How Can I Generate Enough Common Mode Current for Filter Testing?

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Discussion Overview

The discussion revolves around generating sufficient common mode current for testing common mode current filters, specifically focusing on the feasibility of experiments to measure filter efficiency. Participants explore various methods and equipment available for conducting these experiments, including constraints on current levels and the need for reproducibility with accessible materials.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant seeks methods to generate common mode current with a maximum of 1 A, emphasizing the need for easy reproducibility with school materials.
  • Another participant inquires whether the common mode current is DC or AC and asks about the frequency range and circuit configuration.
  • A participant mentions that the generator will provide an AC voltage of 20V peak to peak and describes the filter as an RLC circuit with a common mode choke.
  • Questions arise regarding the measuring equipment available in the school lab, including the presence of oscilloscopes and spectrum analyzers.
  • Some participants suggest searching online for measurement techniques for common mode chokes to gather ideas.
  • There is a discussion about the target application for the choke system and the importance of understanding the frequency range for effective testing.
  • One participant proposes a method to generate common mode current by applying a generator between conductors and measuring voltage across a resistor, while questioning the adequacy of this method for generating sufficient current.
  • Another participant mentions the need to consider the saturation current of the common mode choke and the potential use of ferrite toroids for constructing the chokes.

Areas of Agreement / Disagreement

Participants express various viewpoints on how to generate common mode current and the necessary equipment for testing. There is no consensus on a single method or approach, and multiple competing ideas remain throughout the discussion.

Contextual Notes

Participants have not settled on specific assumptions regarding the circuit configuration, frequency range, or the exact nature of the common mode current to be generated. The discussion reflects a range of ideas and uncertainties about the experimental setup and requirements.

Who May Find This Useful

This discussion may be useful for students and educators in electronics or electrical engineering, particularly those interested in filter design and testing methodologies for common mode currents.

QuentinChe
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Thread moved from the technical forums to the schoolwork forums
Hello!

I'm currently working on a project which aims to study common mode current filters. What I'd like to know is how can I generate enough common mode current in order to be able to make experiment on my filter ( its efficiency to reduce it for example)
I have some constraints:
-the current should not be to high (1 A maximum
-it needs to be easy to reproduce with materials you can find at school

Thank you in advance for your answer
 
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Is the common mode current DC or AC? What range of frequencies?

The source of common mode current will need some where to go, to close the circuit. Is it working against a ground? What is the maximum voltage needed to push the currents into the filter.

It might help if you let us know a bit more about your filter.
Is it designed for signals or for AC power supplies?
Does it have coaxial cables or two parallel wires?
 
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The the generator will deliver an AC tension, 20V peak to peak maximum
Currently I don't know how my circuit will be but from what I've read, common mode is conducted through the ground so I assume it will be the case here
The filter is some kind of RLC circuit with a common mode choke in it, without a coaxial cable

(A filter like this one https://www.coilcraft.com/en-us/other/common-mode-choke-design/. There is more info on the pdf named common mode filter design guide at the bottom of the page)
 
BTW, you can get some good ideas about how to measure CM chokes by doing a Google search on measurement of common mode chokes and checking out some of the hits... :smile:
 
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berkeman said:
BTW, you can get some good ideas about how to measure CM chokes by doing a Google search on measurement of common mode chokes and checking out some of the hits...
Already did that. The goal of the filter is to reduce common mode current, but I have currently no idea how to reproduce enough of it (and not too much too) to be able to quantify the filter's efficiency
 
berkeman said:
What kind of measuring equipment do you have available in the school lab?
 
A lot of things! I'd say pretty much everything you can find in a regular high school plus other equipment we don't use but we can access if we ask for. My school has some kind of electronic department (it's hard to describe because I don't know the equivalent of the formation my school delivers in English
berkeman said:
Do you have an oscilloscope with a differential probe, for example? Do you have a spectrum analyzer with a "tracking generator" output perhaps?
They have that for example

EDIT: after some research, it looks like a HND in electronic
 
Last edited:
QuentinChe said:
They have that for example
Sorry, they have which? A differential probe and an oscilloscope, or a spectrum analyzer with a tracking generator?
 
  • #10
They have both of them sorry
 
  • #11
Holy smokes! That's a well-equipped high school. :smile:

Okay, then do you know how to use a spectrum analyzer and tracking generator to generate a Bode Plot (transfer amplitude versus frequency)?

And as was mentioned by @Baluncore we need to know more about the target application and frequency range for this choke system (both differential mode passband and common mode stop band frequency ranges).
 
  • #12
berkeman said:
Okay, then do you know how to use a spectrum analyzer and tracking generator to generate a Bode Plot (transfer amplitude versus frequency)?
No but I can learn, that's not a problem
berkeman said:
And as was mentioned by @Baluncore we need to know more about the target application and frequency range for this choke system
What do you mean by the target application? And for the frequency range, I don't know yet (I didn't think about it because my problem right now is to know if I'll be able to generate enough common mode current without a phenomenal amount or tension to conduct experiment on the filter, if my experiment is feasible. Then I'll start thinking about it)
 
  • #13
QuentinChe said:
What do you mean by the target application? And for the frequency range, I don't know yet (I didn't think about it because my problem right now is to know if I'll be able to generate enough common mode current without a phenomenal amount or tension to conduct experiment on the filter, if my experiment is feasible. Then I'll start thinking about it)
Well, as was mentioned, CM chokes for switching power supplies are commonly used, since SMPS can drive a lot of CM noise back into the AC Mains powerlines and interfere with other devices that are connected to power. So for those chokes, they obviously need to pass the high-current AC Mains power at 50-60Hz, and you will generally have a CM attenuation band from about 10kHz to about 10MHz.

I've designed CM chokes for twisted pair communication, for both noise immunity and for radiated EMI suppression. They needed to pass the DM communication waveform (anywhere from 10kHz to 10MHz, depending on the communication band), and attenuate CM noise/EMI from just above the communication band up through a few 100MHz.

But it also sounds like part of your investigation is to examine what happens to CM and DM attenuation as you get close to the saturation current of the CM choke? You will want to use pretty small ferrite toroids to wind your CM chokes on then, or else you will need to come up with some moderate power amplifiers to drive in the waveforms.

What ferrite design kits will you be working with? What gauges of magnet wire do you have available? Can you post a datasheet of a typical ferrite toroid that you will be using?
 
  • #14
Easy to generate CM current - you just apply the generator between both conductors in parallel and a ground plane. At the far end you use a scope to measure the voltage across a 47 Ohm resistor. Keep the frequency low enough so the line is a small fraction of a wavelength. Compare voltages at the sending and receiving ends, then CM atteniation = 20 log V1/V2.
 
  • #15
tech99 said:
Easy to generate CM current - you just apply the generator between both conductors in parallel and a ground plane. At the far end you use a scope to measure the voltage across a 47 Ohm resistor. Keep the frequency low enough so the line is a small fraction of a wavelength. Compare voltages at the sending and receiving ends, then CM atteniation = 20 log V1/V2
I've seen on the internet modellisation of the perturbation with Norton equivalent sources, but it seemed complicated... Is your method enough to generate enough common mode current?
For the ferrit toroid, I'm asking arround what we have in stock
 

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