Common Mode Filters - Page 1
Forum section: electronics / repair

author: ThermallyFrigid
author_rank: Frequent Contributor
timestamp: September 13, 2022, 11:47:11 am
content: G'day!I just received a batch of various common mode filters for various projects and unfortunately they didn't measure to specs.How far off can a common mode filter be and still be used as a substitute?Thanks

author: Mario87
author_rank: Frequent Contributor
timestamp: September 13, 2022, 12:23:32 pm
content: The real question is what is the inductance of the 2 sides of the filter? They should be very close to one another so as to cancel out when a common mode current is passed through the filter.Also, how far off from the original spec is acceptable depends on what frequencies you are trying to filter out.You might want to watch this...

author: ThermallyFrigid
author_rank: Frequent Contributor
timestamp: September 13, 2022, 02:33:20 pm
content: Is there any such thing as a general rule of thumb such as......A 50mH Common Mode filter should basically measure within 20% of it's designed value ?

author: capt bullshot
author_rank: Super Contributor
timestamp: September 13, 2022, 03:33:14 pm
content: These inductors will measure to different values depending on the operating conditions - e.g. the inductance might vary depending on the normal current, temperature, material tolerances ...Check the data sheet at what conditions the nominal value is to be expected, and what the tolerance is specified to.To me, they just look OK.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: September 13, 2022, 05:56:44 pm
content: Also the inductance of a particular coil will vary with the frequency applied to the various material characteristics Bullshot mentioned.Measuring at KHz will be different the measuring at MHz

author: Kleinstein
author_rank: Super Contributor
timestamp: September 13, 2022, 09:18:04 pm
content: The tolerances can be quite high, and the inductance does change with temperature, mechanical stress and current level. Especually a higher value is rarely a problem. This would only increase the filter action for the relevant relatively low frequencies.I would not be surprized to see more than 20% tolerance, likely some asymmetric specs, similar to some electrolytic onee, like -20% to +50%.

author: ThermallyFrigid
author_rank: Frequent Contributor
timestamp: September 14, 2022, 12:25:55 pm
content: What I gather from the replies is that Common Mode Filters (even if marked as a specific inductance) do not have any clear method to determine the "Bench" or uninstalled inductanceinstead the measured inductance can be a range as much as 50% different from the advertised or indicated inductance.So it is not possible to know exactly what value inductance a Common Mode Filter has before installing it. Instead it is determined after installing it based on various external factors.At least that is what the replies seem to suggest from reading them all.Thank you.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 14, 2022, 06:40:01 pm
content: At what frequency and amplitude?30% or worse is common enough. The tolerance is the sum of the ferrite itself (permeability might be 10k or so, with a tolerance that accounts for most of that range), plus minor dimension variation of the core shape, plus air gap (which needs to be very close to zero -- some µm is typical), plus, Idunno, turns can be counted perfectly so shouldn't account for much.Amplitude accounts for a pretty big range, depending on material, as permeability is lower at very low signal level (which might be fractional to single volts for values this high, at least at modest frequencies). Hence the term "initial permeability". The curve ticks up at higher signal levels, giving the advertised average permeability; eventually saturation occurs and permeability drops off sharply again.Frequency matters, not just in terms of linearity as above, but also because the combination with winding resistance increases the apparent inductance at very low frequencies. That is, if the meter is measuring |I| for some applied V, but |V| / |I| measures ~constant (a resistance), the apparent inductance goes as 1/f. So if the meter measures at some stupid low frequency like 100Hz, the resistance might dominate, and it reads some ridiculous number.If it measures by reactance properly (Im(V/I)), this should be less of a problem. Still better to read it at a relevant frequency, usually more like 10kHz I think.Tim

author: strawberry
author_rank: Super Contributor
timestamp: September 14, 2022, 06:44:38 pm
content: higher inductance - lower saturation current