The inductance unit, formally known as the Henry (H), measures a component's ability to oppose changes in electrical current by storing energy in a magnetic field. In a real circuit, this unit dictates how effectively an inductor will filter high-frequency noise, smooth out DC ripple in power supplies, or tune an RF resonant tank. People commonly confuse inductance with capacitance (which opposes voltage changes) or simple DC resistance, but inductance is strictly about current inertia—it fights the change in current flow, not the steady-state flow itself. Think of it as the electrical equivalent of a mechanical flywheel: it takes significant force to spin it up, but once spinning, it resists being stopped.
The Inductance Unit Defined: Henrys, Milli, and Micro
One Henry is defined as the inductance that induces one volt of electromotive force (EMF) when the current through it changes at a rate of one ampere per second. Because a single Henry is massive for most modern electronics, you will almost exclusively work with fractional sub-units. The inductance unit you select directly determines the inductive reactance ($X_L = 2 \pi f L$), meaning the physical value you choose will shift the frequency response of your circuit.
| Prefix | Symbol | Multiplier | Typical Application Domain |
|---|---|---|---|
| Henry | H | 1 | Industrial line chokes, massive audio crossovers |
| Millihenry | mH | 10^-3 | Audio filters, EMI suppression, low-frequency ballasts |
| Microhenry | µH | 10^-6 | Switch-mode power supplies (SMPS), DC-DC converters |
| Nanohenry | nH | 10^-9 | RF impedance matching, high-speed digital decoupling |
Worked Example: Sizing the Inductance Unit for a 5V Buck Converter
Let us calculate the exact inductance unit required for a classic step-down (buck) converter using the Texas Instruments LM2596, a widely used 150 kHz switching regulator. We need to drop a 12V nominal input down to a 5V output delivering 2A of continuous current.
The standard formula for buck converter inductance is:
L = [(Vin - Vout) × Vout] / [Vin × f_sw × ΔI_L]
We will target a ripple current ($\Delta I_L$) of 30% of our maximum load, which is 0.6A.
- Vin: 12V
- Vout: 5V
- f_sw: 150,000 Hz
- ΔI_L: 0.6A
The closest standard E12 series value is 33 µH. However, picking the inductance unit is only half the battle; we must also check the saturation current ($I_{sat}$). For a 2A load with 0.6A ripple, the peak current is 2.3A. We need an inductor with an $I_{sat}$ comfortably above 3A to prevent the core from saturating, which would cause the inductance to collapse and the switching IC to overcurrent and fail.
Concrete Pick: The Bourns SRP1265A-330M. It is a shielded 33 µH surface-mount power inductor with a saturation current rating of 9.5A and a low DC resistance (DCR) of 38 mΩ, keeping thermal losses minimal at 2A.
Where You Meet This in Practice
You will encounter different scales of the inductance unit depending on the energy domain and frequency you are working within.
Power Electronics (µH range)
In switch-mode power supplies, buck/boost converters, and LED drivers, microhenry inductors act as the primary energy transfer element. They store energy when the internal MOSFET is on and release it to the load when the MOSFET is off. Shielded ferrite cores are mandatory here to prevent magnetic flux from inducing noise into nearby sensitive analog traces.
Audio and Line Filtering (mH range)
In passive speaker crossover networks and AC mains EMI filters, you deal in millihenrys. A woofer crossover might use a 2.5 mH air-core or iron-powder inductor to block high frequencies from reaching the bass driver. These components handle high RMS currents and must be sized physically large to avoid core saturation and resistive heating.
RF and High-Speed Digital (nH range)
At frequencies above 10 MHz, parasitic capacitance ruins large inductors. Here, you use nanohenry chip inductors for Pi-filters, antenna impedance matching, and decoupling high-speed FPGA power rails. At these scales, the physical PCB trace itself often possesses enough inductance (roughly 1 nH per millimeter of trace) that discrete components are sometimes omitted entirely.
Decision Path: Selecting Your Inductance Value and Part Number
Use this decision tree to narrow down the correct inductance unit scale and terminate at a specific component family for your next build.
| If Your Application Is... | Frequency Range | Target Unit Scale | Core Material | Concrete Part Series Pick |
|---|---|---|---|---|
| DC-DC Buck/Boost Converter | 100 kHz - 3 MHz | µH (1 to 100) | Shielded Ferrite / Powdered Iron | Coilcraft XEL or Bourns SRP series |
| Passive Audio Crossover | 20 Hz - 20 kHz | mH (0.5 to 10) | Air Core or Laminated Steel | Jantzen Audio Air Core coils |
| AC Mains EMI Choke | 50/60 Hz (Common mode) | mH (10 to 50) | Nanocrystalline / High-Perm Ferrite | Würth Elektronik WE-CMB series |
| RF Impedance Matching | > 50 MHz | nH (1 to 100) | Ceramic Multilayer / Wirewound | Murata LQG or Coilcraft 0402HP |
Measuring and Verifying Your Inductance Unit
A standard digital multimeter cannot measure inductance; it only checks DC continuity. To verify the inductance unit of a component, you need a dedicated LCR meter, such as the DER EE DE-5000 or a Keysight U1733C.
Critical to accurate measurement is selecting the correct test frequency on the LCR meter. Measuring a 10 mH audio choke at 1 MHz will yield garbage data due to parasitic self-resonance. Conversely, measuring a 4.7 nH RF inductor at 100 Hz will drown the reading in the meter's noise floor.
- For mH components: Test at 100 Hz or 120 Hz.
- For µH components: Test at 1 kHz or 10 kHz.
- For nH components: Test at 1 MHz or higher.
Whenever possible, measure inductors out of the circuit. Parallel capacitance and alternative current paths on a populated PCB will skew the reading. If you must measure in-circuit, ensure all capacitors are fully discharged and lift one leg of the inductor if the reading seems suspiciously low.
FAQ: Common Inductance Unit Confusions
Q: Can I substitute a higher inductance unit value than the datasheet recommends for my SMPS?
A: Generally, yes, but with a penalty. Increasing the inductance (e.g., swapping a 22 µH for a 47 µH) reduces output voltage ripple, which is good. However, it also slows down the transient response time of the regulator, meaning the power supply will react sluggishly to sudden load spikes. Furthermore, physically larger inductance values often come with higher DC resistance (DCR), which drops your overall efficiency and increases heat. Stick within 20% of the calculated value.
Q: Why does my power inductor get incredibly hot even though its DC resistance is only 20 milliohms?
A: You are likely experiencing core losses, not copper ($I^2R$) losses. Inductor cores dissipate heat when subjected to high-frequency AC ripple. If you selected an inductor optimized for low-frequency filtering and placed it in a 1 MHz switch-mode power supply, the core hysteresis and eddy currents will generate massive heat. Always check the manufacturer's datasheet to ensure the core material is rated for your specific switching frequency.
Q: Is the Henry used in digital electronics, or just analog power circuits?
A: It is heavily used in digital electronics, specifically in the nanohenry (nH) range. High-speed digital buses (like DDR4/DDR5 memory or PCIe) require precise impedance matching. Engineers use nH inductors in termination networks and EMI bead filters to prevent signal reflections and ringing. Additionally, the parasitic inductance of the PCB vias and power planes (measured in nH) is a primary factor in designing the decoupling capacitor network for modern FPGAs and microprocessors.
