At its core, an inductor stores energy in a magnetic field and fundamentally opposes any change in current flow. If you are asking what is an inductor used for on a practical level, the answer splits into four main jobs: filtering out high-frequency noise (chokes), storing and transferring energy in switch-mode power supplies (SMPS), impedance matching in RF circuits, and tuning resonant frequencies in LC oscillators. Think of an inductor like a heavy mechanical flywheel in a water pipe: it takes a lot of pressure to get the water moving, but once it is flowing, the flywheel keeps it moving even if the pump suddenly stops.
This opposition to current change is quantified by the formula V = L × (di/dt). For example, if a 10µH inductor in a buck converter experiences a current ramp of 2A/µs when the MOSFET switches on, it generates 20V of back-EMF. Understanding this behavior is critical before you select a part for your next PCB layout or breadboard build.
Inductor Types and Selection Matrix
Not all inductors are created equal. The core material dictates the saturation current, temperature stability, and frequency response. Below is a data-dense comparison of the five most common inductor constructions you will encounter in modern electronics, complete with real-world part examples.
| Core Material | Construction | Typical Tolerance | Tempco (ppm/°C) | Saturation Behavior | Typical Use Case | Example Part |
|---|---|---|---|---|---|---|
| Air / Ceramic | Wirewound on non-magnetic core | ±2% to ±5% | +20 to +80 | None (Linear) | RF tuning, HF filters, VCOs | Coilcraft 0603CS (10nH) |
| Ferrite (Unshielded) | Drum core with exposed windings | ±10% to ±20% | ±100 to ±500 | Hard saturation (Sharp drop) | Low-cost SMPS, basic chokes | Würth Elektronik WE-PD2 |
| Ferrite (Shielded) | Molded or enclosed drum core | ±20% to ±30% | ±200 to ±600 | Hard saturation | High-density SMPS, noise-sensitive boards | Bourns SRP1260 (15µH) |
| Powdered Iron | Toroidal (Distributed air gap) | ±5% to ±15% | +20 to +350 | Soft saturation (Gradual roll-off) | High-power RF, PFC chokes, audio crossovers | Micrometals T50-2 (Red/Black) |
| Metal Alloy (Composite) | Molded metal powder in resin | ±20% | ±100 to ±300 | Very soft saturation | High-current POL converters, automotive | Coilcraft XEL3530 (1.5µH) |
Which Type for Which Job?
Use air/ceramic cores when you need high Q-factor and precise tuning above 10MHz; they will not saturate, but their inductance values are low (nH range). Choose shielded ferrite or metal alloy composites for switch-mode power supplies (100kHz to 3MHz). Metal alloys handle massive current spikes without the hard, catastrophic inductance drop-off seen in standard ferrites. Reserve powdered iron toroids for high-power, lower-frequency applications (like 50/60Hz line filtering or amateur radio amplifiers) where you need the core to gracefully handle overcurrent without instantly dropping to zero inductance.
Decoding Inductor Markings and Codes
Unlike resistors, inductor markings are notoriously inconsistent across manufacturers, but two dominant systems exist for surface-mount and axial through-hole parts.
SMD 3-Digit and 4-Digit Codes
Most SMD power and RF inductors use a 3-digit code where the first two digits are the significant figures and the third digit is the multiplier (number of zeros), expressed in microhenries (µH).
- 100 = 10 × 10^0 = 10µH
- 101 = 10 × 10^1 = 100µH
- 472 = 47 × 10^2 = 4700µH (4.7mH)
For values under 10µH, manufacturers use an 'R' to denote the decimal point. 4R7 means 4.7µH, and R10 means 0.10µH (100nH). Always verify with a datasheet, as some RF inductors specify values in nanohenries (nH) using the same 3-digit format (e.g., a Coilcraft 0603CS marked '100' might be 10nH, not 10µH).
