What Is the Unit for Inductance? Understanding the Henry in Real Circuits

The 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 are designing a switching power supply or filtering noise on an ESP32 I2C bus, understanding this unit is the difference between a stable voltage rail and a fried microcontroller. Inductance dictates how aggressively a component opposes changes in current, acting as an electrical flywheel that smooths out spikes and stores energy in a magnetic field.

The Henry (H) in Plain English and Math

While the base unit is the henry, a 1 H inductor is physically massive—typically a heavy iron-core choke used in high-power AC mains filtering or audio crossovers. In modern PCB-level electronics and embedded systems, we almost exclusively deal with fractional values. You will specify components in millihenries (mH), microhenries (µH), or nanohenries (nH).

What it changes in a real circuit: Inductance changes the slew rate of current. In a pure DC circuit, an ideal inductor is just a piece of wire with near-zero resistance. But in AC circuits or switching DC environments (like PWM motor control or buck converters), it introduces inductive reactance ($X_L = 2 \pi f L$). This reactance chokes high-frequency signals while allowing low-frequency or DC signals to pass unimpeded.

To visualize this without relying on fluid dynamics, think of a heavy mechanical flywheel. It takes a lot of torque (voltage) to get the flywheel spinning (current flowing). But once it is spinning, it takes a massive amount of braking force to stop it. If you try to stop it instantly by opening a switch, the flywheel's momentum will snap the shaft—generating a massive, destructive voltage spike (inductive kickback) that can destroy MOSFETs if not clamped by a flyback diode.

Worked Numeric Example: Sizing a Buck Converter Inductor

Let’s calculate the required inductance for a 12V to 5V buck converter stepping down power for a Raspberry Pi 5, which can pull up to 5A. We will design for a 3A continuous load to keep the example straightforward, using a standard switching frequency and ripple target.

• Input Voltage ($V_{in}$): 12V
• Output Voltage ($V_{out}$): 5V
• Switching Frequency ($f$): 500 kHz (500,000 Hz)
• Maximum Load Current ($I_{out}$): 3A
• Target Ripple Current ($\Delta I_L$): 30% of max load = $0.3 \times 3A = 0.9A$

First, determine the duty cycle ($D$):
$D = V_{out} / V_{in} = 5 / 12 = 0.4167$

Next, apply the standard buck converter inductance formula:
$L = \frac{V_{out} \times (1 - D)}{f \times \Delta I_L}$

Plug in our real values:
$L = \frac{5 \times (1 - 0.4167)}{500,000 \times 0.9}$
$L = \frac{5 \times 0.5833}{450,000}$
$L = \frac{2.9165}{450,000} = 6.48 \times 10^{-6} \text{ H}$

This gives us 6.48 µH. In practice, you would select the closest standard E-series value, such as a 6.8 µH shielded power inductor (for example, a Würth Elektronik 7447742068). Crucially, you must also check the saturation current rating. If you chose a tiny 0805-package 6.8 µH RF inductor rated for only 200mA, the core would magnetically saturate under the 3A load, the inductance would drop to near zero, and your switching IC would instantly overcurrent and fail.

Where You Meet Inductance in Practice

You will encounter the henry and its sub-units across several distinct domains on the bench and in the field:

• Switching Power Supplies (SMPS): Buck, boost, and buck-boost converters rely on µH-range power inductors to store magnetic energy during the switch-on phase and transfer it to the load during the switch-off phase. According to SparkFun's electronics tutorials, selecting the right saturation current and DC resistance (DCR) is critical for thermal management.
• EMI/RFI Filtering: Common-mode chokes (typically in the mH range) are placed on USB data lines, Ethernet jacks, or AC mains inputs. They present high impedance to high-frequency noise while letting the differential signal or 60Hz power pass.
• Motor Control: The stator windings of BLDC and stepper motors have inherent inductance (often 1 to 5 mH per phase). This physical property limits how fast the drive current can ramp up, directly dictating your maximum stepping speed before the motor loses torque.
• Audio Crossovers: Passive speaker crossovers use large air-core or iron-core inductors (ranging from 0.5 mH to several Henries) as low-pass filters to route bass frequencies to woofers while blocking tweeter-damaging highs.

Common Confusions: Inductance vs. Related Concepts

When reading datasheets or troubleshooting with an oscilloscope, it is easy to mix up related terms. Here is what people commonly confuse with inductance:

• Inductance vs. Impedance: Inductance ($L$, measured in Henries) is a fixed physical property of the component based on its core, turns, and geometry. Impedance ($Z$, measured in Ohms) is the actual resistance to AC current flow at a specific frequency. A 10 µH inductor has a fixed inductance, but its impedance changes from near-zero at DC to thousands of ohms at RF frequencies.
• Inductance vs. Capacitance: These are duals. Capacitors oppose changes in voltage by storing energy in an electric field. Inductors oppose changes in current by storing energy in a magnetic field.
• Self-Inductance vs. Mutual Inductance: Self-inductance is what we calculate for a single coil. Mutual inductance (also measured in Henries) defines how the changing magnetic field of one coil induces a voltage in a neighboring coil—the foundational principle behind transformers and wireless charging pads.

Frequently Asked Questions