Electronics > Beginners

How does a noise filter work?

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max.wwwang:
I watched some videos recently about the noise filtering circuits, common mode/differential mode etc etc., but still could not quite get the point. A simple example is shown in the photo, which I understand is for noise filtering purpose. These are the two wires between the SLA battery and the PCB in a UPS unit and we can see there is a ring through which both wires go. I think the sum of the two wires will always be zero so the ring will have no effect. I know I must be wrong. Any easy explanation will be appreciated.

Vovk_Z:
Search for "common noise".

EEEnthusiast:
It works on magnetic reluctance.. The common mode current (the current which is flowing in both the conductors in the same direction) would produce a magnetic field which will be in the same direction. This magnetic field will oppose the flow of current which caused it. Thus it will minimise the common mode current.

In differential case, the magnetic fields will be in opposite direction and will cancel out. Since there is no magnetic field created, there will be no opposition to the differential current.
Thus the common mode choke passes differential currents with less opposition while cancelling the common mode current.

(The noise created due to external interference occurs as common mode. For the interference, both the conductors are equal and it induces the same noise current on them)

max.wwwang:
Thanks for your help. I understand the general behavior of an inductor (and a magnetic ring like this). I understand if this is a filter (or a choke) it's wired in a common mode way (not differential). Do you mean its purpose is to reduce the noise which may exist in both wires in the same direction (both coming in or going out of the battery). In this case the combined NET current in both wires (treating them as one single wire) is not zero (the noise), and this net current would always be zero if without such noise. This net current would be similar to the net combined current in the winding of a similar ring in a RCD which detects the arithmetic sum of current in the incoming and outgoing wires (leakage current existing if not zero)?

If this is correct, why should such noise be removed just here (not everywhere else)? why is it so important that it should be dealt with? I think I mentioned this is a part of a UPS - between the battery and the board.

EEEnthusiast:
In this case the common mode noise is removed in order to reduce the EMI emissions from the wire. For the common mode noise, the entire wire acts as an antenna. For the differential currents, the forward and reverse currents would cancel out any fields created, thereby EMI is not a big concern.

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