Inductance is the property of an electrical conductor that opposes a change in the current flowing through it by generating a self-induced electromotive force (EMF). Measured in Henries (H), it dictates how a circuit reacts when you switch it on, switch it off, or feed it alternating current. Unlike resistance, which fights the current itself, inductance fights the change in current.
The Core Definition of Inductance (and What It Actually Changes)
When current flows through a wire, it creates a magnetic field. If that current changes, the magnetic field expands or collapses, which in turn induces a voltage that pushes back against the change. This is Faraday’s law of induction in action. In a real circuit, this property changes three critical behaviors:
- DC Ramp-Up: It prevents current from spiking instantly when a switch closes, forcing it to ramp up over time.
- AC Phase Shift: It causes the current waveform to lag behind the voltage waveform, altering the power factor.
- Energy Storage: It stores energy in the magnetic field, which must be safely dissipated or transferred when the circuit opens.
The Math That Matters: A Worked Numeric Example
Abstract definitions do not help you pass a design review or troubleshoot a blown MOSFET. Let us look at how inductance behaves in both AC and DC domains with real numbers.
AC Domain: Inductive Reactance
In AC circuits, inductance manifests as inductive reactance ($X_L$), measured in ohms. The formula is $X_L = 2\pi fL$.
$X_L = 2 \times 3.14159 \times 60 \times 0.05 = 18.85\Omega$
If 120V AC is applied across this reactor, the current is limited to roughly 6.3A ($120V / 18.85\Omega$), regardless of the wire's low DC resistance.
DC Domain: The RL Time Constant
In DC circuits, we care about the time constant ($\tau$), which dictates how fast the current reaches its steady state. The formula is $\tau = L / R$.
$\tau = 0.6 / 120 = 0.005$ seconds (5ms).
Because it takes $5\tau$ to reach 99% of the final current, the relay takes 25ms to fully pull in the contacts after voltage is applied.
Where You Meet Inductance in Practice
You will encounter inductance in almost every electrical and electronic system, either as an intentional component or a parasitic nuisance.
- Switch-Mode Power Supplies (SMPS): Buck, boost, and buck-boost converters rely on inductors to store energy during the MOSFET's on-time and transfer it to the load during the off-time. Without the inductor, a buck converter would just be a short circuit.
- Motors and Transformers: The windings in AC motors and transformers are essentially massive inductors. This is why motors draw high inrush currents and require power factor correction.
- EMI/RFI Filtering: Common-mode chokes use high inductance to block high-frequency noise from escaping a power supply or entering a sensitive microcontroller circuit.
- Relays and Contactors: The coils that pull the mechanical contacts are inductive. When the control switch opens, the collapsing magnetic field generates a massive voltage spike, which is why flyback diodes or snubber circuits are mandatory.
Common Confusions: What People Get Wrong
Bench mistakes often happen when builders confuse inductance with other passive properties. Here is how to keep them straight.
| Property | Symbol / Unit | What It Opposes | Energy Storage Medium | DC Behavior at Steady State |
|---|---|---|---|---|
| Resistance | R / Ohms ($\Omega$) | Opposes current flow | Dissipates as heat | Acts as a fixed current limiter |
| Inductance | L / Henries (H) | Opposes change in current | Magnetic field | Acts as a short circuit (wire) |
| Capacitance | C / Farads (F) | Opposes change in voltage | Electric field | Acts as an open circuit (block) |
The Trap: Measuring an inductor with a standard multimeter in resistance mode will only show the DC resistance (DCR) of the copper wire, which might be 0.2$\Omega$. This tells you absolutely nothing about its inductance or its ability to handle AC ripple current.
Decision Path: Sizing an Inductor for a DC-DC Buck Converter
Choosing the wrong inductor for a switching regulator leads to core saturation, overheating, and destroyed switching ICs. Let us size an inductor for a modern 5V, 3A buck converter running at a 500kHz switching frequency (e.g., using a TI TPS5430 or MPS MP2315).
Step 1: Calculate Target Inductance
We want a ripple current ($\Delta I_L$) equal to 30% of the max load (0.9A). Assuming $V_{in} = 12V$ and $V_{out} = 5V$:
$L = \frac{V_{out} \times (V_{in} - V_{out})}{V_{in} \times f_{sw} \times \Delta I_L}$
$L = \frac{5 \times (12 - 5)}{12 \times 500,000 \times 0.9} = \frac{35}{5,400,000} = 6.48\mu H$
We round to the nearest standard value: 6.8µH.
Step 2: Determine Saturation Current ($I_{sat}$)
The peak current is the load current plus half the ripple: $3A + (0.9A / 2) = 3.45A$. The inductor's saturation current must exceed this, ideally by a 20% margin. Target $I_{sat} > 4.1A$.
| Condition / Constraint | Evaluation | Resulting Action |
|---|---|---|
| Is $I_{sat} < 3.5A$? | Yes | REJECT: Core will saturate at peak load, causing MOSFET overcurrent failure. |
| Is it an unshielded drum core? | Yes | REJECT: Radiated EMI will fail FCC/CE compliance in dense 2026 IoT PCB layouts. |
| Is DCR > 50m$\Omega$? | Yes | REJECT: Excessive $I^2R$ losses will drop efficiency below 90% and overheat the part. |
| Shielded, $\ge 6.8\mu H$, $I_{sat} > 4.5A$, DCR < 30m$\Omega$ | Yes | SELECT: Coilcraft XEL6060-682MEC (6.8µH, 6.2A $I_{sat}$, 16.5m$\Omega$ DCR). |
FAQ: Quick Answers on Inductor Behavior
Q: Why do inductors spark or blow transistors when disconnected?
A: When you break the circuit, the current tries to drop to zero instantly. The inductor opposes this by generating whatever voltage is necessary to keep the current flowing. If there is no path (like a flyback diode), the voltage spikes into the hundreds of volts, ionizing the air across the switch contacts (a spark) or punching through the silicon of your driving MOSFET.
Q: Does wire thickness change the inductance value?
A: Not directly. Inductance is primarily determined by the number of turns, the coil diameter, and the core material. However, thicker wire lowers the DC resistance (DCR) and increases the current-carrying capacity (ampacity), which is critical for power applications to prevent the inductor from melting.
Q: How do I measure inductance without an LCR meter?
A: You cannot measure it accurately with a standard multimeter. However, you can estimate it using an oscilloscope and a function generator. Wire the inductor in series with a known resistor to form an RL voltage divider, apply a high-frequency sine wave, and measure the voltage drop across both components to calculate the reactance, then back-calculate the inductance.
