An inductor is a passive electronic component that stores energy in a magnetic field when electrical current flows through it. Fundamentally, it opposes any change in current, governed by the equation V = L(di/dt). If a resistor opposes current flow, an inductor opposes changes in current flow. In practical circuit design, this property makes inductors indispensable for filtering noise, storing energy in switch-mode power supplies (SMPS), and tuning resonant RF circuits.
While the physics definition is straightforward, selecting the right physical part for a specific job requires navigating core materials, saturation currents, and cryptic SMD markings. This guide breaks down the practical realities of inductor selection, decoding, and substitution on the workbench.
Inductor Core Types: Which Type for Which Job?
The core material dictates an inductor's behavior under DC bias, its operating frequency range, and its physical size. Choosing the wrong core material is the most common reason a power supply fails EMI testing or an RF filter misses its target frequency. Below is a comparison of the four primary inductor constructions you will encounter.
| Core Type | Construction | Typical Tolerance | Tempco (ppm/°C) | Best Application |
|---|---|---|---|---|
| Air Core | Copper wire wound on non-magnetic ceramic/plastic form | ±2% to ±5% | +50 to +150 | High-Q RF filters, VHF/UHF tuning, crossover networks |
| Ferrite Core | Manganese-zinc (MnZn) or nickel-zinc (NiZn) magnetic ceramic | ±10% to ±20% | Variable (often negative at high temp) | SMPS energy storage, EMI chokes, broadband transformers |
| Iron Powder | Insulated iron particles compressed under high pressure | ±10% | Highly stable up to 100°C | High DC bias applications, PFC circuits, differential mode chokes |
| Ceramic (LTCC) | Low-Temperature Co-fired Ceramic with printed internal traces | ±5% to ±10% | ±100 | Ultra-compact SMD RF matching, GHz-range cellular/WiFi modules |
Selection Rule of Thumb: If you are designing a buck converter stepping down 12V to 3.3V at 2A, you need a ferrite or iron powder core (like the Coilcraft XEL or TDK SPM series) to handle the high DC bias without saturating. If you are building a 2.4GHz antenna matching network for an ESP32, you need a ceramic multilayer or air-core inductor (like the Murata LQP series) to maintain a high Q-factor and minimize parasitic capacitance.
Decoding Physical Markings and Inductor Codes
Unlike resistors, which universally use a straightforward color band or 3-digit SMD code for ohms, inductor markings vary wildly by form factor and manufacturer. Here is how to read the code on the physical part sitting on your bench.
SMD Power and RF Inductors
Surface-mount inductors typically use a 3-digit or 4-digit code where the unit of measure is microhenries (µH) or nanohenries (nH). Always check the datasheet for the base unit, but these rules apply 90% of the time:
| Marking | Decoding Logic | Actual Value |
|---|---|---|
| 100 | 10 × 10⁰ µH | 10 µH |
| 101 | 10 × 10¹ µH | 100 µH |
| 472 | 47 × 10² µH | 4,700 µH (4.7 mH) |
| 4R7 | 'R' acts as a decimal point (µH) | 4.7 µH |
| R47 | 'R' acts as a leading decimal (µH) | 0.47 µH |
| 4N7 | 'N' acts as decimal for nanohenries | 4.7 nH (Common in RF) |
Molded Axial (Through-Hole) Inductors
These look exactly like 1/4W resistors but use color bands to indicate microhenries (µH). The first two bands are significant digits, the third is the multiplier, and the fourth (usually silver or gold) is tolerance. For example, a Brown-Black-Brown band sequence translates to 1-0-1, meaning 10 × 10¹ = 100 µH. For a comprehensive breakdown of standard passive component color codes, refer to the All About Circuits inductor reference guide.
Real-World Failure Modes and Visual Symptoms
Inductors rarely fail silently. When they do fail, the symptoms usually manifest on the PCB or in the driving semiconductor. Here are the three most common failure modes I see in repair and prototyping.
If an inductor's DC bias exceeds its saturation current ($I_{sat}$), the core loses its magnetic permeability. The inductance drops to near-zero, effectively turning the inductor into a short piece of wire. In a buck converter, this causes instantaneous, massive current spikes that will vaporize your switching MOSFET. Always design for a peak current at least 20% below the inductor's $I_{sat}$ rating.
