The Direct Answer: What Does an Inductor Actually Do?

An inductor resists changes in electrical current by storing energy in a magnetic field. If a capacitor resists changes in voltage, an inductor is its dual: it resists changes in current. When current flows through the coiled wire, it generates a magnetic field. If the current tries to increase, the collapsing/expanding magnetic field induces a back-EMF (voltage) that opposes the increase. If the current tries to drop, the collapsing field induces a voltage that tries to keep the current flowing.

The governing equation is V = L(di/dt). The voltage across the inductor (V) equals its inductance in Henries (L) multiplied by the rate of change of current over time (di/dt). The energy stored in that magnetic field is calculated as E = ½ × L × I².

The Bench Analogy: Think of an inductor as a heavy mechanical flywheel connected to a motor. When you first apply power, the flywheel’s inertia resists spinning up (current builds slowly). Once it is spinning at full speed, if you suddenly cut the power, the flywheel’s momentum keeps it turning, driving the motor as a generator to keep current flowing into the circuit. This 'electrical inertia' is exactly what smooths out the chopped waveforms in switching power supplies.

Inductor Types and Selection Criteria

Not all coils are created equal. Choosing the wrong core material for a switching regulator will result in massive efficiency losses or catastrophic saturation. Here is how the main types break down when you are selecting parts for a build.

Type / Core Construction Tolerance Tempco (ppm/°C) Typical Use Case
Shielded Drum Ferrite core with magnetic shielding compound ±20% (M) Varies by ferrite mix DC-DC buck/boost converters, high-density PCBs where EMI must be contained.
Unshielded Drum Exposed ferrite bobbin, wire wound openly ±20% (M) Varies by ferrite mix Cost-sensitive power supplies where EMI and physical clearance are not strict.
Toroidal Wire wound around a donut-shaped iron powder or ferrite ring ±10% to ±20% Low (Iron powder) High-current AC/DC line filtering, audio crossovers, large bench power supplies.
Ceramic Multilayer Printed spiral traces sandwiched in ceramic layers ±5% to ±10% Highly stable High-frequency RF matching, GHz-range signal filtering (low current).
Ferrite Bead Solid ferrite cylinder over a straight wire N/A (Impedance spec) N/A High-frequency noise suppression on power rails and data lines (not for energy storage).

Which type for which job? If you are building a switching regulator (like a buck converter), you need a shielded or unshielded drum core rated for power. If you are filtering high-frequency noise off a 3.3V logic rail, use a ferrite bead. If you are designing an RF antenna matching network for an ESP32, use a ceramic multilayer inductor for its tight tolerance and high Q-factor.

Decoding Physical Markings and Sourcing Substitutes

When you are digging through your parts bins, reading SMD inductor markings can be confusing because they borrow from resistor coding but apply to microhenries (µH).

How to Read the Markings

  • Three-Digit Code: The first two digits are the significant figures, and the third is the multiplier (number of zeros). A marking of 100 means 10 × 10⁰ = 10µH. A marking of 101 means 10 × 10¹ = 100µH.
  • The 'R' Decimal Code: The letter 'R' acts as the decimal point. A marking of 4R7 means 4.7µH. A marking of R22 means 0.22µH.
  • Color Codes: Older through-hole molded inductors (like the classic Vishay IM-series) use 4-band resistor-style color codes, but the result is in microhenries. Brown-Black-Brown-Silver = 100µH ±10%.

How to Substitute Safely When the Exact Part is Missing

You blew an inductor and need a replacement from your stock, but you don't have the exact BOM part. You can substitute safely if you respect three hard limits:

  1. Never go lower on Saturation Current (Isat): This is the current at which the core magnetically saturates and inductance drops by 20-30%. If your circuit peaks at 4A, your substitute must have an Isat of at least 4.5A. Substituting a lower Isat part will cause immediate failure.
  2. Match or beat the RMS Current (Irms): This is the continuous DC current the part can handle before it overheats due to its internal DC Resistance (DCR). A lower DCR is always safer for a substitute.
  3. Inductance Tolerance: In a switching regulator, a ±20% swing in inductance changes your ripple current. Substituting a 10µH part with a 15µH part is usually fine (it lowers ripple), but dropping to 4.7µH might push your peak currents past the IC's internal limit.
Warning: Never substitute a ferrite bead for a power inductor. A ferrite bead is designed to dissipate high-frequency energy as heat and will act as a dead short at DC. Putting one in the energy-storage path of a buck converter will instantly destroy your switching IC.

