Electronics > Projects, Designs, and Technical Stuff

lowish frequency EMC filtering - lowest ESR or highest Capacitance

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Simon:

--- Quote from: Mr Evil on March 19, 2020, 05:34:24 pm ---
--- Quote from: Kleinstein on March 19, 2020, 03:05:29 pm ---...For the capacitor one may get better of combining 2 caps. An electrolytic cap for the lower frequencies and a relatively small ceramic one (with low ESR) for the higher ones...
--- End quote ---
Be careful doing this. The capacitance of the smaller one will resonate with the self-inductance of the larger one, and capacitors with low ESR are worse as they provide less damping. If you're unlucky, that will happen at exactly the frequency you're trying to filter out, giving worse results than no filtering at all.

--- End quote ---

It's on the end of a piece of wire so there is inductance/resistance from the load to the filter. Basically, suck it and see. I keep warning them that until we do one and test it we have nothing to go on.

trobbins:
Is the filter aimed at an attenuation spec, or some other kind of benchmark performance, or just suck it and see?

I don't see any identification of a supply - is it dc or ac, and what is it?  We could presume you are describing electrolytic caps, and used in a polarised circuit, but there are other plausible presumptions.

T3sl4co1l:

--- Quote from: Simon on March 19, 2020, 08:08:11 pm ---
--- Quote from: T3sl4co1l on March 19, 2020, 06:31:31 pm ---What does it connect between?

Tim

--- End quote ---

A fan motor and it's supply.

--- End quote ---

Yeh, what kinds?  Is it filtering supply noise or brush noise or what?

Tim

Simon:
It's a 24V vehicle supply running a little 120W fan. We aim to stop the interference from the "noisy" fan motor going up to the supply.

T3sl4co1l:
Does that need to be at the fan, then, or where the cable comes into, I guess you have a box you're making that it goes to--?  How long is the fan lead?  What supply emissions level is required?



--- Quote from: Simon on March 19, 2020, 08:22:25 pm ---It's on the end of a piece of wire so there is inductance/resistance from the load to the filter. Basically, suck it and see. I keep warning them that until we do one and test it we have nothing to go on.

--- End quote ---

Well heck, don't wait up, measure it yourself if you can!  Get a ~5uH choke, 1uF coupling capacitor into 50 ohm scope or analyzer, and see what it does!

Once you know the impedance, source intensity, and load intensity, you can design a filter.

Tim

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