Inductance is the property of an electrical conductor that opposes a change in the current flowing through it by storing energy in a magnetic field. When builders ask "what is the inductance" of a specific coil or trace, they are usually trying to solve one of two problems: managing the frequency-dependent impedance in an AC or switching circuit, or surviving the massive voltage spikes generated when a DC current is suddenly interrupted.
The Core Definition and the Resistance Confusion
The most common mistake hobbyists and junior technicians make is confusing inductance with resistance. A resistor opposes all current equally and permanently burns electrical energy as heat. An inductor only opposes changes in current. If you apply a steady 5V DC across a perfect inductor, it acts as a dead short. But if you try to switch that 5V on and off at 100 kHz, the inductor fights the change, creating a high impedance that restricts the AC ripple while passing the DC baseline.
To visualize this, imagine a heavy water wheel placed in a stream. When the water (current) is flowing steadily, the wheel spins freely and offers little resistance. But if you suddenly try to slam a gate shut to stop the water, the heavy momentum of the wheel keeps pushing water forward, creating a massive pressure spike (voltage spike) against the gate. Conversely, if the water is stopped and you suddenly open the gate, the heavy wheel takes time to start spinning, delaying the flow of water. This mechanical inertia is exactly how inductance behaves electrically.
The Math That Matters: Impedance and Flyback Spikes
On the workbench, you rarely need to calculate the physical inductance of a hand-wound coil from its geometry. Instead, you use the inductance value (measured in Henrys, H) to calculate two critical circuit behaviors: AC reactance and inductive kickback.
1. AC Inductive Reactance ($X_L$)
In a switching power supply or an audio filter, inductance creates an AC resistance called reactance. The formula is:
$X_L = 2 \pi f L$
Worked Example: You are designing a low-pass filter for an audio line and place a 10 mH (0.010 H) inductor in series. You want to know how much it will block a 1 kHz noise signal versus a 20 Hz bass note.
At 1 kHz: $X_L = 2 \times 3.1416 \times 1000 \times 0.010 = 62.8 \Omega$.
At 20 Hz: $X_L = 2 \times 3.1416 \times 20 \times 0.010 = 1.25 \Omega$.
The inductor easily passes the bass but severely chokes the 1 kHz noise.
2. Flyback Voltage Spikes ($V = L \frac{di}{dt}$)
This is the formula that destroys microcontrollers. When you open a switch (like a transistor turning off) in series with an inductor, the current tries to drop to zero instantly. Because $dt$ (time) approaches zero, the induced voltage ($V$) approaches infinity.
Worked Example: You are driving a 12V relay coil with an Arduino GPIO pin via a 2N2222 transistor. The relay coil has an inductance of 50 mH (0.050 H) and draws 100 mA (0.1 A). When the transistor turns off, the current drops to zero in roughly 1 microsecond ($0.000001$ s).
$V = 0.050 \times (0.1 / 0.000001) = 5,000 Volts$.
Without a flyback diode to absorb this energy, that 5,000V spike will instantly punch through the 2N2222's collector-emitter junction and fry your ATmega328P microcontroller.
Where You Meet Inductance in Practice
You will encounter inductance in three primary areas of DIY and professional electronics:
- Switching Power Supplies (Buck/Boost): Inductors act as temporary energy buckets. The switching IC turns on to store energy in the inductor's magnetic field, then turns off to dump that energy into the output capacitor. The inductance value directly dictates the ripple current and switching frequency.
- EMI and RFI Filtering: Common-mode chokes (two inductors wound on the same core) are placed on USB lines, AC mains inputs, and motor leads. They present high impedance to high-frequency noise (which causes EMI) while allowing the DC or 50/60Hz AC power to pass unimpeded.
- Motor and Relay Control: Any coiled wire creates inductance. Stepper motors, DC brushed motors, and relay coils are all massive inductors. Managing the flyback energy generated by these components is mandatory for circuit survival.
