The inductor SI unit is the Henry (H), named after American scientist Joseph Henry. One Henry is defined as the inductance that induces one volt of electromotive force (EMF) when the current changes at a rate of one ampere per second ($V = L \cdot di/dt$). Because a full Henry represents a massive amount of inductance for modern electronics, practical components are almost exclusively rated in millihenries (mH, $10^{-3}$), microhenries (µH, $10^{-6}$), or nanohenries (nH, $10^{-9}$).
Whether you are designing a 500 kHz synchronous buck converter or tuning an RF matching network, understanding how the Henry translates to physical component behavior, markings, and failure modes is critical for bench work and PCB layout.
The Henry in Practice: Numeric Examples and SPICE Conversions
To ground the theory, consider a modern USB-C PD power delivery buck converter stepping 20V down to 5V at 3A. The switching frequency is typically 500 kHz. Using the standard inductor sizing formula $L = \frac{(V_{in} - V_{out}) \cdot D}{f_{sw} \cdot \Delta I_L}$, you might calculate a required inductance of 4.7 µH.
In terms of the base SI unit, 4.7 µH is $4.7 \times 10^{-6}$ H. If the current ramps up by 1 Ampere in 2 microseconds, the voltage drop across this inductor is:
$V = (4.7 \times 10^{-6}) \cdot (1 / 2 \times 10^{-6}) = 2.35 \text{ Volts}$.
For authoritative reference on SI base and derived units, the NIST Reference on Constants, Units, and Uncertainty remains the definitive standard for metrology and unit definitions.
Inductor Construction Types and Selection Matrix
Selecting the right inductor goes far beyond just matching the µH value. The core material dictates the temperature coefficient (tempco), saturation current ($I_{sat}$), and high-frequency losses. Here is a selection matrix to determine which type fits your specific application.
| Core Type | Construction | Typical Tolerance | Tempco (ppm/°C) | Typical Use Case |
|---|---|---|---|---|
| Air Core | Copper wire wound on non-magnetic ceramic/plastic | ±2% to ±5% | ±50 (highly stable) | RF tuning, VHF/UHF filters, high-Q resonant tanks |
| Ferrite (Drum) | Copper wound on high-permeability NiZn or MnZn drum | ±10% to ±20% | ±1000 to ±3000 | DC-DC power converters, EMI chokes (low cost) |
| Shielded Composite | Wire embedded in magnetic alloy powder resin | ±20% | ±1500 | High-density point-of-load (PoL) regulators, noise-sensitive boards |
| Multilayer Ceramic | Ferrite paste and silver traces co-fired (LTCC) | ±5% to ±10% | ±100 to ±500 | Ultra-compact wearables, IoT devices, high-frequency decoupling |
Which type for which job? If you are building a switching power supply, shielded composite or ferrite drum cores are mandatory to handle high DC bias without saturating. If you are building a ham radio transmitter, air core or multilayer ceramic is required to maintain a stable resonant frequency as the component heats up.
Reading Physical Inductor Markings and Codes
Unlike resistors, which use a standardized 4-band color code, inductors rely heavily on printed alphanumeric SMD codes or physical dimensions. Misreading these can result in a catastrophic core saturation event on the bench.
SMD Letter-Number Codes
- The 'R' Decimal System: A marking of
4R7means 4.7 µH. The 'R' acts as the decimal point.R47means 0.47 µH. - The 3-Digit Multiplier System: Similar to ceramic capacitors, the first two digits are the significant figures, and the third is the multiplier (number of zeros) in microhenries.
100= 10 × $10^0$ = 10 µH (Note: 100 is 10µH, NOT 100µH).101= 10 × $10^1$ = 100 µH.472= 47 × $10^2$ = 4700 µH (4.7 mH).
Through-Hole Color Bands
While rare in modern power electronics, axial RF chokes still use MIL-PRF-39010 style color bands. Read from the lead wire inward: the first two bands are significant digits, the third is the multiplier (in µH), and the fourth is tolerance (Gold = ±5%, Silver = ±10%). For deeper specifications on surface mount coding standards, refer to the IEC 60062 standard for marking codes for passive components.
