The SI unit of inductance is the henry (H), defined as the amount of inductance that causes a voltage of one volt when the current changes at a rate of one ampere per second. In a physical circuit, inductance changes the time-domain behavior of current by opposing any sudden increase or decrease, temporarily storing electrical energy as a magnetic field rather than dissipating it as heat.
When you are designing a power supply, wiring a motor controller, or debugging a noisy sensor line, the henry is the metric that dictates whether your switching components survive the transient spikes or go up in magic smoke. According to the National Institute of Standards and Technology (NIST), the henry is one of the derived SI units fundamental to electromagnetic theory, yet its practical implications on the workbench are often misunderstood by hobbyists transitioning from basic DC resistive circuits.
What the Henry Actually Measures in a Circuit
To understand the henry without getting lost in Maxwell's equations, think about how a component reacts when you flip a switch. A resistor reacts instantly. An inductor reacts sluggishly. The henry quantifies that sluggishness. Specifically, it measures a coil's ability to generate a back-electromotive force (back-EMF) when the current flowing through it changes.
The governing equation for this behavior is:
V = L × (di / dt)
Where V is the induced voltage, L is the inductance in henries, di is the change in current, and dt is the change in time. Notice that if the current is steady (di = 0), the induced voltage is zero. The inductor only 'wakes up' when the current changes.
The Flywheel Analogy: Imagine water flowing through a pipe, and inside that pipe is a heavy, mechanical water wheel. When you first open the valve, the water wheel's inertia resists the flow, and it takes time for the water to reach full speed. Once the wheel is spinning, if you suddenly slam the valve shut, the heavy wheel keeps spinning, violently forcing water forward and creating a massive pressure spike that can burst the pipe. In this analogy, water flow is current, pressure is voltage, and the mass/inertia of the wheel is the inductance measured in henries.
Worked Numeric Example: Calculating Inductive Voltage
Let's move from theory to the bench with a concrete numeric example. Suppose you are designing a buck converter and need to select a snubber or flyback diode for the switching node. You are using a 47 µH (microhenry) power inductor.
During the switching cycle, your MOSFET turns off, and the current through the inductor drops from 2.0 Amps to 0 Amps. Because modern MOSFETs switch incredibly fast, this current transition happens in just 50 nanoseconds (ns).
Let's calculate the voltage spike the inductor will generate if there is no freewheeling path for the current:
- Convert to base SI units: L = 47 × 10⁻⁶ H. di = 2.0 A. dt = 50 × 10⁻⁹ s.
- Calculate the rate of current change (di/dt): 2.0 A / (50 × 10⁻⁹ s) = 40,000,000 A/s.
- Apply the formula (V = L × di/dt): V = (47 × 10⁻⁶ H) × 40,000,000 A/s.
- Result: V = 1,880 Volts.
Even though your circuit is only running at 12V or 24V, the inductor generates a theoretical 1,880V spike trying to keep the current flowing. In reality, parasitic capacitance and component breakdown will clamp this voltage, but it is more than enough to instantly destroy a standard 30V or 60V rated switching MOSFET. This is exactly why a Schottky diode is mandatory across the inductor path—to provide a safe route for that current to decay naturally.
Where You Meet the Henry in Practice
You rarely deal with whole henries in modern electronics; a 1H inductor would be the size of a microwave oven and weigh several pounds. Instead, you will encounter sub-multiples of the SI unit of inductance depending on the application. Here is a breakdown of where you will find specific ranges of the henry on the workbench:
| Unit Multiplier | Symbol | Typical Value Range | Common Applications |
|---|---|---|---|
| Microhenry | µH | 0.1 µH to 1,000 µH | Switch-mode power supplies (buck/boost), RF chokes, high-frequency EMI filtering. |
| Millihenry | mH | 1 mH to 1,000 mH | Audio crossover networks, line-frequency filters, solenoid coils, relay coils. |
| Henry | H | 1 H to 100+ H | Large industrial motor chokes, high-voltage power factor correction, antique telephony. |
When sourcing components, always check the core material. A 100 µH inductor with a ferrite core will handle high frequencies but might saturate (lose its inductance) at high DC currents. An iron-powder core of the same henry value will handle more DC current but will suffer from high core losses at switching frequencies above 100 kHz.
Real-World Scenario Walkthrough: The Blown MOSFET
To see what happens when the SI unit of inductance is ignored in a practical build, let's look at a common DIY failure mode: a PWM motor speed controller.
