Inductors work by storing energy in a magnetic field when electrical current flows through a coiled conductor. The core metric is inductance, measured in Henries (H), which dictates how strongly the component opposes changes in current. In practical circuits, they act as energy reservoirs in switch-mode power supplies (SMPS), low-pass filters to smooth DC rails, and chokes to block high-frequency AC noise while passing DC.
The Core Mechanism: Energy Storage in a Magnetic Field
The governing equation for an inductor is V = L(di/dt). This means the voltage across the inductor is proportional to the rate of change of current through it. If current is steady (DC), di/dt is zero, and the inductor acts like a plain wire (limited only by its DC Resistance, or DCR). If current changes rapidly, the inductor generates a voltage to oppose that change.
Think of an inductor like a heavy steel flywheel connected in-line with a water pipe. When you open the valve, the flywheel’s mechanical inertia resists the initial flow of water (current rise). Once the water is flowing steadily, the flywheel spins freely. But if you suddenly slam the valve shut, the flywheel’s momentum forces water forward, creating a massive pressure spike (voltage spike) that can burst the pipe. In electronics, this 'burst pipe' is inductive kickback, which is why we place flyback diodes or RC snubbers across relay coils and SMPS switches to safely dissipate that stored magnetic energy.
Inductor Types and Selection Criteria
Choosing the right inductor requires matching the core material to your frequency, current, and spatial constraints. Below is a breakdown of common types, their construction, and exactly which job they are built for.
| Type | Core Construction | Typical Tolerance | Tempco (ppm/°C) | Best Application (Which Type for Which Job) |
|---|---|---|---|---|
| Molded Power | Ferrite or Powdered Iron | ±20% | High (±1000+) | General purpose SMPS (Buck/Boost), where high current and low cost matter more than tight tolerance. |
| Shielded SMD Composite | Alloy Powder / Carbonyl Iron | ±20% | Very Low (±100) | High-density DC-DC converters (GaN/SiC designs), noise-sensitive RF paths. Excellent soft saturation. |
| Toroidal | Ferrite Tape or Powdered Iron Ring | ±10% to ±20% | Medium | Audio crossovers, differential mode EMI chokes, high-Q tank circuits. Low radiated magnetic flux. |
| Air Core (Ceramic) | Non-magnetic (Air/Ceramic/Plastic) | ±2% to ±5% | Near Zero | VHF/UHF RF tuning, impedance matching networks. Zero core saturation, but low inductance density. |
Selection Rule of Thumb: For power conversion, always check two current ratings: Isat (the current where inductance drops by 20-30%, causing loss of regulation) and Irms (the current that causes a 40°C temperature rise due to copper I²R losses). Your peak circuit current must stay below Isat, and your continuous load must stay below Irms. Refer to manufacturer tools like the Coilcraft Inductor Finder to cross-reference these exact thresholds for specific part numbers like the XEL or MSS series.
Decoding Inductor Markings and SMD Codes
Unlike resistors, inductor markings are notoriously inconsistent across manufacturers, but they generally follow a few standard coding schemes based on the package size.
SMD Power and RF Inductor Codes
- 3-Digit Code (Microhenries): The first two digits are the significant figures, and the third is the multiplier (number of zeros).
Example:100= 10 × 10⁰ = 10 µH.101= 10 × 10¹ = 100 µH. - R-Notation (Decimal placement): The letter 'R' replaces the decimal point for values under 10 µH.
Example:4R7= 4.7 µH.R22= 0.22 µH. - N-Notation (Nanohenries): Used for high-frequency RF chip inductors. The 'N' acts as the decimal point, and the unit is nH.
Example:8N2= 8.2 nH.N80= 0.80 nH.
Through-Hole Color Bands
Axial leaded inductors often use the standard 4-band resistor color code, but the base unit is microhenries (µH), not ohms.
Example: Brown (1) - Black (0) - Brown (×10) - Silver (±10% tolerance) = 10 × 10 = 100 µH ±10%. Note that some RF chokes use a 'dot' system or print the value directly; always verify with an LCR meter if the bands look faded or non-standard.
Failure Modes and Visual Diagnostics
Inductors are passive, but they fail violently when pushed past their physical limits. Here is how to diagnose them on the bench, along with guidelines for safe substitution.
Common Failure Modes
- Thermal Runaway / Insulation Breakdown:
Visual Symptom: The epoxy coating or plastic overmold is discolored (yellowed or charred), cracked, or emits a sharp phenolic smell.
