Dumb question re screened cable and ferrites... - Page 1
Forum section: electronics / projects

author: alanambrose
author_rank: Frequent Contributor
timestamp: June 29, 2023, 01:56:14 pm
content: I should know the answer to this but I don't...For noise suppression purposes on mains input cable - is there any point adding clamp-on ferrites around screened SY-style mains cable?The answer probably has something to do with E and H fields but....

author: Terry Bites
author_rank: Super Contributor
timestamp: June 29, 2023, 03:34:53 pm
content: Unscreened and screened cable will be similarly affected by the ferrite. Flux cutting at play.The conductors inside a ferrite lump experience tighter magnetic coupling. They form a common mode transformer in which each "winding" has one turn.Not a great one, but enough to get you under an emc compliance limit.It may help reduce interference getting in or leaking out, it depends on the nuisance frequency. At lower frequencies more inductance is required.I had a very noisy power TV supply interfering with my BT headphone transmitter. I cured the PSU noise by winding a couple of turns of it's mains lead on a ferrite ring, that amounts to the exactly same thing- it worked wonders.www.coilcraft.com/en-us/edu/series/a-guide-to-understanding-common-mode-chokes/

author: alanambrose
author_rank: Frequent Contributor
timestamp: June 29, 2023, 04:31:57 pm
content: Thanks Terry, much appreciated.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 29, 2023, 05:25:18 pm
content: Generally no: the series (CM) impedance added by the core is orders of magnitude less than the CMC already inside the equipment.Having a CMC, is an assumption; many smaller items skimp on this (charger adapters are a common example), as well as unapproved imports.Most of the time, the purpose of the ferrite is to introduce impedance and resistance, so that common-mode currents on cables are reduced, and resonances are dampened so that the peaks (at certain frequencies, namely 1/4 or 1/2 wave multiples of the cable lengths) are reduced, to the same end. This doesn't do too much to CMRR overall -- again, the impedance is small, typically 20-100 ohms, with 200-300 ohms available with multiple turns and/or very large cores or stacks thereof -- but by bringing down the peaks at the unlucky frequencies, the EMC response is flattened and therefore the worst-case condition is improved. Typically the improvement is a few dB overall (cable CM impedance is in the 100-200 ohm range, roughly), with peaks coming down by maybe 10, even 20dB.The important part is, the function is hardly at all as a transformer (CM choke), but just introducing loss to that path. For damping purposes, it could equally well be shunt R+C distributed along the cable; but that is inconvenient to arrange. The series R||L impedance of the bead can be placed beside low-impedance end(s) of the cable (i.e., connectors / boxes / modules / circuit boards) to the same effect.Quote from: Terry Bites on June 29, 2023, 03:34:53 pmThat's as unbelievable, that the power cord was the culprit (or, more exactly, a sufficient point of application for the bead), as that it was interfering with BT (2.45GHz!) at all!Tim

author: alanambrose
author_rank: Frequent Contributor
timestamp: June 30, 2023, 02:59:53 pm
content: OK thanks for your detailed reply - I had to read it through a couple of times to make sure I fully understood