Failure Modes: Visual Symptoms and Bench Testing
Inductors are generally robust, but they fail in distinct ways when pushed past their thermal or magnetic limits. A standard multimeter measuring DC resistance (DCR) will not catch every failure; you need an LCR meter to verify the actual Henry value.
- Shorted Turns (Insulation Breakdown):
- Visual Symptom: Darkened or blistered epoxy coating, sometimes a faint smell of burnt varnish. The physical package may look intact.
- Bench Test: DCR drops slightly, but the LCR meter shows inductance significantly lower than the rated µH. The Q-factor plummets. This happens when the enamel wire insulation melts, bypassing turns.
- Open Circuit (Thermal Overload):
- Visual Symptom: Cracked ferrite core or a visibly snapped wire at the termination pad.
- Bench Test: Infinite resistance on a multimeter. Infinite/OL on an LCR meter.
- Core Saturation (Operational Failure):
- Visual Symptom: None. The component looks perfectly fine.
- Bench Test: Inductance measures correctly at small signal (1V AC), but when subjected to the circuit's actual DC bias current, the effective inductance drops by 30% to 80%. This causes massive current spikes in switching regulators, often destroying the downstream MOSFET.
Safe Substitution Rules When the Exact Part is Missing
Supply chain shortages often force makers and engineers to substitute inductors. Never substitute blindly based on the µH value alone. Follow this hierarchy to ensure safe substitution:
- Inductance Value: For power filtering, ±20% variance is acceptable. For RF oscillators or active filters, you must match within ±2% or use a trimmable air-core.
- Saturation Current ($I_{sat}$): NEVER substitute with a part that has a lower $I_{sat}$. $I_{sat}$ is the DC current at which inductance drops by a specified amount (usually 20% or 30%). If your circuit draws 4A peak, your substitute must have an $I_{sat}$ of at least 5A.
- RMS Current ($I_{rms}$): This dictates thermal heating based on the wire's DCR. The substitute's $I_{rms}$ rating must exceed the maximum continuous load current.
- Shielding: If the original BOM called for a shielded inductor (e.g., in a medical device or audio preamp), do not substitute an unshielded drum core. The radiating magnetic flux will induce audible hum or EMI failures in adjacent traces.
- DCR (DC Resistance): Lower is always better for power efficiency. Substituting a part with higher DCR will increase $I^2R$ losses and cause the part to run hotter.
Inductor SI Unit and Application FAQ
Why is the inductor SI unit called the Henry instead of the Weber?
The Weber (Wb) is the SI unit of magnetic flux, representing the total magnetic field passing through an area. The Henry (H) is the unit of inductance, which defines the relationship between that magnetic flux and the current generating it ($L = \Phi / I$). One Henry equals one Weber of flux per Ampere of current. They measure related but distinct physical phenomena.
Does the SI unit of inductance change with frequency?
No, the Henry as a base SI unit is a constant definition. However, the effective inductance of a physical component changes drastically with frequency. At high frequencies, core material permeability drops (causing µH to decrease), and parasitic winding capacitance creates a self-resonant frequency (SRF). Above the SRF, the component stops behaving as an inductor and acts as a capacitor. Always check the manufacturer's S-parameter or impedance-vs-frequency graph.
How do I calculate impedance using the Henry in AC circuits?
While the Henry defines the component's physical property, its opposition to alternating current is called inductive reactance ($X_L$), measured in Ohms ($\Omega$). The formula is $X_L = 2 \pi f L$, where $f$ is frequency in Hertz and $L$ is inductance in base SI units (Henries). For example, a 10 µH ($10 \times 10^{-6}$ H) inductor at 1 MHz presents $X_L = 2 \cdot \pi \cdot 1,000,000 \cdot 0.000010 = 62.8 \Omega$ of reactance.
