EMI Filter Designer (LC)
Design a simple LC low-pass EMI filter for power inputs.
How to Use
- Enter target cutoff f₀.
- Pick L → C auto-computed from LC = 1/(2πf₀)².
Show Work
Formulas
History of EMI Filtering
Radio-frequency interference from electrical equipment became a regulatory issue in the 1930s as broadcast radio and emerging television services were disrupted by sparks from motor brushes, arc lamps, and early switched-mode power converters. The FCC codified the first conducted-emission limits in the 1940s, and CISPR (International Special Committee on Radio Interference) formed in 1934 to align regulations internationally.
The switched-mode power supply revolution of the 1970s made EMI filtering universal. A 100 kHz SMPS fundamental plus its harmonics radiates into the 150 kHz – 30 MHz CISPR 11/22 conducted EMI test band, requiring line filters on every product sold. The "π filter" (two capacitors bracketing an inductor) became standard for AC inputs, with common-mode chokes wound on ferrite toroids handling the common-mode noise that dominates at higher frequencies.
Modern low-voltage DC rail filters — downstream of a buck converter, upstream of a sensitive analog circuit — use simpler LC topology: a ferrite bead or inductor in series, a ceramic capacitor (typically 10 µF MLCC) to ground. Cutoff is placed a decade below the switcher frequency (e.g., 100 kHz switcher → 10 kHz cutoff) to achieve 40 dB of attenuation at the switcher fundamental.
About This Calculator
Enter the target cutoff frequency (typically 1/10th the switcher frequency or at the low end of the noise band you need to attenuate) and an available inductor value. The tool solves for the capacitor: C = 1 / (L · (2π·f₀)²). Roll-off is 40 dB/decade above f₀ for any 2nd-order LC — doubling frequency above cutoff = 12 dB more attenuation.
Cautions: verify the inductor's saturation current exceeds your load current (otherwise L drops and the filter stops working at load); add a small series R or RC snubber across the L or across the C to damp the LC resonance peak (undamped LCs can actually amplify line disturbances at f₀); and check that the capacitor is rated for line-frequency continuous voltage. Everything runs client-side; no values leave your browser.
About the EMI Filter Designer (LC)
Meet the EMI Filter Designer (LC): a free, no-fuss tool for electronics and circuit design with nothing to install and no sign-up. Design a simple LC low-pass EMI filter for power inputs.
How it works
Put each value in its box and read the answer as you go. Because it recalculates live, you can play with the inputs to see how each one moves the result — handy for checking your own working or planning ahead. Everything happens on your device, so it is fast and private.
Want the deeper story? The Knowledge Base explains the ideas behind the tools in more detail.
Frequently Asked Questions
Why LC?
Second-order roll-off at 40 dB/decade with no resistive loss.
How do I use the EMI Filter Designer (LC)?
Just type your numbers. The answer shows up right away — there is no button to press. Change anything and it updates by itself.
Do I need to install or sign up for anything?
Not at all — it runs in the browser with nothing to install and no account. After it loads once, it even works without an internet connection.
Is my information private?
Yes. Everything happens in your browser. Nothing you type is sent to a server or saved anywhere.
Common Use Cases
Power Input
Conducted EMI (CISPR 11/FCC).
DC Rail
Downstream of switcher.
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