How Current Across an Inductor Behaves: Types, Markings, and FAQs

Strictly speaking in circuit theory, current flows through a component while voltage drops across it. However, on the bench and in practical engineering, the phrase "current across an inductor" is a common colloquialism for the current flowing from terminal to terminal and how the component's magnetic field reacts to it. The fundamental rule governing this behavior is that an inductor opposes any change in current. Mathematically, this is expressed as V = L(di/dt), meaning the voltage induced is proportional to the rate of change of the current.

If you try to interrupt the current across an inductor instantaneously, di/dt approaches infinity, generating a massive voltage spike (flyback) that will easily punch through a MOSFET's drain-source junction. Understanding how to select, read, test, and substitute these passive components is critical for everything from RF filtering to buck converter design. Here is a practical, bench-level guide to managing inductors in your circuits.

Inductor Types and Selection Criteria

Choosing the right inductor isn't just about matching the microhenry (µH) value. The core material dictates the saturation current, frequency response, and thermal behavior. Below is a comparison of the most common inductor types you will encounter in power and signal circuits.

Which type for which job? If you are building a 500 kHz buck converter, an iron powder or shielded composite SMD inductor (like the Coilcraft XEL series) is mandatory to handle the high DC bias without saturating. If you are building an FM radio front-end, an air-core or low-loss ferrite inductor is required to maintain a high quality factor (Q).

Decoding Physical Markings and SMD Codes

Unlike resistors, inductor markings are notoriously inconsistent across manufacturers, but they generally follow a few established patterns. Knowing what the markings mean saves you from guessing with an LCR meter every time you dig through your component bins.

Through-Hole Axial Color Bands

Axial inductors use a color band system nearly identical to the 4-band resistor code, but the base unit is microhenries (µH), not ohms.
Example: A band sequence of Brown (1), Black (0), Brown (×10) translates to 10 × 10 = 100 µH. The fourth band (often silver or gold) indicates tolerance (±10% or ±5%).

SMD Inductor Codes

Surface mount inductors use a 3-digit or 4-digit alphanumeric code.

• 3-Digit Code: The first two digits are significant figures, and the third is the multiplier (number of zeros). A marking of 101 means 10 × 10¹ = 100 µH. A marking of 470 means 47 × 10⁰ = 47 µH.
• R-Notation: The letter 'R' acts as a decimal point. A marking of 4R7 means 4.7 µH. R22 means 0.22 µH.
• 4-Digit Code: Used for tighter tolerances or specific manufacturer series, where the first three digits are significant and the fourth is the multiplier.

Failure Modes: Visual Symptoms and Bench Testing

Inductors are generally robust, but they are not immune to abuse, especially in power electronics. Here is how they fail and how to spot it.

• Shorted Turns (The Silent Killer): When the thin enamel insulation on the copper windings breaks down due to thermal cycling or voltage spikes, adjacent turns short together. Visual Symptom: Often none; the epoxy or core looks perfectly fine. Bench Test: The DC resistance (DCR) drops slightly, but the inductance plummets. In a switching circuit, a shorted-turn inductor will run blazing hot and cause the switching IC to overcurrent and shut down.
• Open Circuit: The wire snaps, usually right at the terminal pad where mechanical stress and thermal expansion meet. Visual Symptom: A hairline crack in the solder joint or a visibly detached wire on bobbin types. Bench Test: Infinite resistance on a multimeter.
• Core Saturation (Functional Failure): This isn't a permanent physical failure, but a design flaw. If the peak current exceeds the inductor's saturation current ($I_{sat}$), the core's permeability drops to near that of air. Visual Symptom: None on the part itself, but your oscilloscope will show the inductor current waveform changing from a clean linear ramp to a sharp, exponential spike at the peak of the switching cycle.
• Thermal Degradation: Operating continuously above the rated RMS current ($I_{rms}$) bakes the component. Visual Symptom: Yellowing or browning of the heat-shrink sleeve, cracked epoxy coating, or a burnt smell when desoldering.

Safe Substitution When the Exact Part is Missing

When you are prototyping or repairing a board and the exact BOM inductor is out of stock, you can substitute safely if you respect four critical parameters. According to All About Circuits, ignoring these parameters is the leading cause of DIY power supply failures.

1. Inductance (L): For general EMI filtering, ±20% is perfectly acceptable. For resonant converters or precise timing circuits, you must match the value within ±5%.
2. Saturation Current ($I_{sat}$): Never substitute a part with a lower $I_{sat}$. If your circuit peaks at 3A, and you swap in an inductor with a 2.5A $I_{sat}$, the core will saturate, inductance will collapse, and your switching MOSFET will likely explode. Always round up.
3. RMS Current ($I_{rms}$): This is the thermal limit of the wire. Substituting a part with a lower $I_{rms}$ will result in overheating and eventual open-circuit failure. Round up.
4. DC Resistance (DCR): Lower is better. If you substitute an inductor with a higher DCR, you will increase $I^2R$ losses, reducing your overall efficiency and increasing the part's operating temperature.

Physical Footprint: If you must use a physically larger inductor to get the required $I_{sat}$, ensure the taller profile doesn't interfere with enclosures and that the larger pad size doesn't lift the SMD pads during rework.

Frequently Asked Questions

Why does the current across an inductor cannot change instantaneously?

This is dictated by the conservation of energy and the formula V = L(di/dt). To change the current instantaneously, the time differential (dt) would be zero. Dividing by zero means the required voltage (V) would have to be infinite. Since infinite voltage doesn't exist in the real world, the magnetic field must collapse or build up over a finite amount of time, forcing the current to ramp smoothly rather than step instantly.

How do you measure the current across an inductor in a switching circuit?

Do not use a standard multimeter in series for high-frequency switching nodes; the lead inductance and meter bandwidth will ruin the measurement. Instead, use an oscilloscope with a dedicated AC/DC current probe clamped around a loop containing the inductor. Alternatively, insert a low-value, low-inductance shunt resistor (e.g., 10mΩ) in series with the inductor's ground return path and measure the voltage drop across the shunt with a differential probe.

What happens to the current across an inductor when DC voltage is applied?

When a constant DC voltage is first applied, the current starts at zero and ramps up linearly (di/dt = V/L). It does not instantly jump to the maximum value. Eventually, the current will level off and be limited only by the inductor's internal DC resistance (DCR) and the source's current capacity, at which point it acts like a standard piece of wire and the magnetic field becomes static.

Can I put two inductors in parallel to handle more current?

Yes, but with strict caveats. Placing two identical 10µH, 2A inductors in parallel gives you an equivalent inductance of 5µH (assuming zero mutual coupling) and doubles your current handling to 4A. However, if they are placed physically close together, their magnetic fields will interact (mutual inductance), drastically altering the total inductance. If you must parallel them, place them at 90-degree angles to each other or use physically shielded SMD types to minimize crosstalk, as detailed in Electronics Tutorials guides on magnetic coupling.