings dictates the inductor's behavior, saturation limits, and frequency response. Choosing the wrong core for a specific job will result in excessive heat, EMI failures, or catastrophic core saturation. Here is the selection matrix for the four most common core types you will encounter on the bench.
| Core Type | Construction | Typical Tolerance | Tempco (ppm/°C) | Typical Use Case |
|---|---|---|---|---|
| Air Core | Copper wire wound on a non-magnetic ceramic or plastic form | ±2% to ±5% | ~0 (Minimal) | RF tuning, high-frequency filters (>100MHz), antenna matching |
| Ferrite (MnZn/NiZn) | Sintered iron oxide ceramic (e.g., TDK PC44 material) | ±10% to ±20% | -200 to +500 | Switch-mode power supplies (SMPS), EMI chokes (10kHz - 2MHz) |
| Powdered Iron | Iron particles suspended in an insulating resin binder | ±10% to ±15% | +50 to +200 | High-current DC chokes, Power Factor Correction (PFC) circuits |
| Ceramic (SMD) | Multilayer LTCC or wirewound on a non-magnetic ceramic substrate | ±5% to ±10% | +100 to +300 | High-frequency signal filtering, RF impedance matching networks |
Selection Criteria: Use ferrite (like the Wurth WE-PD series) when you need high inductance in a small footprint for power conversion, but beware of sharp saturation curves. Use powdered iron (like the Bourns 2200LL toroids) when you need a 'soft' saturation curve that gracefully rolls off rather than abruptly shorting out at peak currents. Use air or ceramic strictly for RF and high-speed signal paths where core losses and hysteresis would destroy signal integrity.
Decoding Inductor Markings and Color Codes
Unlike resistors, inductor markings are notoriously inconsistent across manufacturers, but they generally follow two dominant standards depending on the package type.
SMD (Surface Mount) 3-Digit and R-Codes
For SMD power inductors (e.g., 6x6mm shielded drum cores), manufacturers use a 3-digit code or an 'R' decimal indicator, reading in microhenries (µH):
- 4R7 = 4.7µH (The 'R' acts as the decimal point).
- 100 = 10 × 10⁰ = 10µH. (The first two digits are significant figures, the third is the multiplier).
- 101 = 10 × 10¹ = 100µH.
- 222 = 22 × 10² = 2200µH (or 2.2mH).
Bench Warning: Do not confuse a '100' marking on an inductor (10µH) with a '100' marking on an SMD resistor (10Ω). Always verify with an LCR meter if the part is pulled from an unmarked tape.
Through-Hole Axial / Molded Color Codes
Molded axial inductors often use a 4-band color code similar to resistors, but the base unit is microhenries (µH), and the bands are read from left to right starting with the band closest to the lead.
- Band 1 & 2: Significant digits.
- Band 3: Multiplier (number of zeros).
- Band 4: Tolerance (Silver = ±10%, Gold = ±5%, No band = ±20%).
Example: An inductor with Brown-Black-Brown-Silver bands translates to 1 - 0 - ×10 - ±10%. This equals 100µH ±10%.
Failure Modes and Visual Symptoms
Inductors are generally robust, but they fail predictably when pushed beyond their thermal or magnetic limits. According to Wurth Elektronik's REDEXPERT design guidelines, exceeding thermal limits is the primary cause of field failures in power magnetics.
If an inductor's internal wire insulation melts due to overheating, adjacent copper windings can touch. This creates a 'shorted turn' inside the magnetic field. The inductor will read a much lower DC resistance (DCR) on your multimeter, but in-circuit, it will act as a massive short, drawing extreme current and potentially destroying your driving MOSFET or diode.
- Thermal Runaway / Insulation Breakdown: Visual Symptom: The outer heat-shrink sleeve or tape wrap is discolored (brown/black), blistered, or emits a sharp, acrid burning-plastic smell. The magnetic core may show stress fractures from thermal expansion.
