How does a noise filter work? - Page 1
Forum section: electronics / beginners

author: max.wwwang
author_rank: Frequent Contributor
timestamp: February 09, 2020, 08:34:26 am
content: 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.

author: Vovk_Z
author_rank: Super Contributor
timestamp: February 09, 2020, 10:01:51 am
content: Search for "common noise".

author: EEEnthusiast
author_rank: Frequent Contributor
timestamp: February 09, 2020, 12:01:42 pm
content: 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)

author: max.wwwang
author_rank: Frequent Contributor
timestamp: February 10, 2020, 09:06:11 am
content: 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.

author: EEEnthusiast
author_rank: Frequent Contributor
timestamp: February 10, 2020, 02:46:42 pm
content: 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.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: February 11, 2020, 07:09:36 am
content: Thanks. This demo is really amazing, and the effect of a tiny ferrite bead could be so fascinating!

author: EEEnthusiast
author_rank: Frequent Contributor
timestamp: February 11, 2020, 07:50:32 am
content: And the reason they are preferred over inductors for EMI is that an inductor will not dissipate the RF energy. It will just move it from the wire to itself. The ferrite actually converts the RF energy to heat.

author: radiolistener
author_rank: Super Contributor
timestamp: February 11, 2020, 08:05:26 am
content: The noise has very wide bandwidth. The inductor works as a simple low-pass filter, because it's resistance depends on the frequency (higher frequency = higher inductor resistance). So, inductor just acts like attenuator for high frequency components of noise in the signal.Ferrite core is good for such filters, because it has heat loss and dissipate part of noise energy into heat. So, it works not only like ideal inductor (which reflects noise energy back to the noise source), but also as a noise absorber.

author: RoGeorge
author_rank: Super Contributor
timestamp: February 11, 2020, 09:13:27 am
content: Exactly that. ^^^At first, I thought that an EMI filter will absorb the higher noise frequencies, and dissipate their energy as heat inside the ferrite core, but this is not entirely true. In my measurements most of the energy was, in fact, reflected back into the wires, and only a very small part of it was absorbed as heat by the ferrite core.How much is absorbed as heat in the ferrite core and how much is reflected back into the wires depends a lot with ferrite material and with the frequency.In my sloppy measurements, all of the "noises" were reflected back into the wires. For sure there was some energy absorbed by the ferrite, too, and turned into heat, but the absorbed part was so small that I couldn't measure its value.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: February 12, 2020, 07:45:40 am
content: Thanks for all the answers. Back to the application in this case - a ferrite ring around the wires connecting the battery and the board in a UPS - why is it needed here?Quote from: EEEnthusiast on February 10, 2020, 02:46:42 pmIt's simply a charging/discharging process, both would be pretty stable (I guess). How could there be EMI emissions in these 2 wires?

author: RoGeorge
author_rank: Super Contributor
timestamp: February 12, 2020, 09:58:45 am
content: In this case the noises are produced by the electronic PCB, and if the noises travel into the battery, than the whole battery will act like a big transmission antenna, radiating all kind of parasitic into air.Since we want to minimize any radio noises, we will want to stop switching noises to get into the battery, so we put that ferrite bead on the wires to stop the noises getting into the battery and later into air as radio waves.

author: EEEnthusiast
author_rank: Frequent Contributor
timestamp: February 12, 2020, 10:03:25 am
content: The battery is powering a UPS which creates a lot of switching noise. This noise can radiate from the wires connecting the batteries to the PCB as its length is long. The ferrite bead acts as a damper for these switching signals in order to minimise the EMI.

author: radiolistener
author_rank: Super Contributor
timestamp: February 13, 2020, 10:47:56 pm
content: Quote from: max.wwwang on February 12, 2020, 07:45:40 amThere are a lot of pulses in charger circuit, because it is based on Switched-Mode Power Supply. Any current change (any pulse) in the wire leads to EMI emission from this wire. So, there is a filter to minimize EMI produced by current pulses in the wire...

author: max.wwwang
author_rank: Frequent Contributor
timestamp: February 15, 2020, 06:11:02 am
content: Quote from: RoGeorge on February 12, 2020, 09:58:45 amStopping the battery itself acting as an antenna makes sense. So the key here, as I understand, is: any wiring with a good length (with good potential as antenna).

author: max.wwwang
author_rank: Frequent Contributor
timestamp: February 15, 2020, 06:19:22 am
content: Quote from: EEEnthusiast on February 12, 2020, 10:03:25 amQuote from: radiolistener on February 13, 2020, 10:47:56 pmI understand the fact that in the switching circuits where could be high frequency noise components which we would not like it to propagate to outside of the unit. The problem is, because of the wiring (two wires go through the ferrite ring), any internal niose in one wire would cancel that in the other) then there would be no effect of cancelling the noise that is generated in the unit itself.May be I was wrong, this should be called differential mode? Whatever the correct name is. How it works remains the same.Furthermore, I recently came across some other UPS units and saw designs where these two wires actually wind around the ring (2-3 turns). I guess there is no right number of turns. We should have as many turns as is practicable to get the best noise cancelling effect?