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
Key specs
Inductance
HµH for power filtering, mH for common-mode mains filters.
Rated current
AAbove this the core saturates and inductance collapses.
DC resistance
ΩCopper loss — causes voltage drop and heat.
Impedance @ frequency
ΩFor EMI chokes, quoted at 100 MHz or the noise band.
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.