- Mechanical Cracking (SMD Ferrite): Visual Symptom: A hairline fracture running horizontally through the ferrite drum core, usually originating right at the solder fillet pad. This is caused by PCB flexure during depaneling or connector insertion. It causes an intermittent open circuit.
- Core Saturation: Visual Symptom: None on the inductor itself. The inductor looks perfectly fine, but the switching MOSFET it drives has exploded or shows a dead short from drain to source. Saturation causes the inductance to drop to near-zero, turning the inductor into a plain wire and allowing infinite di/dt current to spike through the switch.
How to Safely Substitute an Inductor
When the exact BOM part (e.g., a TDK SPM5032T-4R7M) is out of stock, you cannot simply grab any 4.7µH inductor from your bin. Power inductors and RF inductors are not interchangeable. To safely substitute an inductor, you must match or exceed five critical parameters:
- Inductance (L): Must be within ±20% of the original. In a buck converter, going too low increases output ripple; going too high degrades transient response and can cause subharmonic oscillation.
- Saturation Current (I_sat): The current at which inductance drops by 20% to 30%. Your substitute must have an I_sat rating higher than the peak switching current of your circuit. If the original was 4A, use a 4.5A or 5A part.
- RMS Current (I_rms): The continuous DC current the part can handle before exceeding its temperature rise limit (usually ΔT = 40°C). Your substitute must exceed the maximum continuous load current.
- DC Resistance (DCR): Lower is better for efficiency, but physically larger. Ensure the substitute's DCR is equal to or lower than the original to prevent excess I²R heating.
- Self-Resonant Frequency (SRF): The frequency where the inductor's parasitic parallel capacitance resonates with its inductance. The substitute's SRF must be significantly higher than your circuit's switching frequency. As Analog Devices notes in their magnetics selection guides, operating near or above the SRF turns your inductor into a capacitor, completely defeating its filtering purpose.
Shielding Note: If the original part was a shielded inductor (magnetic flux contained within a metal or composite core, like the Wurth WE-LQS), do not substitute an unshielded drum core inductor in noise-sensitive circuits (like near an ADC or RF transceiver), or you will introduce severe EMI.
Frequently Asked Questions
What is an inductor used for in a DC-DC buck converter?
In a buck converter, the inductor acts as the primary energy storage and transfer element. When the high-side switch turns on, the inductor stores energy in its magnetic field while supplying current to the load. When the switch turns off, the collapsing magnetic field maintains current flow to the load through the freewheeling diode (or synchronous low-side MOSFET). Without the inductor, a buck converter would just be a switch slamming full input voltage into the output capacitor, causing massive current spikes and failing to step down the voltage smoothly. For deeper mathematical modeling of this behavior, Electronics Tutorials provides excellent foundational calculus on inductor charge/discharge cycles.
What is an inductor's self-resonant frequency (SRF) and why does it matter?
Every physical inductor has parasitic capacitance between its adjacent wire windings and between the windings and the core. This creates a parallel LC tank circuit. The Self-Resonant Frequency (SRF) is the exact frequency where the inductive reactance (X_L) and capacitive reactance (X_C) cancel each other out, resulting in a massive impedance peak. Below the SRF, the component acts as an inductor. Above the SRF, the parasitic capacitance dominates, and the component acts as a capacitor. When selecting an inductor for a 2MHz switching regulator, you must ensure the SRF is well above 2MHz (typically >10MHz), otherwise the inductor will fail to block high-frequency switching noise.
What is an inductor's saturation current versus RMS current?
These are two entirely different limits that confuse many hobbyists. Saturation current (I_sat) is a magnetic limit. It is the peak instantaneous current required to align all magnetic domains in the core. Exceeding I_sat causes the inductance to plummet, leading to destructive current spikes. RMS current (I_rms) is a thermal limit. It is the continuous root-mean-square current that causes the copper windings to heat up by a specified amount (usually 40°C above ambient) due to I²R losses. A safe design ensures the peak ripple current stays below I_sat, and the continuous DC load current stays below I_rms.
