Inductance Converter

Inductance describes how strongly a coil or wire resists a change in the current flowing through it. Push current into an inductor and it stores energy in a magnetic field; try to cut that current suddenly and the inductor fights back with a voltage spike. The unit is the henry (H), named after Joseph Henry, who worked out electromagnetic induction in the 1830s around the same time as Faraday.

A henry is a lot of inductance, so real parts are almost always fractions of one.

Metric prefixes:

• 1 H = 1,000 mH (millihenrys)
• 1 H = 1,000,000 µH (microhenrys)
• 1 H = 1,000,000,000 nH (nanohenrys)

Typical values:

• Power transformer winding: 1-100 H
• Filter choke or small inductor: 1 µH to 10 mH
• A short PCB trace: 1-100 nH

The reason nanohenrys matter at all is frequency. At 60 Hz a few nanohenrys is invisible. At gigahertz radio frequencies that same trace inductance can detune a circuit completely, which is why RF and high-speed designers fuss over trace length while a power-supply engineer rounds it to zero.

A common mistake is treating the inductor value alone as the whole story. What a circuit actually feels is reactance, which is inductance times frequency. The same 10 µH coil acts like a near-short at audio frequencies and a near-open at radio frequencies.

That is what the frequency box above is for. Inductive reactance is XL = 2πfL, in ohms, and the numbers move fast: a 10 µH coil measures 62.8 mΩ at 1 kHz, 62.8 Ω at 1 MHz, and 62.8 kΩ at 1 GHz. Same part, six orders of magnitude of behaviour, decided entirely by what you feed it.

Reading the number once you have it:

There is a limit to how far that scaling holds. Every real inductor has stray capacitance between its own turns, and above its self-resonant frequency the part stops behaving like an inductor and starts behaving like a capacitor. Datasheets list the self-resonant frequency for exactly this reason, and it is the first spec worth checking when a filter behaves backwards from what the maths predicted.