- Thermal Overload (Copper Loss): Visual Symptom: Discolored or yellowed epoxy encapsulation, melted solder mask on the PCB pads, or a distinct burnt-varnish smell. This happens when the RMS current exceeds the inductor's thermal rating ($I_{rms}$), causing the internal DC resistance (DCR) to heat the copper wire beyond its limits.
- Mechanical Cracking (Ferrite Fracture): Visual Symptom: A hairline crack running through the center of a shielded SMD ferrite drum, or lifted PCB pads. Ferrite is essentially compressed ceramic dust; it is highly brittle. If the PCB flexes during depanelization or if the enclosure is dropped, the inductor body can snap, breaking the internal wire connection.
- High-Frequency Core Losses: Visual Symptom: The inductor is too hot to touch ( >80°C), but the epoxy isn't melted and the current is within $I_{rms}$ specs. This occurs when using a low-frequency core material (like MnZn ferrite) in a high-frequency switching circuit (e.g., >1MHz). The magnetic domains in the core cannot flip fast enough, generating massive internal hysteresis heat. Use the Würth Elektronik RED EXPERT tool to simulate core losses before selecting a part.
Safe Substitution Rules When the Exact Part is Missing
You are building a prototype, and the exact 4.7µH SMD inductor specified in the BOM is on a 12-week backorder. Can you substitute it? Yes, but you must follow these strict engineering rules to avoid destroying your circuit.
- Match or Exceed Both Current Ratings: Inductors have two current ratings. $I_{sat}$ (saturation current) is the point where inductance drops by 20-30%. $I_{rms}$ (thermal current) is the point where the part heats up by 40°C. Your substitute must have an $I_{sat}$ higher than your peak switch current, and an $I_{rms}$ higher than your continuous output current.
- Keep Inductance Within ±20% for SMPS: In a switching regulator, the inductor value sets the ripple current and the control loop crossover frequency. Substituting a 10µH part for a 4.7µH part will reduce ripple but may cause subharmonic oscillation or slow transient response. Stick to the same nominal value.
- Check the DCR (DC Resistance): A substitute with a much higher DCR will drop voltage and waste power as heat. Conversely, in RF filter applications, a lower DCR is critical for maintaining a high Q-factor.
- Verify the Shielding: If the original part was a shielded inductor (like the Coilcraft MSS series) and you substitute an unshielded drum core, the stray magnetic flux will couple into nearby sensitive traces, causing EMI failures or noise in your ADC readings.
Frequently Asked Questions
What happens if I use an inductor with a lower current rating?
If you exceed the $I_{rms}$ rating, the inductor will overheat, potentially melting the internal wire or the PCB pads. If you exceed the $I_{sat}$ (saturation) rating, the inductance value collapses toward zero. In a power supply, this removes the current-limiting effect of the inductor, causing a massive short-circuit current spike that will instantly destroy your switching transistor or diode.
How do I measure an inductor's value without an LCR meter?
You can estimate inductance using an oscilloscope, a function generator, and a known resistor. Build a simple series RL circuit. Apply a square wave from the generator and measure the voltage across the resistor with the scope. Measure the time constant ($\tau$) — the time it takes for the voltage to reach 63.2% of its maximum value. Since $\tau = L / R$, you can calculate the inductance as $L = \tau \times R$. This won't give you the high-frequency Q-factor, but it will verify the base µH value.
Why does my switching power supply inductor get too hot to touch?
Inductor heating comes from two sources: copper losses ($I^2R$ heating from the DC current) and core losses (hysteresis and eddy currents from the AC ripple). If your DC load is well below the $I_{rms}$ rating but the part is still burning hot, your AC ripple current is too high, or you are switching at a frequency outside the core material's optimal range. Check your switching frequency against the manufacturer's core loss curves.
Can I put two inductors in parallel to increase current handling?
Generally, no. Placing two inductors in parallel halves the total inductance ($L_{total} = L / 2$), which might ruin your SMPS ripple calculations. More dangerously, unless the two inductors are perfectly matched in DCR and physically spaced far apart to prevent mutual magnetic coupling, they will not share current equally. One inductor will saturate before the other, leading to a cascading thermal failure. If you need more current handling, buy a single inductor rated for the higher current.