Bench Scenario: The Melted Buck Converter

To understand what happens when you ignore inductor physics, let us walk through a real-world bench failure involving a classic LM2596 step-down (buck) converter module.

The Setup: A hobbyist needed to drop a 12V battery down to 5V to power a 3A servo array. They used a generic LM2596 breakout board. The board came stock with a 33µH unshielded inductor (marked '330'). Looking at the inductor's tiny datasheet, its saturation current (Isat) was rated at just 2.0A.

The Numbers: The LM2596 switches at roughly 150kHz. At a 12V input and 5V output, the duty cycle is about 41%. The inductor ripple current (ΔIL) is calculated as:
ΔIL = (Vout × (1 - Duty)) / (L × f)
ΔIL = (5V × 0.59) / (33µH × 150,000Hz) = 0.59A ripple.
With a 3A DC load, the peak current through the inductor is 3A + (0.59A / 2) = 3.29A peak.

The Outcome: The circuit required the inductor to handle 3.29A peaks, but the physical core saturated at 2.0A. Once the current crossed 2.0A, the ferrite core could not hold any more magnetic flux. The inductance effectively dropped to near zero (just the resistance of the bare wire). With the inductance gone, the current spiked violently, limited only by the parasitic resistance of the PCB traces and the internal MOSFET.

What Went Wrong: The LM2596's internal current limit tried to react, but the di/dt spike was too fast. The internal switch overheated and shorted out, passing the full 12V straight to the 5V servo rail, frying the microcontrollers downstream. The inductor itself survived but smelled sharply of burning enamel.

The Fix: Replace the 33µH/2A inductor with a Wurth Elektronik 74477420 (10µH, 4.5A Isat) or a Coilcraft DO3316P-103ML (10µH, 2.9A Isat - marginal, better to use the 15µH version rated for 4A). Always calculate peak current, not just average DC load, when sizing power inductors. For deeper math on buck converter inductor sizing, the Coilcraft Inductor Basics library provides excellent step-by-step worksheets.

Failure Modes and Visual Diagnostics

Inductors are generally robust, but they do fail. When troubleshooting a dead power supply, here is how to diagnose inductor health using your eyes and a multimeter.

1. Thermal Overload (Melted Enamel)

  • Visual Symptom: The outer heat-shrink sleeve is melted, or the copper wire winding looks blackened and smells like burnt plastic. The ferrite core may show scorch marks.
  • Cause: Exceeding the Irms rating. The DCR (DC resistance) of the wire generated more heat (I²R losses) than the component could dissipate.
  • Test: Measure DC resistance. It will often still read as a short (near 0Ω), but the part is compromised and must be replaced.

2. Mechanical Shock (Cracked Core)

  • Visual Symptom: A visible hairline fracture running through the ferrite drum or shielding compound. Common in unshielded SMD inductors that were dropped or subjected to heavy PCB flexing.
  • Cause: Ferrite is essentially compressed ceramic dust. It is incredibly brittle. Dropping the board or using an ultrasonic cleaner with the wrong frequency can shatter the core.
  • Test: An LCR meter will show a massive drop in inductance value because the physical air gap introduced by the crack alters the magnetic permeability. Discard the part.

3. Open Circuit (Wire Break)

  • Visual Symptom: The component looks perfectly fine from the outside. No burns, no cracks.
  • Cause: A microscopic break in the copper winding, usually right at the termination pad where the wire is soldered to the metal cap. This happens due to thermal cycling (expansion/contraction) or a transient current spike that acted like a fuse.
  • Test: A multimeter in continuity/resistance mode will read 'OL' (Open Loop) or infinite resistance. For a comprehensive look at testing passive components, All About Circuits offers solid foundational theory on why opens occur in inductive windings.

4. Magnetic Saturation (The Invisible Failure)

  • Visual Symptom: None. The inductor looks pristine. However, the switching IC attached to it is dead, or the output voltage has massive, erratic ripple.
  • Cause: As detailed in the buck converter scenario, the core saturated. The inductor didn't physically break; it just stopped acting like an inductor during the peak of the switching cycle.
  • Test: You cannot test for saturation with a standard DC multimeter. You must use an oscilloscope with a current probe to view the inductor current waveform. If the current ramp suddenly turns into a vertical spike midway through the switching cycle, your inductor is undersized for the load.