The standard unit of inductor measurement is the Henry (H), named after American scientist Joseph Henry. One Henry is defined as the inductance that induces one volt of electromotive force when the current through it changes at a rate of one ampere per second. In practical bench and PCB work, a full 1 Henry inductor is massive and rarely used outside of specialized audio crossover networks or heavy industrial filtering. Instead, you will almost exclusively work with sub-units: millihenrys (mH), microhenrys (µH), and nanohenrys (nH).
Understanding these units is only the starting point. To actually use inductors in power supplies, RF matching networks, and signal filters, you need to know how to read their physical markings, choose the right core material, and safely substitute parts when your exact BOM component is out of stock.
The Henry and Its Sub-Units: A Data-Dense Reference
Before selecting a part, you need to map the schematic value to real-world physical constraints. The table below bridges the gap between theoretical units and the actual components you will solder to a board.
| Unit | Symbol | Multiplier | Typical Circuit Application | Common Package / Size | Typical DCR Range |
|---|---|---|---|---|---|
| Henry | H | 1 | Audio crossovers, heavy mains filtering, Tesla coils | Large toroids, bobbin cores (20mm+) | 0.5 Ω to 10+ Ω |
| Millihenry | mH | 10-3 | Low-frequency SMPS, EMI chokes, line filters | Radial leaded, large SMD (12x12mm) | 0.05 Ω to 2 Ω |
| Microhenry | µH | 10-6 | DC-DC buck/boost converters, RF chokes | Standard SMD (6x6mm), shielded drums | 0.01 Ω to 0.5 Ω |
| Nanohenry | nH | 10-9 | RF impedance matching, VHF/UHF filters, decoupling | 0402 / 0603 ceramic multilayer | < 0.1 Ω (often mΩ) |
Note: DCR (DC Resistance) is a critical parasitic parameter. Even if the unit of inductor matches your schematic, a high DCR will cause excessive I²R heating and voltage drop in power circuits.
Decoding Inductor Markings and Color Codes
Unlike resistors, inductors lack a single universal marking standard, but three dominant systems cover 95% of the parts you will encounter. Knowing what the markings mean prevents catastrophic misplacements in power circuits.
The 3-Digit EIA Code (SMD Power Inductors)
Most surface-mount power inductors use a three-digit code where the first two digits are the significant figures and the third digit is the multiplier (number of zeros), expressed in microhenrys (µH).
- 101 = 10 × 10¹ µH = 100 µH
- 470 = 47 × 10⁰ µH = 47 µH (Note: the zero means 'no extra zeros', not 470 µH)
- 152 = 15 × 10² µH = 1500 µH (or 1.5 mH)
The 'R' Decimal Notation
For values under 10 µH, the letter 'R' replaces the decimal point, and the unit remains microhenrys.
- 4R7 = 4.7 µH
- R47 = 0.47 µH (470 nH)
Color Bands (Leaded RF Inductors)
Molded axial inductors often use color bands identical to the resistor color code, but the resulting value is read in microhenrys (µH). A silver or gold band at the end indicates tolerance (Silver = 10%, Gold = 5%). For example, a brown-black-brown-gold band reads as 1-0-0 µH ±5%, which is a 100 µH inductor.
Inductor Construction Types: Which Type for Which Job?
Selecting the correct core material is just as critical as selecting the correct unit of inductor. The core dictates saturation current, temperature stability, and EMI shielding. Use this comparison table to select the right construction for your specific application.
| Core Type | Construction Details | Typical Tolerance | Tempco (ppm/°C) | Typical Use Case |
|---|---|---|---|---|
| Ferrite Drum/Shielded | High-permeability ceramic ferrite core, often enclosed in a magnetic shield can. | ±10% to ±20% | +1000 to +3000 | DC-DC buck converters, high-current power rails where EMI must be contained. |
| Powdered Iron | Iron powder particles insulated and bound in a resin matrix, forming a distributed air gap. | ±10% to ±15% | +50 to +300 | Switch-mode power supplies requiring high saturation current and soft saturation curves. |
| Ceramic / Air Core | Wire wound on a non-magnetic ceramic former or suspended in air/epoxy. | ±2% to ±5% | +10 to +50 | RF matching networks, VHF/UHF filters, high-Q resonant tanks where core losses ruin performance. |
| Multilayer Ceramic | Ferrite paste and conductive traces printed and co-fired in alternating layers (like an MLCC). | ±10% to ±30% | +200 to +1000 | High-frequency decoupling, low-current signal filtering in ultra-compact 0402/0603 footprints. |
Safe Substitution: When You Don't Have the Exact Part
Supply chain shortages frequently force bench substitutions. When the exact part is missing, you must evaluate three parameters before soldering a replacement. According to Coilcraft's power inductor selection guidelines, substituting blindly based on inductance alone is the leading cause of prototype failures.
The Substitution Hierarchy
- Match Inductance (L): Stay within ±20% of the original value for power converters. For RF matching networks, you must match within ±2% or recalculate the matching pi-network.
- Verify Current Ratings: Inductors have two current ratings. Irms is the thermal limit (where the part gets too hot due to DCR). Isat is the magnetic limit (where inductance drops by 20-30%). Your substitute must exceed both the original part's Irms and Isat.
- Check Physical Footprint and DCR: A larger inductor might fit the pads but have a lower DCR, which can actually alter the control loop compensation in voltage-mode buck converters. Conversely, a smaller part with higher DCR will overheat.
Series and Parallel Combinations
If you lack a specific value, you can combine standard values. Assuming the inductors are placed far enough apart to prevent mutual magnetic coupling:
- Series: Ltotal = L1 + L2. (Useful for increasing inductance, but DCR also adds up, increasing heat).
- Parallel: 1/Ltotal = 1/L1 + 1/L2. (Useful for achieving non-standard lower values while splitting the current load and halving the effective DCR).
Failure Modes and Visual Symptoms
Inductors are generally robust, but they do fail, usually due to thermal or mechanical stress. Recognizing these failure modes saves hours of debugging. For deeper diagnostic theory, All About Circuits provides excellent foundational models on inductor parasitics and failure states.
Thermal Runaway and Enamel Breakdown
- Cause: Sustained operation above the Irms rating, or high-frequency AC ripple causing excessive core eddy currents.
- Visual Symptom: The outer heat-shrink sleeve or epoxy coating is discolored (yellowed or charred). On unshielded drum cores, you may see melted or blistered copper winding enamel.
- Electrical Symptom: The part may read the correct inductance on an LCR meter at low voltage, but under load, shorted turns inside the winding cause the effective inductance to drop and DCR to fluctuate.
Mechanical Cracking (SMD Ferrites)
- Cause: PCB flexure during depaneling, or thermal shock from an improperly profiled reflow oven or aggressive hand-soldering.
- Visual Symptom: A hairline fracture running vertically through the ferrite core body, often originating from the solder fillet at the terminal pad.
- Electrical Symptom: Intermittent open circuit. The circuit works when the board is flat, but drops out when the enclosure is flexed or pressed.
Core Saturation Damage
- Cause: A transient current spike exceeded Isat, causing the downstream switching transistor to fail and short the rail.
- Visual Symptom: The inductor itself may look perfectly fine, but the associated MOSFET or diode will be cracked, vented, or blown apart. Always suspect inductor saturation if a new power supply design instantly destroys its switching FET on first power-up.
By mastering the unit of inductor measurement, decoding physical markings, and respecting core material limits, you transition from simply swapping parts to engineering reliable, high-performance magnetic circuits.
