I am trying to figure out the role of each of the lettered components in this line filter circuit. The line filter is connected to a rectifier, which is connected to a high frequency power inverter, which is connected to a step down transformer.

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Not sure if the above will display so here is the link https://ibb.co/ci0CmJ

What I think so far (which may be wrong!): K1 is the common mode line choke which filters out common mode signals and lets differential signals pass.

C1 helps remove some of the high frequency noise coming from the mains and ensures any is not transmitted back into the mains.

C3 and C4 ground high frequency signals, but I am not sure how. They look like they are voltage dividers there.

I am not sure about C2 and R1.

Why do capacitors in parallel act as low pass filters, and is this what C2 and R1 are doing, and if so, how?

Also, from what I have read, for capacitors to act as voltage reservoirs to produce a steady voltage from an ac source, they have to be bigger than what is used in this circuit - is that correct?

  • \$\begingroup\$ To put an image in line, you need to link to the actual image file, not a page containing the image. \$\endgroup\$
    – The Photon
    Commented May 18, 2018 at 3:05

2 Answers 2


You basically have the details right. C1 and C2 are X or X2 rated capacitors allowing them to be across the AC mains. K1 is the common mode inductor mostly to block EMI noise which is mostly common mode from leaving the switching power supply and getting out into your house wiring

R1 is the required 'bleed' resistor to make sure when the AC is OFF or unplugged the capacitors have a discharge path.

C3 and C4 are Y rated and meant to be a balanced HF noise filter and a active ground reference for HF noise. 35.5 nF is substantial noise filtering, especially above 30 KHZ where most SMPS supplies operate.

An X or Y rated capacitor cannot fail by shorting out, even with a nail driven through them. They must be UL certified to be X or Y rated.

Note that a 10 mH common mode choke is a large value, blocking HF noise from either direction. Combined with the capacitors this is heavy duty filtering.

  • \$\begingroup\$ thank you. Is it significant that C3 and C4 are in a voltage divider style setup? Is that how such filters are usually designed, or is it just this setup? \$\endgroup\$
    – marsupials
    Commented May 18, 2018 at 3:27
  • \$\begingroup\$ The balanced filter is not mandatory. It is up to the design engineer to find the best noise filter scheme. To save a few pennies most use a capacitor to neutral only. C3 and C4 are in series so locally they add almost 18 nF of filtering. \$\endgroup\$
    – user105652
    Commented May 18, 2018 at 5:05

This is standard line filter with very balanced Common Mode choke that operates over the spectrum of the application for switched currents . It raises the impedance of both Line and Neutral or L1 & L2 such that in combination with C3,4 noise generated going out or in has the same series impedance shunted to Earth ground by those 2 caps . This filter has bidirectional noise reduction properties.

In addition to reducing Differential Mode noise C1 & L1 & C2 form. “Pi” low pass filter (LPF) which is also bidirectional for reducing switcher noise going out called “egress” and line HV transients called “ingress”.

The specs will be rated for f attenuation [dB], line current [A] and the caps C3,C4 create line leakage current typ. 200uA allowed by most safety standards . The R is included to absorb Voltage on caps when lug is removed.

C1,2 are called X rated caps and C3,4 called Y rated caps made from metallize Polyfilm dielectric rated for high voltage transients (3kV?) and chosen to not fail in the shorted condition.

  • \$\begingroup\$ Just to clarify, you mean the transformer and C1 and C2 both form a low pass filter? \$\endgroup\$
    – marsupials
    Commented May 18, 2018 at 3:40
  • \$\begingroup\$ They all form a LPF, but C1,2 are Diff Mode and C3,4 are CM . Both bidirectional. A low pass filter consists of any series impedance and a shunt capacitance and the impedance ratio of both at each frequency determines the amount of attenuation. \$\endgroup\$ Commented May 18, 2018 at 5:02

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