Decision Tree: How to Pick the Right Inductor
Selecting an inductor isn't just about the Henry value. You must choose the right core material and physical form factor. Use this decision path to terminate your selection process with a concrete part number.
| Application Scenario | If Your Requirements Are... | Then Choose This Core/Type | Concrete Part Pick (Reference) |
|---|---|---|---|
| Buck Converter (DC-DC Step Down) | High current (1A-5A), low frequency (100kHz-1MHz), needs to prevent magnetic field from interfering with nearby Hall sensors or RF. | Shielded Ferrite Core. Look for high saturation current ($I_{SAT}$) greater than your peak switch current. | Bourns SRN6045TA-4R7N (4.7µH, 3.2A Isat, shielded SMD) |
| RF Choke (VHF/UHF Antenna Matching) | Very high frequency (100MHz+), low current, minimal parasitic capacitance required to keep Self-Resonant Frequency (SRF) high. | Air-Core or High-SRF Ceramic. Avoid ferrite, which becomes lossy and drops in permeability at VHF. | Coilcraft 0402HP-100X (10nH, ceramic, 4.5GHz SRF) |
| Audio Line EMI Filter | Low current (<100mA), needs high inductance in a tiny footprint to block MHz noise from switching regulators. | Multilayer Ceramic Chip Inductor. Cheap, tiny, but low saturation current. Do not use in power rails. | Murata LQH32CN100K53 (10µH, 150mA max, unshielded) |
| AC Mains Input Filter | High voltage (120/240VAC), needs to block common-mode noise from entering or leaving the device. | Toroidal Common Mode Choke. Two windings on a high-permeability ring core. | Wurth Elektronik 744824310 (10mH, 3A, 250VAC rated) |
Parasitics: The Hidden Specs That Destroy Circuits
A real-world inductor is not just an ideal inductance ($L$). According to detailed models from manufacturers like Analog Devices, every physical coil includes three parasitic elements that will ruin your design if ignored:
- DC Resistance (DCR): The physical resistance of the copper wire used to wind the coil. A 10µH inductor with a 500mΩ DCR carrying 2A will dissipate $I^2R = 4 \times 0.5 = 2$ Watts of heat. In a tiny SMD package, this will cause thermal shutdown or desolder the pads. Always check the DCR against your thermal budget.
- Saturation Current ($I_{SAT}$): The current level at which the inductor's magnetic core material becomes fully magnetized. Once saturated, the inductance drops drastically—often to near zero. If your buck converter inductor saturates during a load transient, the switching IC will see a dead short to ground and instantly burn out the internal MOSFET. Always pick an $I_{SAT}$ rating at least 20% higher than your peak inductor current.
- Self-Resonant Frequency (SRF): The physical coil windings act as a tiny capacitor in parallel with the inductance. At the SRF, the inductor stops acting like an inductor and becomes a parallel resonant tank, offering maximum impedance. Above the SRF, it actually becomes a capacitor, completely defeating its purpose as a high-frequency choke. Ensure your operating frequency is well below the component's SRF.
Frequently Asked Questions
Can I measure inductance with a standard multimeter?
No. Standard multimeters only measure DC resistance (DCR). To measure actual inductance in Henrys, you need a dedicated LCR meter (like the Der EE DE-5000 or Keysight U1733C) which applies an AC test signal at a specific frequency (usually 1 kHz or 100 kHz) to calculate the reactance.
Does the physical orientation of an unshielded inductor matter on a PCB?
Yes. Unshielded inductors radiate a magnetic field. If you place two unshielded inductors close together, their fields will couple, causing crosstalk and altering the effective inductance. If you must use unshielded parts, rotate one 90 degrees relative to the other to minimize mutual inductance, or switch to shielded SMD parts.
Why do relay coils have a diode across them?
As demonstrated in the flyback math example above, the inductance of the relay coil generates thousands of volts when the driving transistor turns off. The reverse-biased flyback diode (like a 1N4007 or 1N4148) provides a safe, low-resistance path for the collapsing magnetic field to dissipate its stored energy back into the coil, clamping the voltage spike to a safe ~0.7V above the supply rail.
