When designing power supplies, filters, or oscillators, the choice between an inductor and capacitor hinges on a single physical principle: inductors resist changes in current ($V = L \frac{di}{dt}$), while capacitors resist changes in voltage ($I = C \frac{dv}{dt}$). If you need to smooth voltage ripple, you need a capacitor. If you need to choke high-frequency AC noise on a DC line or store energy in a switching converter, you need an inductor. When building an inductor capacitor (LC) network, both work together to set a specific resonant frequency. This guide cuts through the theory and gives you the exact part numbers, substitution rules, and diagnostic frameworks you need on the bench.

The Inductor vs Capacitor Decision Matrix

Use this decision table to determine which component solves your specific circuit problem, terminating in a concrete default part selection for prototyping and production.

Circuit Goal Primary Component Selection Criteria Concrete Default Pick
Smooth 120Hz ripple on a rectified DC supply Capacitor High capacitance, moderate ESR acceptable, high voltage rating Nichicon UHW Series (Aluminum Electrolytic, e.g., 470µF 50V)
Filter high-frequency switching noise on a 5V DC rail Inductor High impedance at target frequency, low DC resistance (DCR) TDK SPM5032T Series (Shielded Wirewound, e.g., 4.7µH)
Decouple high-speed digital ICs (ESP32, FPGA) Capacitor Ultra-low ESL, fast transient response, placed <2mm from pin KEMET X7R MLCC (e.g., 100nF 0402 16V)
Snubber for relay coil inductive kickback Capacitor + Resistor High dV/dt capability, self-healing dielectric Cornell Dubilier 940C Series (Polypropylene Film)
Energy storage for a buck converter power stage Inductor High saturation current ($I_{sat}$), shielded to prevent EMI Coilcraft XEL Series (e.g., XEL4030-102, 1µH, 15A $I_{sat}$)

Type Comparison: Construction, Tolerance, and Tempco

Not all passive components are created equal. A 10µF ceramic capacitor behaves entirely differently from a 10µF tantalum under load. The table below maps the physical construction and environmental stability of the most common families.

Component Family Construction Typical Tolerance Tempco / Stability Typical Use Case
MLCC (C0G/NP0) Class I Ceramic Dielectric ±5% or better 0 ±30 ppm/°C (Ultra-stable) RF tanks, precision timing, filters
MLCC (X7R/X5R) Class II Ceramic Dielectric ±15% to ±22% ±15% over temp range; severe DC bias derating General decoupling, bulk bypass
Aluminum Electrolytic Etched foil with liquid/polymer electrolyte ±20% High tempco; capacitance drops at low temps Bulk input/output filtering, audio coupling
Tantalum (Polymer) Tantalum pentoxide with polymer cathode ±10% to ±20% Stable over temp; no DC bias derating Space-constrained bulk decoupling
Shielded Wirewound Copper wire on ferrite core, enclosed in magnetic shield ±20% Stable until $I_{sat}$ is reached, then drops sharply DC-DC converter power inductors
Toroidal (Iron Powder) Wire wound on iron-powder or ferrite ring ±10% Soft saturation curve, high thermal stability High-current EMI chokes, PFC circuits

Decoding Physical Markings and Codes

Reading the markings on surface-mount and through-hole passives is a mandatory bench skill. Misreading a code by one order of magnitude will destroy your prototype.

Capacitor Markings

  • 3-Digit EIA Code (MLCC/Film): The first two digits are significant figures; the third is the multiplier in picofarads (pF). Example: '104' = 10 × 10⁴ pF = 100,000 pF = 100 nF (0.1 µF).
  • Letter Suffixes: A letter after the number indicates tolerance. 'J' = ±5%, 'K' = ±10%, 'M' = ±20%. Example: '104K' is a 100nF capacitor with 10% tolerance.
  • Electrolytic Polarity: Through-hole radial electrolytics have a painted stripe with minus signs indicating the negative cathode lead. SMD aluminum electrolytics use a black semi-circle on the top of the can to denote the positive anode.

Inductor Markings

  • SMD Numeric Code: Similar to resistors, but the base unit is microhenries (µH). Example: '100' = 10 × 10⁰ µH = 10 µH. '471' = 47 × 10¹ µH = 470 µH.
  • The 'R' Decimal Indicator: When the value is under 10 µH, 'R' acts as the decimal point. Example: '4R7' = 4.7 µH. 'R22' = 0.22 µH.
  • Color Bands (Through-Hole): Read identically to 4-band resistors, but the result is in µH. A brown-black-brown-silver inductor is 10 × 10¹ µH = 100 µH with 10% tolerance.
Bench Tip: Never trust the printed marking on a salvaged MLCC. The mechanical stress of desoldering can cause micro-cracks. Always measure salvaged ceramics with an LCR meter at 1 kHz and 1 Vrms before reusing them.

