In strict circuit theory, every physical inductor is a passive component because it cannot generate power or introduce energy into a system; it only stores energy temporarily in a magnetic field and dissipates some as heat. So, how can some inductors be active and others passive? The term 'active inductor' does not refer to a magical magnetic coil. Instead, it refers to a synthetic inductor—an active solid-state circuit (typically a gyrator using op-amps, transistors, or switched-capacitor networks) designed to mimic the impedance behavior of a physical coil without the bulk, weight, or magnetic interference.
On the bench, you will almost exclusively deal with passive physical inductors for power and RF applications. However, understanding the distinction between physical passive coils and synthetic active equivalents is critical when debugging audio crossovers, integrated circuits, or low-frequency filters where physical coils are impractical. Here is a deep dive into how these components differ, how to read their markings, and how to handle failures and substitutions.
Physical Passive vs. Synthetic Active Inductors: Which Type for Which Job?
Choosing between a physical passive inductor and an active synthetic circuit depends entirely on your frequency range, current requirements, and spatial constraints. Physical inductors handle high power and high frequencies but suffer from parasitic capacitance and electromagnetic interference (EMI). Active inductors excel in low-frequency, low-power applications where massive physical inductance (multi-henry ranges) would otherwise require impossibly large coils.
| Type | Construction | Tolerance | Tempco (ppm/°C) | Typical Use |
|---|---|---|---|---|
| Wirewound Ferrite (Passive) | Copper wire wound on a ferrite or powdered iron core | ±10% to ±20% | +100 to +300 | DC-DC buck/boost converters, power filtering |
| Multilayer Ceramic (Passive) | Co-fired silver/palladium electrodes in ceramic dielectric | ±5% to ±10% | -200 to +200 | High-frequency RF tuning, signal line filtering |
| Synthetic Gyrator (Active) | Op-amp or transistor circuit with a capacitor and resistors | ±1% (resistor dependent) | ±50 (resistor dependent) | Audio crossovers, IC internal filtering, low-freq EQ |
Which type for which job? If you are designing a 5A switching power supply, you must use a passive wirewound inductor (like the Coilcraft XEL or TDK SPM series) because active circuits cannot handle the raw current without massive, inefficient power dissipation. Conversely, if you are designing an active audio equalizer requiring a 10H inductor at 50Hz, a passive coil would be the size of a brick and highly susceptible to 60Hz mains hum. An active gyrator circuit solves this elegantly.
Decoding Inductor Markings and Physical Identification
Unlike resistors, which universally use a color band or clear numeric system for ohms, inductor markings can be highly inconsistent across manufacturers like Murata, TDK, and Würth Elektronik. However, most surface-mount device (SMD) passive inductors follow a standardized EIA 3-digit code, measured in microhenries (µH).
The 3-Digit EIA Code System
The first two digits represent the significant figures, and the third digit is the multiplier (number of zeros). The base unit is always microhenries (µH).
- 100 = 10 × 10^0 = 10 µH
- 101 = 10 × 10^1 = 100 µH
- 472 = 47 × 10^2 = 4,700 µH (or 4.7 mH)
The 'R' Decimal Indicator
For values less than 10 µH, the letter 'R' replaces the decimal point to prevent printing errors on microscopic components.
- 4R7 = 4.7 µH
- R22 = 0.22 µH
Color Bands on Leaded Inductors
Axial leaded inductors often use color bands similar to resistors, but they are read in microhenries, not ohms. A brown-black-brown-silver band translates to 1-0-×10 with a 10% tolerance, meaning 100 µH ±10%. Always verify with an LCR meter, as some manufacturers deviate from standard color codes for proprietary RF chokes.
Failure Modes: Visual Symptoms and Bench Testing
Inductors rarely fail without a trace. When they do, the physical evidence on the PCB usually tells the story before you even unclip your multimeter probes.
1. Thermal Overload (Exceeded I_rms)
Visual Symptom: The epoxy coating or plastic overmold is discolored (yellowed or charred), and the surrounding PCB solder mask is blistered or browned. Bench Test: The DC Resistance (DCR) will often read slightly higher than the datasheet specification due to copper annealing, or the part may read open-circuit if the internal wire fused.
2. Core Saturation and Cracking (Exceeded I_sat)
Visual Symptom: Hairline fractures visible on the ferrite core, especially near the air gap or the mechanical stress points of the leads. In operating power circuits, this is often accompanied by an audible high-pitch 'whine' or buzzing. Bench Test: Inductance drops drastically under load. On an LCR meter at 1kHz, it might read nominal, but the core's magnetic permeability is permanently degraded.
