The voltage across an inductor is not a fixed value like a battery; it is strictly a function of how fast the current through it changes. Governed by the formula V = L(di/dt), an inductor opposes any change in current by generating a counter-electromotive force (CEMF). If you apply 5V to a 10µH inductor with negligible resistance, the current ramps up at 500,000 Amps per second. This article breaks down the math, compares core materials, decodes SMD markings, and provides a concrete decision tree to select the right inductor for your next power supply or RF filter.
The Physics and Formula: Voltage Across an Inductor
Faraday’s law of induction dictates that a changing magnetic field induces a voltage. In a discrete inductor, this is expressed as:
V = L × (di / dt)
Where:
V = Voltage across the inductor (Volts)
L = Inductance (Henries)
di = Change in current (Amperes)
dt = Change in time (Seconds)
Worked Numeric Example: Imagine a buck converter switching node connected to a 15µH inductor. During the switch's 'on' time, the current rises from 1.0A to 2.5A (di = 1.5A) over a period of 3 microseconds (dt = 0.000003s).
V = 0.000015 × (1.5 / 0.000003) = 7.5 Volts.
This 7.5V is the net voltage dropped across the inductor's magnetic field during that switching interval.
When a circuit carrying DC current through an inductor is suddenly opened (dt approaches zero), the voltage across the inductor spikes toward infinity to maintain current flow. This 'flyback' voltage can easily exceed 500V in a simple 12V relay coil, destroying MOSFETs or causing lethal shocks in high-voltage ignition systems. Always use a flyback diode, snubber network, or TVS diode across inductive loads.
Inductor Core Types and Comparison Matrix
The physical core material dictates the inductor's saturation current, frequency response, and thermal stability. Measuring the voltage across and inductor thermal limits requires understanding how these cores behave under load. Below is a comparison of the four most common core types found on the bench.
| Core Type | Construction | Typical Tolerance | Tempco (ppm/°C) | Best Use Case |
|---|---|---|---|---|
| Ferrite | Sintered iron oxide/ceramic mix, often shielded | ±20% (M) | Highly non-linear | Switching power supplies (Buck/Boost), EMI chokes |
| Iron Powder | Insulated iron particles pressed into a toroid/shape | ±10% to ±15% | +100 to +350 | Audio crossovers, high-current DC filtering |
| Ceramic | Non-magnetic dielectric core, air-like properties | ±2% to ±5% (G, J) | +100 to +250 | RF matching networks, VHF/UHF filters, high-Q circuits |
| Air Core | Self-supporting copper coil, no magnetic material | ±5% to ±10% | ~0 (Copper only) | High-power audio, high-frequency RF where saturation is unacceptable |
When to choose which: If your circuit operates below 1MHz and handles amps of current (like a DC-DC converter), choose Ferrite. If you are designing a 100MHz RF filter where tight tolerance and high Q-factor matter more than current capacity, choose Ceramic.
Decoding SMD and Through-Hole Markings
Unlike resistors, inductor markings are notoriously inconsistent across manufacturers, but the majority follow a modified three-digit EIA code or a direct alphanumeric stamp.
SMD Inductor Codes
- 4R7: The 'R' acts as a decimal point. Value = 4.7µH.
- 100: First two digits are significant figures, third is the multiplier (10^0). Value = 10 × 1 = 10µH.
- 101: Value = 10 × 10^1 = 100µH.
- 2N2: The 'N' acts as a decimal for nano-Henries. Value = 2.2nH (common in 0402 RF inductors).
Tolerance Letters
You will often see a trailing letter on through-hole radial inductors or larger SMD pads:
- M = ±20% (Standard for power ferrites)
- K = ±10%
- J = ±5%
- G = ±2% (Precision RF)
Failure Modes and Visual Diagnostics
Inductors rarely fail 'open' under normal conditions unless subjected to severe mechanical shock. Their failure modes are usually thermal or magnetic, and diagnosing them requires looking at the surrounding circuit as much as the part itself.
1. Core Saturation (Magnetic Failure)
The Physics: When the current exceeds the inductor's saturation current ($I_{sat}$), the magnetic core can no longer hold additional flux. The inductance plummets, effectively turning the inductor into a low-resistance wire.
Visual Symptoms: The inductor itself looks perfectly fine. However, the switching MOSFET or IC driving it will be scorched, cracked, or blown open due to the massive current spike that occurred when the inductor saturated.
Fix: Replace the inductor with a part that has a higher $I_{sat}$ rating, or add an air gap to the core.
2. Thermal Runaway (Winding Failure)
The Physics: Exceeding the RMS current rating ($I_{rms}$) causes $I^2R$ heating in the copper windings. The enamel insulation melts, causing inter-winding shorts, which lowers inductance and increases current further.
Visual Symptoms: The inductor's plastic casing or heat-shrink wrap is melted, blistered, or discolored brown/black. A distinct 'burnt electronics' smell is present. An ohmmeter will read a lower-than-expected DC Resistance (DCR).
Fix: Upgrade to a physically larger inductor with a thicker wire gauge (lower DCR) and higher $I_{rms}$ rating.
3. Mechanical Fracture
The Physics: Ferrite is essentially a brittle ceramic. Thermal cycling or PCB flexion can crack the core.
Visual Symptoms: A visible hairline crack running through the ferrite drum or shield. The inductance value will drop significantly because the crack acts as an unintended air gap.
Fix: Replace the part and check the PCB for flexing or solder joint stress.
Safe Substitution and the Selection Decision Tree
When the exact BOM inductor is out of stock, you cannot just swap in any part with the same micro-Henry rating. Use these hard rules for safe substitution:
- Inductance (L): Must be within ±10% of the original for power supplies. RF filters require exact matches.
- Saturation Current ($I_{sat}$): Must be ≥ the original part. Never substitute a lower $I_{sat}$.
- DC Resistance (DCR): Should be ≤ the original part. Lower DCR means less heat.
- Shielding: Never replace a shielded inductor with an unshielded one in a switching regulator; the radiated EMI will cause the circuit to fail FCC/CE compliance and may interfere with nearby sensitive traces.
The Inductor Selection Decision Tree
| Application Scenario | Primary Constraint | Required Core / Type | Concrete Part Recommendation |
|---|---|---|---|
| DC-DC Buck/Boost Converter (1A - 5A) | High $I_{sat}$, low EMI, compact footprint | Shielded Ferrite (SMD) | Würth Elektronik 744774215 (15µH, 2.9A $I_{sat}$, shielded) |
| RF Impedance Matching (10MHz - 2GHz) | High Q-factor, tight tolerance, low parasitic capacitance | Ceramic Core (SMD) | Coilcraft 0402CS-10NX (10nH, ±2% tol, ceramic) |
| Audio Speaker Crossover (Low Pass) | No core saturation at high wattage, low DCR | Air Core or Iron Powder (Through-hole) | Dayton Audio 2.0mH Air Core (14 AWG wire, zero saturation) |
| Snubber / EMI Choke on AC Mains | High voltage isolation, high inductance | Toroidal Iron Powder or Common Mode Choke | Wurth 744825601 (Common Mode, 10mH, 6A) |
By anchoring your selection to the physical constraints of your circuit—specifically the peak current ($I_{sat}$) and the operating frequency—you eliminate the guesswork. Remember that the voltage across an inductor is merely the symptom of the changing current; designing for the current limits and core material ensures your circuit survives the voltage transients inherent to magnetic components.