Failure Modes and Visual Diagnostics

Passive components fail in predictable ways. Recognizing the visual symptoms saves hours of oscilloscope debugging.

Capacitor Failures

  • MLCC Flex Cracking:
    • Visual Symptom: Hairline crack in the ceramic body near the metal termination, often visible only under 10x magnification. On a powered board, it manifests as a dead short or intermittent open.
    • Cause: PCB bending during depaneling, connector insertion, or thermal shock.
    • Prevention: Use 'soft-termination' (flexible polymer electrode) MLCCs like the KEMET FT-CAP series near board edges and connectors.
  • Electrolytic Venting:
    • Visual Symptom: Bulging top vent, crusty brown or white residue leaking from the bottom rubber seal, or a completely split casing.
    • Cause: Overvoltage, reverse polarity, or excessive ripple current causing internal heating and electrolyte vaporization.
  • Tantalum Thermal Runaway:
    • Visual Symptom: Charred black epoxy, sometimes a literal hole burned through the PCB. Smells acrid.
    • Cause: Voltage spikes exceeding the rated maximum. Tantalum dioxide faults create a low-resistance short that draws massive current, igniting the component.
Safety Warning: Large aluminum electrolytic capacitors in power supplies can hold lethal charges for days after power is removed. Always discharge them using a high-wattage bleeder resistor (e.g., 100Ω 10W) on an insulated stick before touching the board. Never short them with a screwdriver.

Inductor Failures

  • Core Saturation:
    • Visual Symptom: None initially. The inductor looks perfectly normal, but the circuit fails. If pushed hard, the wire enamel darkens and smells like burning plastic.
    • Cause: Current exceeding the saturation limit ($I_{sat}$). The ferrite core loses permeability, inductance drops to near zero, and the component acts like a short piece of wire, spiking the current and destroying the switching MOSFET.
  • Ferrite Cracking:
    • Visual Symptom: Visible physical crack through the ferrite core or shield material.
    • Cause: Mechanical shock or dropping the board. Cracks alter the magnetic path and drastically reduce inductance.

Safe Substitution Rules When the Exact Part is Missing

When your BOM is out of stock, you must substitute carefully. Swapping passives blindly is the fastest way to introduce EMI failures or thermal shutdowns.

Capacitor Substitution Rules

  1. Voltage Rating: You can always substitute a higher voltage rating, never lower. A 25V cap can replace a 16V cap.
  2. Dielectric Upgrades: You can substitute a C0G/NP0 for an X7R, and an X7R for an X5R. You cannot substitute an X7R for a C0G in a timing or RF circuit; the X7R's piezoelectric effect and voltage coefficient will detune your oscillator.
  3. The DC Bias Trap: If substituting MLCCs, check the manufacturer's DC bias curve. A generic 10µF 0805 X5R capacitor rated for 10V might only provide 2µF of actual capacitance when 10V DC is applied. If your circuit relies on that 10µF for loop stability (like an LDO output), the substitution will cause the regulator to oscillate. Always pick a physically larger package or a higher voltage rating to maintain capacitance under bias.
  4. ESR Requirements: Do not substitute a low-ESR ceramic for an electrolytic in older linear regulator circuits (like the LM1117) that specifically require high ESR for loop stability. If you must use ceramic, add a 1Ω series resistor to emulate ESR.

Inductor Substitution Rules

  1. Saturation Current ($I_{sat}$): The substitute's $I_{sat}$ must be equal to or greater than the peak current of the circuit (usually $1.3 \times I_{out}$ in a buck converter). Never substitute a lower $I_{sat}$.
  2. Shielding: You can substitute a shielded inductor for an unshielded one. You cannot substitute an unshielded inductor for a shielded one in a switching power supply; the radiated magnetic flux will couple into nearby high-impedance analog traces, destroying your signal-to-noise ratio and failing FCC/CE EMI certification.
  3. DC Resistance (DCR): If the substitute has a significantly higher DCR, calculate the $I^2R$ losses. If it exceeds the component's thermal rating, the inductor will overheat and desolder itself from the pads.

For deeper mathematical modeling of inductor saturation and power stage design, refer to the Texas Instruments SLVA061 application note on buck power stages, and for MLCC mechanical failure prevention, review the KEMET flex-cracking technical guide.

Final Design Rule: When pairing these components to build an LC filter, always calculate the resonant frequency ($f_r = \frac{1}{2\pi\sqrt{LC}}$). Ensure $f_r$ is at least one decade below your switching frequency or noise target to guarantee attenuation rather than amplification. If your calculated $f_r$ lands on your switching node, you will build an oscillator, not a filter.