The fundamental inductor unit is the Henry (H), named after American scientist Joseph Henry. By definition, a circuit has an inductance of one Henry when a current change of one ampere per second induces an electromotive force (EMF) of one volt across the component ($V = L \frac{di}{dt}$). While the Henry is the SI base unit, a 1H inductor is physically massive and rarely seen outside of heavy industrial power factor correction banks or massive audio crossover networks. In practical electronics, we operate in fractional sub-units.
Choosing the right part requires more than just matching the inductance value. You must navigate core materials, saturation currents, physical markings, and failure modes. This guide provides the exact data and bench-tested frameworks needed to specify, decode, and substitute inductors in modern AC/DC and DC/DC circuits.
The Henry and Practical Inductor Unit Conversions
Before selecting a component, you must translate schematic values into the physical parts available in your inventory. The schematic might call for 4,700nH, but the BOM and the physical part will label it as 4.7µH. Below is the definitive conversion and application matrix for the four practical tiers of the inductor unit.
| Unit Name | Symbol | Multiplier (Henry) | Typical Application Range | Example Benchmark Part |
|---|---|---|---|---|
| Nanohenry | nH | $10^{-9}$ (0.000000001 H) | RF impedance matching, GHz bandpass filters, VCO tanks | Coilcraft 0402HP (e.g., 2.2nH) |
| Microhenry | µH | $10^{-6}$ (0.000001 H) | DC-DC buck/boost converters, EMI common-mode chokes | Vishay IHLP-2525 (e.g., 4.7µH) |
| Millihenry | mH | $10^{-3}$ (0.001 H) | Line-frequency filtering, audio crossovers, low-freq SMPS | TDK B82793 (e.g., 10mH) |
| Henry | H | $10^{0}$ (1.0 H) | Industrial power factor correction, massive line reactors | Custom wound iron-core reactors |
Bench Rule of Thumb: For switching power supplies operating between 100 kHz and 2 MHz, your target inductor unit will almost always fall in the 1.0µH to 47µH range. For RF circuits operating above 100 MHz, you will be working exclusively in the 1nH to 100nH range.
Core Materials and Selection Criteria
Inductance is only half the story; the core material dictates how the component behaves under load, temperature, and high frequencies. Selecting the wrong core material will result in premature saturation, excessive EMI, or thermal runaway. Use this comparison matrix to match the core type to your specific circuit topology.
| Core Type | Construction & Material | Typical Tolerance | Tempco / Saturation Curve | Primary Use Case |
|---|---|---|---|---|
| Air Core | Copper wire wound on non-magnetic ceramic/plastic form | ±2% to ±5% | Linear (no saturation), extremely stable over temp | UHF/VHF RF circuits, high-Q resonant tanks |
| Ferrite (Unshielded) | MnZn or NiZn ceramic bobbin core, exposed winding | ±20% to ±30% | Hard saturation, high permeability, moderate temp drift | Cost-sensitive DC-DC converters, general EMI chokes |
| Powdered Iron | Insulated iron particles pressed into a toroid or drum | ±10% to ±20% | Soft saturation (distributed air gap), high thermal stability | High-current switching regulators, PFC chokes |
| Shielded Composite | Metal alloy powder encapsulated in magnetic epoxy resin | ±20% | Very soft saturation, high $I_{sat}$, low profile, low EMI | Space-constrained high-current POL converters, noise-sensitive boards |
Decoding Physical Markings and Inductor Codes
Unlike resistors, inductor markings are not globally standardized across all form factors, but the industry heavily relies on the 3-digit EIA code for SMD power magnetics and color bands for axial/radial leaded parts.
SMD 3-Digit and 4-Character Codes
For standard SMD power inductors (measured in microhenries), the first two digits represent the significant figures, and the third digit is the multiplier (number of zeros). The base unit for this code is always the microhenry (µH).
- 100 = 10 × $10^0$ = 10µH
- 101 = 10 × $10^1$ = 100µH
- 472 = 47 × $10^2$ = 4700µH (or 4.7mH)
When the value includes a decimal, the letter R acts as the decimal point. A marking of 4R7 means 4.7µH, and R47 means 0.47µH.
