The SI unit for inductance is the henry (H), defined as the amount of inductance that induces one volt of electromotive force when the current through it changes at a rate of one ampere per second. When you look at a schematic, the henry tells you how aggressively a component will fight any attempt to change the current flowing through it. Unlike resistance, which opposes current itself, inductance opposes the change in current.
What the Henry Actually Changes in a Real Circuit
In a real circuit or installation, inductance dictates the rise and fall times of current and stores energy in a magnetic field. When current flows through an inductor, energy is stored in that magnetic field. If you try to increase the current, the inductor generates a back-EMF (voltage) that fights the increase. If you try to decrease the current—like opening a switch on a relay coil or a motor winding—the collapsing magnetic field tries to maintain the current flow. This generates a massive voltage spike, often hundreds of volts, which is why we use flyback diodes to clamp the spike and protect switching transistors.
To visualize this, think of water flowing through a pipe with a heavy, submerged paddlewheel. The water (current) takes time and pressure (voltage) to get the heavy wheel (inductor) spinning. Once it is spinning, the wheel's momentum keeps the water flowing forward even if the pump suddenly shuts off, causing a pressure spike (water hammer) against the closed valve.
Worked Numeric Example: Sizing a Buck Converter Inductor
The most common place hobbyists and engineers calculate the SI unit for inductance from scratch is in switch-mode power supply (SMPS) design. Let's calculate the required inductance for a DC-DC buck converter stepping 12V down to 5V at a 2A maximum load, switching at 500 kHz.
The standard formula for buck converter inductance is:
L = ((V_in - V_out) × D) / (f_sw × ΔI_L)
- Duty Cycle (D): V_out / V_in = 5 / 12 = 0.4167
- Target Ripple Current (ΔI_L): A standard design rule is to set the ripple current to 30% of the maximum load current. 2A × 0.30 = 0.6A.
- Switching Frequency (f_sw): 500,000 Hz.
Plugging in the numbers:
L = ((12 - 5) × 0.4167) / (500,000 × 0.6)
L = 2.9169 / 300,000
L = 0.000009723 H or 9.72 µH
In practice, you would select a standard 10 µH shielded power inductor. However, the henry value is only half the story. You must also check the saturation current (I_sat). If the core saturates at 1.5A, your 10 µH inductor will effectively drop to near 0 µH at the 2A peak, causing your switching MOSFET to overcurrent and fail. Using a tool like the Coilcraft Power Inductor Finder, you would filter for 10 µH with an I_sat rating of at least 3A (e.g., the MSS1210-103KEB).
Where You Meet This in Practice
While the base SI unit for inductance is the henry, a full 1H inductor would be the size of a car battery. On the bench, you will almost exclusively deal with fractional sub-units. Here is where you will encounter them in real-world builds:
| Application | Typical Unit | Example Component & Value | Function |
|---|---|---|---|
| Switch-Mode Power Supplies | Microhenrys (µH) | Buck converter choke (4.7 µH - 47 µH) | Stores and transfers energy during the switching cycle to regulate DC voltage. |
| EMI / RFI Filtering | Millihenrys (mH) | Common mode choke (10 mH - 50 mH) | Blocks high-frequency noise from entering or leaving a device on AC mains lines. |
| Audio Speaker Crossovers | Millihenrys (mH) | Air-core woofer coil (1.5 mH - 4.7 mH) | Acts as a low-pass filter, blocking high frequencies from reaching the subwoofer. |
| Industrial Motor Windings | Henrys (H) or mH | Large 3-phase stator winding (0.5 H) | Creates the rotating magnetic field; dictates the motor's starting current surge. |
Common Confusions: Inductance vs. Reactance vs. Capacitance
People commonly confuse inductance with inductive reactance and capacitance. Understanding the distinction is critical for AC circuit analysis and fundamental component theory.
- Inductance (L, Henrys): The physical, fixed property of the component based on its core material, number of turns, and coil geometry. It does not change with frequency.
- Inductive Reactance (X_L, Ohms): The effective AC "resistance" the inductor presents to the circuit. It is calculated as
X_L = 2πfL. While the inductance (Henrys) stays the same, the reactance (Ohms) increases as the AC frequency (f) goes up. - Capacitance (C, Farads): The dual of inductance. While an inductor stores energy in a magnetic field and opposes changes in current, a capacitor stores energy in an electric field and opposes changes in voltage.
Frequently Asked Questions
What is the SI unit for inductance and how is it measured on a bench?
The SI unit for inductance is the henry (H), named after Joseph Henry and formally recognized by the NIST SI Units framework. On a bench, it is measured using an LCR meter. The meter applies an alternating voltage at a specific test frequency (e.g., 100 Hz, 1 kHz, or 100 kHz), measures the resulting current and phase shift, and calculates the inductance. For precision work, you must also apply a DC bias current during measurement, as the henry value of an inductor drops significantly as its magnetic core approaches saturation.
Why do schematics use millihenrys and microhenrys instead of full henrys?
A one-henry inductor requires an enormous number of wire turns around a large, heavy magnetic core to achieve that much flux linkage. In modern electronics, especially high-frequency switch-mode power supplies and RF circuits, the required physical inductance is tiny. A 10 µH (0.000010 H) inductor can be manufactured as a tiny surface-mount component (SMD) measuring just 3mm x 3mm. Using sub-units like millihenrys (10^-3 H), microhenrys (10^-6 H), and nanohenrys (10^-9 H) avoids writing strings of leading zeros on schematics.
How do I calculate total inductance for inductors in series and parallel?
Inductors combine exactly like resistors, assuming they are placed far enough apart that their magnetic fields do not interact (mutual inductance is zero).
In Series: The total inductance is the sum of the individual values. L_total = L1 + L2 + L3...
In Parallel: The reciprocal of the total inductance is the sum of the reciprocals of the individual values. 1/L_total = 1/L1 + 1/L2 + 1/L3...
Keep in mind that putting inductors in parallel halves the current rating per branch but also reduces the total inductance, while putting them in series increases the total inductance but the maximum current is limited by the inductor with the lowest saturation rating.
