The standard unit for inductance is the henry (H), defined as the property of a closed circuit where an electromotive force of one volt is induced when the current changes uniformly at one ampere per second. In a real circuit, inductance dictates how strongly a component opposes changes in current, storing energy in a magnetic field rather than dissipating it as heat like a resistor. While resistance fights current flow, inductance fights current acceleration, acting much like a mechanical flywheel that resists changes in rotational speed.
The Henry (H): Breaking Down the Unit for Inductance
Because a full one-henry inductor would be physically massive and highly impractical for modern printed circuit boards, you will almost exclusively work with fractional sub-units. The henry scales down by factors of 1,000, mapping directly to the frequency domains and physical footprints of modern electronics.
| Sub-Unit | Symbol | Decimal Value | Typical Application Domain |
|---|---|---|---|
| Millihenry | mH | 10-3 H | Audio crossovers, 50/60Hz mains filtering, low-frequency chokes |
| Microhenry | µH | 10-6 H | Switch-mode power supplies (SMPS), RF impedance matching, buck/boost converters |
| Nanohenry | nH | 10-9 H | UHF/VHF RF circuits, high-speed digital signal integrity, parasitic trace modeling |
When sourcing components, you will typically find inductance values printed in microhenries for power applications. For example, a standard 22µH shielded ferrite inductor is a staple in 5V and 12V buck converter designs.
Worked Example: Sizing an Inductor for a Buck Converter
Let’s move from theory to the workbench. Suppose you are designing a custom carrier board for an ESP32-WROOM-32 and need to step down a 12V industrial supply to a clean 5V rail using a switching regulator. Your controller switches at 500 kHz, and you want to limit the inductor ripple current (ΔIL) to 300 mA (0.3 A) to maintain a stable voltage.
We use the standard buck converter inductor sizing formula:
L = [ Vout × (Vin - Vout) ] / [ Vin × fsw × ΔIL ]
- Plug in the real values: L = [ 5 × (12 - 5) ] / [ 12 × 500,000 × 0.3 ]
- Simplify the numerator: 5 × 7 = 35
- Simplify the denominator: 12 × 500,000 × 0.3 = 1,800,000
- Divide: 35 / 1,800,000 = 0.00001944 Henries
- Convert to microhenries: 0.00001944 × 1,000,000 = 19.44 µH
Since 19.44 µH is not a standard E-series value, you would select the next closest standard value, which is 22 µH. A part like the Bourns SRP1265A-220M would be an excellent choice here, as it provides the correct unit for inductance while supporting the necessary saturation current without core saturation.
Where You Meet This in Practice
Understanding the unit for inductance is only half the battle; knowing how physical construction alters behavior is what separates hobbyists from professionals. When you look at a schematic, the henry value tells you the magnetic storage capacity, but the physical package tells you the limits.
- Switch-Mode Power Supplies (SMPS): In buck, boost, and SEPIC topologies, inductors act as temporary energy buckets. You will almost always use the microhenry (µH) range here. The critical spec isn't just the inductance, but the saturation current (Isat). If your peak current exceeds Isat, the magnetic core saturates, the inductance drops to near zero, and your switching MOSFET will likely explode from overcurrent.
- EMI and RFI Filtering: Common-mode chokes on USB lines or AC mains inputs often use the millihenry (mH) or high microhenry range. Here, the goal is to present massive impedance to high-frequency noise while letting DC or 60Hz AC pass freely.
- Audio Crossovers: Passive speaker networks use large, wire-wound air-core or iron-core inductors in the millihenry range to block high frequencies from reaching a woofer. These are physically massive because they must handle tens of watts of audio power without saturating or adding resistive losses.
Common Confusions: Inductance vs. Reactance vs. Impedance
The most frequent mistake on the bench is confusing the physical property (inductance) with its electrical effect in an AC circuit (reactance and impedance).
Inductance (L) is measured in Henries. It is a fixed physical property of the component, determined by the number of wire turns, the core material permeability, and the coil geometry. A 10µH inductor is 10µH whether it is sitting in a drawer or connected to a 1 MHz radio transmitter.
Inductive Reactance (XL) is measured in Ohms (Ω). It is the opposition to alternating current, and it scales linearly with frequency. The formula is XL = 2πfL. A 10µH inductor has almost zero reactance at 60Hz (0.003Ω), but presents over 62Ω of reactance at 1 MHz. People commonly confuse the unit for inductance with reactance, assuming an inductor has a fixed 'resistance' to AC—it does not.
Impedance (Z) is also measured in Ohms (Ω). It is the true, real-world AC opposition, combining the inductive reactance (XL) with the inductor's internal DC Resistance (DCR) from the copper wire. At very high frequencies, parasitic capacitance between the wire windings also joins the impedance equation, eventually causing the inductor to act like a capacitor past its self-resonant frequency (SRF).
Frequently Asked Questions About the Unit for Inductance
How do you measure the unit for inductance with standard bench tools?
You cannot measure inductance with a standard digital multimeter (DMM). A DMM only measures DC resistance (DCR). To accurately measure the unit for inductance, you need an LCR meter (like the Keysight E4980A or a benchtop DER EE DE-5000), which applies a known AC test signal (usually at 1 kHz or 100 kHz) and calculates the phase shift between voltage and current. If you lack an LCR meter, you can use the 'ring-down' method: build an LC tank circuit with a known capacitor, strike it with a square wave, and measure the resonant frequency on an oscilloscope to back-calculate the henry value.
Why do components with the same unit for inductance have vastly different physical sizes?
Two inductors can both be rated at 47µH, but one might be a tiny 0805 SMD chip while the other is a massive through-hole cylinder. The difference is current handling and core material. The tiny chip inductor has a very thin wire and a small ferrite core; it might saturate at just 50mA, making it suitable only for low-power signal filtering. The large inductor uses thick copper wire and a powdered iron core, allowing it to handle 5A of continuous current in a power supply without the magnetic field collapsing. Always check the Isat (saturation current) and Irms (thermal current) ratings alongside the henry value.
What happens if I swap a microhenry for a millihenry unit for inductance in an SMPS?
If you accidentally install a 10mH inductor where a 10µH inductor belongs in a 500kHz buck converter, the circuit will likely fail to regulate. The massive inductance will restrict the current ramp-up rate (di/dt) so severely that the output capacitor cannot charge fast enough during the switching cycle, resulting in a massive voltage droop under load. Furthermore, physical 10mH inductors are not designed for 500kHz switching; their parasitic inter-winding capacitance will cause severe ringing, high switching losses, and potential electromagnetic interference (EMI) failures.
