Electronics > Beginners

How does a noise filter work?

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max.wwwang:
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 pm ---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.

--- End quote ---

It's simply a charging/discharging process, both would be pretty stable (I guess). How could there be EMI emissions in these 2 wires?

RoGeorge:
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.

EEEnthusiast:
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.

radiolistener:

--- Quote from: max.wwwang on February 12, 2020, 07:45:40 am ---It's simply a charging/discharging process, both would be pretty stable (I guess). How could there be EMI emissions in these 2 wires?

--- End quote ---

There 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...

max.wwwang:

--- Quote from: RoGeorge on February 12, 2020, 09:58:45 am ---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.

--- End quote ---

Stopping 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).

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