The unit of inductance is the henry (H), which measures a component's ability to store energy in a magnetic field and oppose changes in electrical current. While a resistor opposes the flow of current regardless of what it is doing, an inductor only cares about what the current is changing into. If the current is steady DC, an ideal inductor acts like a plain piece of wire. But the moment you try to ramp that current up or down, the inductor pushes back, generating a voltage to fight the change. Understanding the henry—and its smaller, more practical sub-units—is the difference between a power supply that runs cool and one that violently destroys its own switching transistors.
The Henry Explained: Base Units and Sub-Multiples
One henry is defined as the amount of inductance that induces one volt of electromotive force (EMF) when the current through it changes at a rate of one ampere per second. In equation form, this is expressed as:
V = L × (di/dt)Where
V is induced voltage, L is inductance in Henrys, and di/dt is the rate of current change in Amps per second. (Source: All About Circuits)
Because one full henry is a massive amount of inductance for modern electronics, you will almost never see a component rated in base henrys unless you are dealing with heavy industrial mains filtering. Instead, we use sub-multiples.
| Unit Name | Symbol | Multiplier | Typical Application |
|---|---|---|---|
| Henry | H | 1 | Mains EMI chokes, large variacs |
| Millihenry | mH | 10-3 (0.001) | Audio crossovers, low-frequency ballasts |
| Microhenry | µH | 10-6 (0.000001) | Switch-mode power supplies (SMPS), RF matching |
| Nanohenry | nH | 10-9 | VHF/UHF RF circuits, PCB trace parasitics |
1 Henry = 1,000 mH = 1,000,000 µH. When reading datasheets, always double-check the multiplier. Confusing a 4.7mH inductor for a 4.7µH inductor will result in a component 1,000 times larger than your circuit expects.
What Inductance Actually Changes in a Real Circuit
To be absolutely clear on what this unit dictates: inductance changes the rate of current change (di/dt) over time. It does not limit the maximum steady-state DC current—that is the job of the wire's physical resistance and the inductor's saturation current rating.
Think of an inductor like a mechanical flywheel in a drive train: it takes significant torque (voltage) to get the heavy mass (current) spinning, but once it is up to speed, it fiercely resists any attempt to brake it. If you suddenly disconnect the circuit, the flywheel's momentum will tear the gears apart (resulting in a massive voltage spike, or inductive kickback).
Worked Numeric Example: The LR Time Constant
Suppose you are designing a relay driver circuit. The relay coil has an inductance (L) of 10 mH (0.01 H) and a DC resistance (R) of 50 Ω. When you apply 12V DC, the current does not instantly jump to 240mA (Ohm's Law: 12V / 50Ω). It ramps up based on the RL time constant (τ = L / R).
- Calculate τ: 0.01 H / 50 Ω = 0.0002 seconds (200 µs).
- Time to steady state: It takes roughly 5 time constants (5τ) for the current to reach 99% of its final value.
- Result: 5 × 200 µs = 1,000 µs (1 millisecond). The relay coil takes exactly 1ms to fully energize.
If you swapped in a 100 mH inductor, that ramp-up time would stretch to 10ms, which might cause the relay to chatter or fail to latch properly in a high-speed PWM application.
Where You Meet Inductance Units in Practice
You will encounter different scales of henrys depending on the frequency and power level of your project. Here is where these units show up on the workbench:
- Switch-Mode Power Supplies (µH): Buck, boost, and buck-boost converters rely on microhenrys. A typical 5V to 3.3V buck converter running at 500 kHz will use an inductor in the 2.2 µH to 10 µH range. The high switching frequency means the current changes incredibly fast, so a small inductance value is sufficient to smooth the current.
- Audio Speaker Crossovers (mH): Passive low-pass filters for subwoofers operate in the audio band (20 Hz - 200 Hz). Because the frequency is low, you need much larger inductance values to create enough reactance to block high frequencies. You will typically see air-core or iron-core inductors rated between 1.5 mH and 5 mH.
- Mains EMI Filtering (mH to H): Common-mode chokes on the AC input of a computer power supply must block high-frequency noise while passing 50/60 Hz mains power. These are often rated in the 10 mH to 50 mH range, sometimes pushing into fractional henrys for heavy industrial motor drives.
Worked Scenario: When the Wrong Microhenry Rating Fries a MOSFET
Matching the unit of inductance is only half the battle. Here is a real-world bench failure that illustrates what happens when you ignore the physical limits behind the henry rating.
The Setup: A maker was building a custom LED driver using an MT3608 boost converter IC. The datasheet called for a 22 µH power inductor. Digging through their parts bin, they found a radial-leaded 22 µH inductor and soldered it onto the board.
The Numbers: The MT3608 has an internal MOSFET with a peak switch current limit of roughly 2.0A. The 22 µH inductor from the parts bin, however, was a signal-grade component with a saturation current (I_sat) of only 500mA (0.5A).
The Outcome: Upon applying power, the converter attempted to draw peak current to charge the inductor. Within microseconds, the current hit 500mA. The inductor's ferrite core instantly saturated. Once saturated, the core loses its magnetic permeability, and the inductance effectively drops from 22 µH down to near 0 µH (acting like a straight piece of wire). Without inductance to limit the di/dt, the current spiked violently past 3A, bypassing the IC's protection thresholds and violently popping the internal MOSFET, destroying the chip and blowing the input fuse.
What Went Wrong: The builder perfectly matched the inductance unit (22 µH) but completely ignored the saturation current rating. An inductor's value in henrys is only valid up to its saturation point. For power applications, you must always select a shielded power inductor where I_sat is at least 1.3 to 1.5 times higher than the converter's peak switch current. (Reference: Electronics Tutorials on Inductor Saturation).
Common Confusions: Inductance vs. Reactance vs. Resistance
When troubleshooting AC circuits or RF designs, it is easy to mix up the units of opposition. Here is how to keep them straight:
- Inductance (Henrys, H): A fixed physical property of the component based on its core material, number of wire turns, and coil geometry. It does not change with frequency.
- Inductive Reactance (Ohms, Ω): How much the inductor opposes alternating current at a specific frequency. Calculated as
X_L = 2πfL. A 10 µH inductor has 0 Ω of reactance at DC, but roughly 62.8 Ω of reactance at 1 MHz. - DC Resistance (Ohms, Ω): The physical resistance of the copper wire used to wind the coil. Often called DCR (DC Resistance) on datasheets. A 10mH inductor might have a DCR of 1.5 Ω, which causes heat loss (
I²R) regardless of frequency.
FAQ: Quick Answers on Inductance Units
Q: Can I measure henrys with a standard digital multimeter?
A: No. Standard multimeters can only measure DC resistance (Ohms). To measure inductance, you need a dedicated LCR meter (like the DER EE DE-5000 or Uni-Solar UT612), which applies an AC test signal at a specific frequency (usually 100 Hz or 1 kHz) to calculate the henry value.
Q: Why do some inductors have the same microhenry rating but drastically different physical sizes?
A: Physical size dictates two things: current handling (wire thickness) and saturation current (core volume). A 4.7 µH inductor the size of a grain of rice might saturate at 300mA, while a 4.7 µH inductor the size of a hockey puck can handle 20A without saturating. The unit of inductance is identical, but the power capability is entirely different.
Q: What happens if I use a millihenry inductor in a microhenry circuit?
A: If you put a 10 mH inductor into a 500 kHz buck converter that expects 10 µH, the inductor will store too much energy per cycle, the control loop will likely become unstable, the converter will operate in discontinuous conduction mode (DCM), and you will see massive voltage ringing and poor efficiency. Always stick to the µH value specified by the IC datasheet.
