Common-Mode Choke Calculator (EMI Line Filter)

Bought an off-the-shelf common-mode choke but not sure how to use it or which capacitors to pair it with? Enter the inductance from the datasheet and your Y and X capacitors to plot the filter attenuation curve and see how many dB of noise you remove from 150 kHz to 30 MHz, with a wiring diagram and step-by-step usage. Winding your own? Calculate turns, wire gauge and DC resistance from the toroid size.

Attenuation curveWiring and usageY/X capacitor cutoffDIY: turns and wire

① Inputs

Your case	Off-the-shelf choke: enter inductance Wind your own: target inductance → turns Wind your own: turns → inductance
Inductance (per winding) From the datasheet, e.g. 10 mH, 20 mH	mH
Turns per winding	turns
Y capacitor (each line to ground) Typically 1–4.7 nF, rated Y1/Y2	nF
X capacitor (line to line) Typically 0.1–1 μF, rated X2	μF
Leakage (differential) inductance Use 1% if not in the datasheet	%
Core material Used to estimate high-frequency roll-off; if unsure, keep the first option
Custom permeability μi and frequency where permeability starts to fall	μikHz
Outer diameter OD	mm
Inner diameter ID	mm
Height H	mm
AL value (optional) If the core datasheet gives AL, enter it here; it takes priority	nH/N²
Rated current (per line)	A
Current density Typically 3–5; use lower values for poor cooling	A/mm²

② Filter attenuation curve

③ Results

How to wire an off-the-shelf common-mode choke

Mains → X capacitor → common-mode choke (both dotted pins on the mains side) → two Y capacitors (L′ to ground, N′ to ground) → X capacitor → equipment.

Read four numbers on the datasheet: inductance per winding (mH), rated current (1.2–1.5× your actual current), DC resistance (lower means less heat) and the impedance curve (enough impedance at the frequencies you need to block).
Four pins, two windings: one winding goes in series with L (L in, L′ out), the other with N (N in, N′ out). The two dotted pins on the datasheet must be on the same side (both on the mains side) so the supply current fluxes cancel.
Avoid wiring mistakes: connecting the two pins of one winding to L and N shorts the mains; reversing one winding stops the flux cancelling, so the core saturates, heats up and stops filtering. Check with a meter after fitting: each winding should read near 0 Ω (its DC resistance), and there should be no continuity between windings.
Choose the capacitors: the X capacitor goes across L and N (differential-mode noise) and must be safety-rated X2; one Y capacitor goes from each of L′ and N′ to protective earth or the metal chassis (common-mode noise) and must be Y1/Y2 rated. Leakage-current limits keep Y values small, typically 2.2–4.7 nF.
Placement: put the filter right at the power inlet, keep input and output wiring apart (no crossing or running side by side) and keep the ground connection short and wide, or noise will bypass the filter.
Use the curve above: conducted-emission limits cover 150 kHz–30 MHz, so you want as much attenuation as possible in that band. If it is not enough, use a larger inductance, larger Y/X capacitors (Y is limited) or add another filter stage.

Principle and formulas

Insertion loss (50 Ω / 50 Ω, CISPR 17 method): IL = 20·log₁₀ | (2 − ω²LC)·50 + j·(ωL + 2500·ωC) | / 100　（dB）
Common mode: L = inductance per winding, C = 2·C_Y (two Y capacitors in parallel to ground)　Differential mode: L = 2·L_leak, C = C_X
CM cutoff f_CM = 1 / (2π·√(L · 2C_Y))　　DM cutoff f_DM = 1 / (2π·√(2L_leak · C_X))
High-frequency estimate: L(f) = L / √(1 + (f / f_c)²), where f_c is the frequency at which core permeability starts to fall
Winding your own: AL = 0.2·μ·H_mm·ln(OD/ID) (nH/N²)　L = AL·N²　wire d = 2·√(I / (J·π))　DC resistance R = 0.0172·length / area

The two windings have the same turns in opposite directions: the outgoing and return supply currents (differential mode) cancel in the core, so high current does not saturate it, while common-mode noise flowing in the same direction sees the full inductance. Differential mode only sees a small leakage inductance, so the X capacitor does most of the differential filtering. Real attenuation also depends on source and load impedance, wiring and grounding; treat the curve as a design reference and confirm by measurement.

FAQ

How many mH should an off-the-shelf choke have?: Start from the lowest frequency you must block: the common-mode cutoff should be below 150 kHz, where conducted-emission measurement starts, so attenuation applies across the whole band. Enter the inductance and Y capacitors here and check the cutoff and curve; AC mains filters commonly use 5–30 mH, with a rated current above your actual current.
How is a common-mode choke different from a normal inductor?: A normal inductor has one winding, so all current creates flux in the core. A common-mode choke has two windings with the same turns in opposite directions; the supply current fluxes cancel, so a small core can give several mH without saturating at high current.
Why does the curve flatten at high frequency?: MnZn ferrite permeability starts to fall above a few hundred kHz to 1 MHz, so inductance drops; the dashed line estimates this effect. For noise above 10 MHz, use a nanocrystalline or NiZn choke or add a ferrite bead. Always check the impedance curve in the choke datasheet.
Can I use larger Y capacitors?: Y capacitors connect line to earth, and larger values mean more leakage current; medical and household equipment have leakage limits (commonly 2.2–4.7 nF each). For better common-mode filtering, increase the choke inductance or add another filter stage.
Why leave a gap between the windings (winding your own)?: The gap raises the voltage withstand between windings for safety approval and keeps leakage inductance consistent. A window fill under 30% is easy to wind; about 40% is the practical limit.

Related design tools

Butterworth low-pass filter Chebyshev low-pass filter Antenna impedance matching Pi/T attenuator

Need a custom EMI filter or RF circuit? WENSHING supplies RF modules and antennas and helps with filter and RF circuit design.

Contact our engineers