Cause: Exceeding the Irms rating, causing the copper winding's DCR to heat up. As copper heats, its resistance increases, generating more heat until the enamel insulation melts, creating shorted turns that destroy the inductance. - Core Saturation (Silent Killer):
Visual Symptom: None. The part looks brand new.
Cause: Exceeding Isat. The magnetic core cannot hold any more flux, so inductance plummets to near-zero. The inductor becomes a low-resistance wire, causing massive current spikes that blow the driving MOSFET. Diagnosis: Use an oscilloscope with a current probe; if the inductor current triangle wave flattens out sharply at the peak, your core is saturating. - Mechanical Fracture:
Visual Symptom: SMD inductor lifted from the PCB pad, or a visible hairline crack running through the ferrite drum core.
Cause: PCB flexure during depanelization or thermal shock during wave soldering. This causes intermittent open circuits or shifts in the air gap, altering the inductance value.
How to Substitute Safely When the Exact Part is Missing
If you are repairing a board or prototyping and lack the exact BOM part (e.g., a Würth WE-LQS or Bourns SRP series), follow this substitution hierarchy to prevent catastrophic failure:
- Inductance (L): Must match within the original tolerance (usually ±20% for power). Do not substitute a 10µH with a 4.7µH in a buck converter; the ripple current will spike and trigger over-current protection.
- Saturation Current (Isat): Must be equal to or greater than the original. Never downsize Isat.
- RMS Current (Irms): Must be equal to or greater than the original to prevent thermal failure.
- DC Resistance (DCR): Should be equal or lower. A higher DCR will drop your output voltage and reduce efficiency.
- Shielding: If the original was a shielded composite inductor, do not replace it with an unshielded drum-core inductor in an RF-sensitive or high-density layout, or you will introduce radiated EMI that fails FCC/CE margins.
Frequently Asked Questions
How do inductors work in a buck converter circuit?
In a buck converter, the inductor acts as an energy-transfer bucket. When the high-side MOSFET turns on, input voltage is applied across the inductor, and current ramps up linearly, storing energy in the magnetic field. When the MOSFET turns off, the collapsing magnetic field forces current to continue flowing in the same direction through the catch diode (or synchronous low-side FET) into the output capacitor and load. The inductor smooths the chopped DC into a continuous, lower-voltage DC stream. For deeper topological analysis, review the TI DC-DC Switching Regulator design resources.
How do inductors work with capacitors to form an LC filter?
Inductors and capacitors are complementary: inductors resist changes in voltage (by storing energy in a magnetic field), while capacitors resist changes in voltage (by storing energy in an electric field). When placed together in an LC low-pass filter, the inductor blocks high-frequency AC noise by presenting a high impedance ($X_L = 2\pi fL$), while the capacitor shunts that same high-frequency noise to ground by presenting a low impedance ($X_C = 1 / 2\pi fC$). At the resonant frequency ($f_r = 1 / 2\pi\sqrt{LC}$), they interact to create a sharp peak or notch, which is heavily utilized in radio tuning and audio crossover networks. A great primer on this interaction is available via Electronics Tutorials.
How do inductors work at high frequencies compared to DC?
At pure DC (0 Hz), an ideal inductor is a short circuit, limited only by the physical resistance of the copper wire (DCR). As frequency increases, the inductive reactance ($X_L$) increases linearly. However, at very high frequencies (VHF/UHF and beyond), parasitic effects take over. The capacitance between adjacent wire windings (parasitic parallel capacitance) creates a self-resonant frequency (SRF). Above the SRF, the inductor stops acting like an inductor and behaves like a capacitor, completely losing its ability to choke high-frequency noise. This is why RF designers use air-core or specially wound single-layer inductors to push the SRF well above their operating band.
How do I test if an inductor is working with a standard multimeter?
A standard multimeter cannot measure inductance (Henries), but it can diagnose the two most common hard failures. First, set your meter to the resistance (Ohms) setting and probe the leads. You should read a very low resistance (typically 0.1Ω to 5.0Ω depending on the DCR spec). If it reads 'OL' (Open Line), the internal wire has snapped. If it reads exactly 0.00Ω on a large power inductor, it may be shorted internally. Second, visually inspect for heat damage. To actually verify the inductance value and check for degraded cores, you must use a dedicated LCR meter set to 1 kHz or 100 kHz, depending on the component's intended operating frequency.
