The unit of inductance is the henry (H), which measures a component's ability to resist changes in electrical current flowing through it. When you look at a schematic, the henry tells you exactly how much voltage an inductor will generate to fight a change in current, making it the critical metric for designing power supplies, filters, and motor drives.
The Henry (H) Defined and Demystified
According to the NIST SI unit definitions, one henry is the inductance of a closed circuit in which an electromotive force of one volt is produced when the electric current varies uniformly at the rate of one ampere per second. In practical bench terms, it is a measure of magnetic inertia.
Where V is induced voltage, L is inductance in Henrys, di is change in current, and dt is change in time.
Because one full henry is a massive amount of inductance for modern electronics, you will almost always work with fractional sub-units:
| Sub-Unit | Symbol | Decimal Value | Common Application |
|---|---|---|---|
| Millihenry | mH | 10-3 H | Audio crossovers, relay coils, low-frequency filtering |
| Microhenry | µH | 10-6 H | Switch-mode power supplies (SMPS), DC-DC converters |
| Nanohenry | nH | 10-9 H | RF impedance matching, high-speed decoupling, parasitic trace inductance |
What Inductance Actually Changes in a Real Circuit
Inductance does not resist current itself; it resists the change in current. Think of an inductor like a heavy mechanical flywheel connected to a motor. When you apply power, the flywheel's mass (inductance) prevents the current from spiking instantly, ramping it up smoothly. When you cut the power, the spinning flywheel refuses to stop immediately, forcefully driving current forward and generating a massive voltage spike to keep the flow going.
This 'kickback' effect is what changes the reality of your circuit layout, often destroying transistors if not managed.
Worked Numeric Example: The Relay Kickback
Let's calculate the exact voltage spike generated when switching off a standard 12V automotive relay using a BJT transistor.
- Inductance (L): 50 mH (0.050 H)
- Steady-state current (I): 100 mA (0.1 A)
- Switching time (dt): The transistor turns off in 10 µs (0.00001 s)
Using our core formula, the change in current (di) is 0.1 A dropping to 0 A.
V = 0.050 H × (0.1 A / 0.00001 s) = 500 Volts.
Common Confusions: Inductance vs. Reactance vs. Resistance
A frequent mistake among hobbyists is confusing the physical property of the component (inductance) with its opposition to alternating current (reactance). Here is how to keep them straight on the bench:
| Property | Unit | What It Opposes | Depends on Frequency? |
|---|---|---|---|
| Resistance | Ohm (Ω) | Steady current flow (DC & AC) | No (ignoring skin effect) |
| Inductance | Henry (H) | Changes in current over time | No (it is a fixed physical property) |
| Inductive Reactance | Ohm (Ω) | Alternating current (AC) flow | Yes (XL = 2πfL) |
Your multimeter's LCR meter function measures inductance (Henrys) by injecting a known AC test signal and calculating the phase shift. It does not measure reactance directly; it calculates the Henry value based on the test frequency.
Where You Meet the Henry in Practice
You will encounter inductance values across three primary domains in electrical and electronics work:
- Switch-Mode Power Supplies (SMPS): Buck, boost, and buck-boost converters rely on microhenry (µH) inductors to store energy in a magnetic field during the switch 'on' time and release it to the load during the 'off' time. The henry value dictates your ripple current and transient response.
- EMI and Noise Filtering: Common-mode chokes (measured in millihenries) block high-frequency noise on AC mains lines or USB data cables while passing the DC or low-frequency power unimpeded.
- Motor Drives and Solenoids: The parasitic inductance of stepper motor windings limits how fast current can rise at high stepping speeds, directly capping your motor's high-RPM torque.
Decision Path: Sizing an Inductor for a Buck Converter
Selecting the right henry value for a DC-DC buck converter is a common pain point. If the inductance is too high, the converter responds sluggishly to load changes. If it is too low, the ripple current overheats the inductor and triggers overcurrent faults. Follow this decision tree to lock in your value.
Scenario: Designing a 12V to 5V buck converter delivering 3A maximum load, switching at 500 kHz.
| Design Parameter | If your target is... | Then calculate / choose... |
|---|---|---|
| Ripple Current (ΔIL) | Standard efficiency balance | Set to 30% of max load (0.9A for a 3A supply) |
| Inductance Formula | Continuous Conduction Mode (CCM) | L = [Vout × (Vin - Vout)] / [Vin × fsw × ΔIL] |
| Raw Calculation | Plug in 5V, 12V, 500kHz, 0.9A | L = 35 / 5,400,000 = 6.48 µH |
| Standard Value Selection | Nearest E-series standard value | Round up to 6.8 µH to slightly reduce ripple |
| Saturation Current (Isat) | Prevent core saturation under fault | Must be > 1.2 × Ilimit (Aim for > 8A) |
The Concrete Part Pick
Based on the 6.8 µH requirement and an 8A+ saturation current threshold, the default recommendation is the Bourns SRP1265A-6R8M. This is a shielded, surface-mount power inductor with a 6.8 µH value, an 11A saturation current rating, and a low DC resistance (DCR) of 14 mΩ. It typically costs around $1.50 per unit on Mouser or Digi-Key and prevents the EMI radiation issues common with unshielded drum-core inductors. For through-hole prototyping, the Würth Elektronik 744774210 (10 µH, 6A) is a reliable fallback if you can tolerate slightly higher ripple.
FAQ: Quick Answers on Inductance Units
Can I replace a 10µH inductor with a 10Ω resistor?
No. A resistor limits steady current flow and dissipates energy as heat (measured in Ohms). An inductor stores energy in a magnetic field and resists changes in current (measured in Henrys). They perform entirely different functions in a circuit, and swapping them will result in a non-functional or shorted circuit.
Why do my multimeter's inductance readings fluctuate on large coils?
Large inductors (like 100mH toroidal chokes) often have high parasitic capacitance between their wire windings. Handheld LCR meters typically test at 1 kHz or 100 Hz. If the meter's test voltage interacts with the coil's self-resonant frequency, the reading will become unstable. For large inductors, trust the manufacturer datasheet or measure at a lower test frequency if your meter supports it.
Does the core material change the unit of measurement?
No, the unit remains the henry regardless of the core. However, the core material (air, ferrite, powdered iron) drastically changes the amount of henrys achieved for a given number of wire turns. A ferrite core can yield 100 times more inductance than an air core of the exact same physical dimensions, which is why core selection is just as critical as the wire gauge.
Understanding the henry is the bridge between abstract circuit theory and reliable, smoke-free hardware. Always calculate your di/dt switching times, respect the saturation current limits, and your inductors will do exactly what they were designed to do.
