Inductor / Coil

• Self-inductance formula — v = L di/dtv = L × di/dt, E = ½LI²

  v = L·di/dt is the defining equation — the coil resists current change and stores ½LI² in its field while doing so.

• Faraday's law of electromagnetic inductionEMF = −N × dΦ/dt

  The voltage appears because the coil's own changing flux links its turns; inductance is just Faraday's law wearing a component package.

• Lenz's lawthe minus sign in EMF = −N × dΦ/dt

  The minus sign explains the direction: the induced voltage always fights whatever you are trying to do to the current.

• Ampère's circuital law∮ B · dl = μ₀ × I(enc)

  Gives the field inside a solenoid or toroid, and hence the L you get from N turns on a given core.

Each law is stated in full — with its diagram, variables and worked meaning — on the electrical laws page.

Through-hole: Often printed value with unit (e.g., 100µH). SMD: 3-digit code (e.g., 101 = 10 x 10^1 = 100nH), sometimes with 'nH' or 'µH' suffix.

Common: 1µH, 10µH, 100µH, 1mH, 10mH. RF values: 10nH, 100nH, 1µH. Values often follow E12/E24 series.

1) Determine required inductance for your circuit (depends on frequency and application). 2) Check current rating exceeds your circuit's max current. 3) Consider DC resistance for power efficiency. 4) Select appropriate core material for your frequency range.

• LC filter (low-pass, high-pass, band-pass)
• Buck/boost converter
• Oscillator tank circuit
• RF matching network
• EMI/RFI suppression

Open circuit (burnt wire), shorted turns, increased DC resistance, core cracking. Physical signs: burnt smell, discoloration, cracks in core or epoxy.

Use LCR meter for accurate inductance measurement. With multimeter: Check for continuity (should show low resistance, not open). Cannot measure inductance with standard multimeter.

Use nearest standard inductance value. For RF, must match SRF. Can combine inductors in series/parallel for custom values (series: L1+L2, parallel: 1/(1/L1+1/L2)).