3. Shorted Turns (Insulation Breakdown)
Visual Symptom: The component looks physically pristine. There are no burn marks or cracks. Bench Test: This is the most deceptive failure. If the thin enamel insulation between adjacent windings melts, the turns short together. Your multimeter will show a DCR that is lower than expected, and an LCR meter will show a massive drop in inductance. Always compare DCR to the datasheet; a 4.7µH inductor that normally has a 20mΩ DCR reading at 5mΩ has shorted internal turns.
Safe Substitution: What to Do When the Exact Part is Missing
Supply chain shortages frequently force engineers and repair technicians to substitute inductors. Swapping a resistor is simple; swapping an inductor in a switching regulator can result in a destroyed MOSFET if you ignore the physics. Follow these three rules for safe substitution.
- Match the Inductance (L) and DCR: Stay within ±20% of the nominal inductance. A slightly higher DCR is acceptable but will reduce overall efficiency and increase thermal load. A significantly lower DCR can cause sub-harmonic oscillation in peak-current-mode controllers.
- Never Compromise Saturation Current (I_sat): I_sat is the current at which the inductor's core saturates and its inductance drops by 20% to 30%. If you substitute an inductor with a lower I_sat in a buck converter, the core will saturate during peak load. When the core saturates, the inductor effectively becomes a short piece of wire, current spikes uncontrollably, and the switching MOSFET will violently fail. Always ensure the substitute's I_sat is equal to or greater than the original.
- Respect the RMS Current (I_rms): I_rms is the thermal limit—the DC current that causes a 40°C temperature rise. If your substitute has a lower I_rms, it will overheat and desolder itself from the pad over time.
- Shielded vs. Unshielded: Never replace a magnetically shielded inductor (like a molded ferrite type) with an unshielded one (like a bobbin-style drum core) if the inductor is located near sensitive analog circuitry, Hall-effect sensors, or RF antennas. The unshielded part will radiate EMI and corrupt nearby signals.
Frequently Asked Questions (FAQ)
How can some inductors be active and others passive in integrated circuits?
Inside silicon ICs, fabricating a physical passive inductor takes up a massive amount of expensive die area and yields very low inductance values (usually in the nanohenry range) with poor quality factors (Q). To solve this, IC designers use 'active inductors'—circuits built from transistors and capacitors that mathematically simulate inductive impedance. They are 'active' because they require a DC power bias to operate the transistors, unlike a passive coil which requires no external power to store magnetic energy.
Which type of inductor should I use for a buck converter vs an audio filter?
For a buck converter, you must use a physical passive inductor with a high saturation current (I_sat) and low DC resistance (DCR), typically a wirewound ferrite or powdered iron core. Active synthetic inductors cannot handle the amp-level switching currents and would introduce unacceptable noise and power loss. For an audio filter (like a graphic equalizer), an active synthetic inductor (gyrator) is vastly superior because it eliminates the physical bulk, weight, and magnetic hum pickup associated with the massive passive coils required for low-frequency audio bands.
What do the three-digit SMD inductor markings mean compared to resistors?
The numbering logic is identical to SMD resistors (two significant digits followed by a multiplier), but the base unit is different. SMD resistors are calculated in Ohms (Ω), while SMD inductors are calculated in microhenries (µH). Therefore, an SMD component marked '102' is a 1,000 Ω (1kΩ) resistor, but if it is an inductor, it is a 1,000 µH (1 mH) inductor. Always verify the component type with an LCR meter if the PCB silkscreen is ambiguous.
How do I safely substitute an inductor when the exact part number is out of stock?
To safely substitute, you must match or exceed the original part's Saturation Current (I_sat) and RMS Current (I_rms) ratings while keeping the nominal inductance (L) within a 20% tolerance. If the original part was magnetically shielded, the substitute must also be shielded to prevent EMI issues. Never downgrade the I_sat rating in a switching power supply, as core saturation will cause catastrophic failure of the driving switch MOSFET.
Can I parallel two passive inductors to get a higher current rating?
Generally, no. While paralleling two identical inductors theoretically halves the inductance and doubles the current handling capability, in practice, slight differences in DC resistance (DCR) will cause uneven current sharing. Furthermore, if the two inductors are placed close together, their magnetic fields will interact (mutual inductance), drastically altering the total inductance and potentially causing localized core saturation. It is always safer and more reliable to source a single inductor rated for the required current.
