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

[QuentinChe]

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

 

[Baluncore]

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?

 

[QuentinChe]

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)

 

[berkeman]

The the generator will deliver an AC tension, 20V peak to peak maximum

What kind of measuring equipment do you have available in the school lab? Do you have an oscilloscope with a differential probe, for example? Do you have a spectrum analyzer with a "tracking generator" output perhaps? Those are handy for plotting transfer functions...

https://blogs.keysight.com/blogs/tech/rfmw.entry.html/2019/04/04/expand_your_spectrum-2myT.html

 

[berkeman]

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...

 

[QuentinChe]

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]

What kind of measuring equipment do you have available in the school lab?

 

[QuentinChe]

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

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

 

[berkeman]

They have that for example

Sorry, they have which? A differential probe and an oscilloscope, or a spectrum analyzer with a tracking generator?

 

[QuentinChe]

They have both of them sorry

 

[berkeman]

Holy smokes! That's a well-equipped high school.

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).

 

[QuentinChe]

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

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)

 

[berkeman]

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?

 

[tech99]

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.

 

[QuentinChe]

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