Gotcha for RF Engineers: If you are working with tiny 0402 or 0603 RF chip inductors (like the Coilcraft 0603CS series), the base unit for the 3-digit code shifts to nanohenries (nH). On an RF part, "101" means 100nH, not 100µH. Always verify the datasheet for the specific package size.
Leaded Color Band Codes
Axial and radial inductors often use a four-band color code similar to resistors, read from the lead inward. The first two bands are significant digits, the third is the multiplier (in µH), and the fourth is tolerance.
- Brown (1) - Black (0) - Brown (x10) - Silver (±10%) = 100µH ±10%
- Red (2) - Red (2) - Black (x1) - Gold (±5%) = 22µH ±5%
Failure Modes and Visual Diagnostics
Inductors are generally robust, but they are not immune to failure. When troubleshooting a dead power supply or a noisy RF stage, you must know how an inductor fails and what to look for.
1. Turn-to-Turn Short (Most Common)
The Mechanism: The thin enamel insulation between the copper windings breaks down due to voltage spikes, thermal cycling, or manufacturing defects. Adjacent turns short together.
The Symptom: The overall DC Resistance (DCR) drops slightly, but the inductance value plummets because the effective number of turns ($N$) is reduced ($L \propto N^2$). The power supply will usually fail to regulate, or the switching IC will trigger overcurrent protection immediately.
Visual Check: Often invisible. You must measure with an LCR meter. If a 10µH inductor reads 2µH on the bench, it has internal shorts.
2. Open Circuit (Catastrophic)
The Mechanism: A massive overcurrent event or prolonged operation above the RMS current rating ($I_{rms}$) causes the copper wire to melt and break.
The Symptom: Infinite resistance on a multimeter. The circuit is completely dead.
Visual Check: Look for charred, discolored, or melted epoxy coating. The component may smell distinctly of burnt plastic or ozone. In severe cases, the ferrite core itself may crack from thermal shock.
3. Core Saturation (Functional Failure)
The Mechanism: This is not a permanent physical failure, but a functional one. If the peak current exceeds the saturation current ($I_{sat}$), the core cannot store any more magnetic flux.
The Symptom: The inductor temporarily acts like a piece of straight wire (near-zero inductance). Current ramps up uncontrollably, usually destroying the driving MOSFET or diode.
Visual Check: The inductor itself may look fine, but the switching transistor will be shorted. Always check the inductor's $I_{sat}$ rating when replacing a blown MOSFET in a buck converter.
Safe Substitution When the Exact Part is Missing
You are on the bench, the BOM calls for a specific 4.7µH shielded composite inductor, and you only have unshielded ferrite drums in your bin. Can you swap them? Use this four-point decision framework to substitute safely without blowing up your prototype.
- Inductance Value: For most DC-DC buck/boost converters, a ±20% deviation in inductance is acceptable. The control loop will compensate by adjusting the duty cycle. Do not substitute a 10µH part for a 2.2µH part; the loop compensation will likely go unstable.
- Saturation Current ($I_{sat}$): The substitute's $I_{sat}$ must be greater than or equal to the original part's $I_{sat}$. $I_{sat}$ is defined as the DC current that causes the inductance to drop by a specific percentage (usually 20% or 30%). Never downgrade $I_{sat}$.
- RMS Current ($I_{rms}$): This dictates the thermal limit of the copper wire. The substitute's $I_{rms}$ must exceed the maximum continuous load current of your circuit. If you substitute a part with a lower $I_{rms}$, it will overheat and fail open over time.
- Shielding and EMI: Never swap a shielded inductor for an unshielded one in a noise-sensitive design (e.g., powering an ADC, RF transceiver, or audio DAC). Unshielded inductors radiate a strong alternating magnetic field. I once spent three days debugging a noisy 16-bit ADC, only to discover a junior tech had substituted a shielded TDK SPM series part with an unshielded drum core; the magnetic flux was coupling directly into the ADC's feedback trace.
For authoritative datasheets and selection tools, always refer to manufacturer portals like the Coilcraft Educational Resources for magnetics theory, or use the parametric search filters on the Vishay Inductor Portfolio to cross-reference $I_{sat}$ and DCR values across different composite and ferrite families.
