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ComponentsPassive Components

Choke / Common-mode Choke

Inductor

An inductor used to block AC while passing DC. A common-mode choke has two windings on one core that cancel for the wanted signal but present high impedance to noise flowing the same way down both wires.

Schematic symbol

Inductor / coil

How choke / common-mode choke appears in a circuit diagram.

Types & variants

Power choke (single winding)Common-mode choke (two windings)ToroidalRod-coreSMD data-line choke

Key specs

Inductance

H

µH for power filtering, mH for common-mode mains filters.

Rated current

A

Above this the core saturates and inductance collapses.

DC resistance

Ω

Copper loss — causes voltage drop and heat.

Impedance @ frequency

Ω

For EMI chokes, quoted at 100 MHz or the noise band.

Laws that govern it

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

    A choke is an inductor chosen for its opposition to fast changes — high di/dt noise sees a high voltage drop, DC sails through.

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

    Common-mode noise on both lines induces opposing flux in the shared core and gets choked; the wanted differential signal cancels and passes.

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

    The core's flux comes from the ampere-turns through it — exceed them and the core saturates and the choke stops choking.

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

How it compares

Markings

Power chokes print inductance (e.g. 100 = 10 µH or direct '100µH'); common-mode chokes usually carry a part number only.

Standard values

10 µH – 1 mH power chokes; 1 – 50 mH common-mode chokes for mains filters.

How to choose

Power choke: inductance from the filter design and rated current ≥ 1.3× your load. Common-mode: highest impedance across your actual noise band that meets the current rating.

Pinout & package

2 leads (power) or 4 leads (common-mode, two windings — dots mark winding polarity).

Example circuits

  • LC output filter on a buck converter
  • Mains EMI filter with X/Y caps
  • USB/ethernet data-line noise suppression

Common failures

Saturation under overload (acts like a wire — downstream noise/ripple appears), open winding from mechanical stress, shorted turns after overheating.

How to test

Continuity/resistance across each winding (should be near 0 Ω but not open). An LCR meter confirms inductance; a saturated or shorted-turn choke reads far below marked value.

Substitutes

Power chokes: any inductor of same value and equal/higher current rating. Common-mode: match impedance curve and current — a ferrite bead pair works for mild cases.

Where to buy

DigiKeyMouserLCSCRobu