Axial Color Bands
Through-hole axial inductors (like the classic molded 'dog bone' style) use a 4-band color code similar to resistors, but the base unit is microhenries (µH).
Example: Brown (1), Black (0), Brown (×10), Silver (±10% tolerance) = 100µH ±10%.
For a comprehensive breakdown of standard passive component markings, refer to the All About Circuits inductor guide.
Failure Modes: Visual Symptoms and Bench Diagnostics
Inductors are generally robust, but they fail when pushed beyond their thermal or magnetic limits. Here is how to identify a dead or dying coil on the bench.
1. Thermal Overload (Melted Enamel)
Visual Symptom: Discolored or blistered epoxy coating, melted copper enamel visible under magnification, and a distinct 'burning varnish' smell.
The Physics: Excessive RMS current causes $I^2R$ heating in the copper windings. The enamel insulation melts, causing adjacent turns to short together. This reduces the effective number of turns, dropping the inductance.
Diagnostic: DCR will read lower than the datasheet spec. LCR meter shows a 10% to 40% drop in inductance.
2. Core Saturation and Cracking
Visual Symptom: A visible hairline crack running through the ferrite drum or toroid core.
The Physics: Driven by excessive peak current (exceeding $I_{sat}$) or mechanical shock (like board flex or ultrasonic cleaning). Once a ferrite core cracks, the physical air gap increases unpredictably, altering the magnetic permeability.
Diagnostic: Inductance may measure fine at low signal levels on an LCR meter, but the part will prematurely saturate under load, leading to catastrophic MOSFET failure in SMPS circuits due to uncontrolled current spikes.
3. SMD Pad Lift and Open Circuits
Visual Symptom: The inductor sits slightly tilted on the PCB, or one termination pad is completely detached from the board.
The Physics: Large SMD inductors (like 1210 or 1812 packages) have high mass. If the PCB undergoes mechanical flexing or thermal shock during wave soldering, the rigid termination tears the copper pad off the FR4 substrate.
Diagnostic: Infinite resistance (Open Loop) on a DMM.
Safe Substitution: What to Do When the Exact Part Is Missing
You cannot simply swap any 10µH inductor for another 10µH inductor. When the exact BOM part is out of stock, you must match five critical parameters to ensure the circuit survives. For deeper engineering criteria on switching regulator magnetics, consult Analog Devices' application notes on inductor selection.
The 5-Point Substitution Checklist
- Inductance Value: For power supply filtering, ±20% is usually acceptable. For RF matching networks or LC oscillators, you must match within ±2% or ±5%.
- Saturation Current ($I_{sat}$): This is the current at which inductance drops by 20% to 30%. Your substitute's $I_{sat}$ must be higher than the peak switching current of your converter. If you substitute a part with a lower $I_{sat}$, the inductor will turn into a dead short during the MOSFET's 'on' time, likely blowing the switch.
- RMS Current ($I_{rms}$): This is the thermal rating. The substitute's $I_{rms}$ must exceed your continuous DC load current to prevent the winding enamel from melting.
- DC Resistance (DCR): Always choose a substitute with an equal or lower DCR. Higher DCR increases $I^2R$ losses, dropping your output voltage and ruining efficiency.
- Self-Resonant Frequency (SRF): Every inductor has parasitic parallel capacitance. Above the SRF, the inductor acts like a capacitor. For SMPS, the SRF should be at least 10x higher than your switching frequency. For RF chokes, the SRF should be near the frequency you are trying to block.
The Shielding Trap
Never substitute an unshielded drum-core inductor in place of a shielded molded inductor on a mixed-signal board. Unshielded inductors radiate a massive alternating magnetic field. If placed within 15mm of a high-gain op-amp, a 16-bit ADC, or a Hall-effect sensor, that radiated flux will induce severe ripple and noise into your sensitive analog traces. Always check the manufacturer's technical documentation to verify magnetic shielding characteristics before approving an alternate part number.