The Setup: A hobbyist builds a simple PWM controller to drive a 12V, 5A DC cooling fan. They use an IRLZ44N logic-level MOSFET, which has a maximum Drain-Source Voltage (Vds) rating of 55V. The fan is connected directly between the 12V supply and the MOSFET drain. No flyback diode is used because the builder assumes the 12V supply will absorb any transients.
The Numbers: The internal windings of the DC fan act as an inductor. Measured with an LCR meter, the fan's inductance is 15 mH (0.015 H). The PWM signal is running at 20 kHz, and the MOSFET turns off in roughly 100 nanoseconds. The steady-state running current is 5 Amps.
The Outcome: The moment the PWM signal goes low, the MOSFET channel closes. The fan's inductance attempts to maintain the 5A current flow. The voltage at the MOSFET drain spikes violently. The builder sees a brief flash from the fan, followed by the distinct smell of burning silicon. The MOSFET has shorted internally, and the fan runs at 100% speed permanently.
What Went Wrong: Let's look at the math. The current attempts to drop from 5A to 0A in 100ns.
V = 0.015 H × (5 A / 100 × 10⁻⁹ s)
V = 0.015 × 50,000,000
V = 750,000 Volts (theoretical)
Obviously, the voltage doesn't reach 750 kV. Instead, it rises until it finds a path. It hits the 55V Vds limit of the IRLZ44N in a fraction of a nanosecond, forcing the MOSFET into avalanche breakdown. The energy stored in the fan's magnetic field (E = ½LI² = 0.187 Joules) dumps directly into the silicon die as heat, exceeding the component's thermal mass and destroying the junction. Adding a simple 1N5819 Schottky diode in reverse-parallel across the fan would have clamped the spike to roughly 12.5V, keeping the MOSFET well within its safe operating area.
Common Confusions: Inductance vs. Resistance vs. Capacitance
People commonly confuse inductance with resistance because both are measured in series with a load and both 'limit' current. However, their mechanisms are entirely different. Furthermore, inductance is often conflated with capacitance, as both store energy, but they do so in opposite domains.
- Resistance (Ohms): Opposes steady-state current. Dissipates energy as heat. Does not care about time or frequency (ignoring skin effect).
- Inductance (Henries): Opposes changes in current. Stores energy in a magnetic field. Acts as a short circuit to DC, but high impedance to high-frequency AC.
- Capacitance (Farads): Opposes changes in voltage. Stores energy in an electric field. Acts as an open circuit to DC, but low impedance to high-frequency AC.
If you put a 10-ohm resistor and a 10-millihenry inductor in series with a DC motor, the resistor will permanently drop the voltage and limit the top speed of the motor, wasting power as heat. The inductor will do absolutely nothing once the motor reaches a steady speed, but it will severely limit the inrush current when you first flip the switch, protecting your battery from a voltage sag.
FAQ: The SI Unit of Inductance
Why are most inductors rated in microhenries instead of henries?
A single henry is a massive amount of inductance. To achieve 1H at low frequencies, you need thousands of turns of wire around a high-permeability iron core, resulting in a component with high parasitic resistance and poor high-frequency performance. In modern switch-mode power supplies operating at 100 kHz to 2 MHz, the required inductance to smooth the current ripple is mathematically very small—usually between 1 µH and 100 µH. Using smaller physical cores with fewer turns allows for faster switching and smaller physical footprints.
How do I measure the henry value of an unmarked coil?
You cannot measure inductance with a standard multimeter; a multimeter only measures DC resistance. You need an LCR (Inductance, Capacitance, Resistance) meter. Devices like the DER EE DE-5000 apply a small AC test signal (often at 1 kHz or 100 kHz) and measure the phase shift between voltage and current to calculate the henry value. Be aware that the measured inductance of a coil with a ferromagnetic core will drop significantly if you measure it while it is saturated with a high DC bias current.
Does the henry value change with frequency?
The physical geometry of the coil doesn't change, but the effective inductance does. At very high frequencies, parasitic capacitance between the wire windings creates a parallel resonant circuit. At the self-resonant frequency (SRF), the inductor stops acting like an inductor and behaves like a high-impedance parallel LC tank. Above the SRF, it actually becomes capacitive. Always check the manufacturer's datasheet for the SRF to ensure the component will function as an inductor at your circuit's operating frequency, as detailed in All About Circuits' AC theory guides.
