Power Cable Ferrite

On Saturday, January 9, 2021 at 2:23:23 PM UTC-8, snipped-for-privacy@gmail.com w rote:

reduce EMI. I assume that works on both radiated and conducted emissions. Does the position on the cable make a difference in the radiated emissions? The action is essentially of an in line inductor and so stops the current, but...

It's a LOSSY inductor, damps the oscillation that makes a cable a good ante nna. So, it can be effective anywhere except at a node of the standing-wave that it's intended to suppre ss. Last time I used one to get a device past EMI testing, it didn't matter which size I used, and it even worked on adjacent cables to the (PC sound) cable that was mainly causing the radiati on.

Ferrite beads are a lossy impedance, of modest value.

They aren't filters, at least not good ones. Typical attenuation of EMI is only a few dB overall. Compare typical impedances (30-200 ohms) to system impedance (e.g. 50 ohms), or typical cable impedances in free space (100-300 ohms, ballpark).

Even a large stack of them, makes only a poor CMC. They have little effect on conducted (

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It seems I did not understand the use of ferrite. Our unit is powered from a PSU at 12V similar to units used for laptops. The entire case is metal but I don't know if the intent is to ground the case to anything, the suppl y is isolated from ground. I don't even know if they intend to connect the circuit ground to the case. I will need to bring that up as an issue. I' ll not worry about the ferrite as that can be added at any time easily enou gh.

Thanks to you and Whit

Isn't the connector end a node? (Whenever I've used a ferrite 'slug' I've stuck it near the connector... 'cause that's what I see elsewhere.)

George H.

Yes, because the connector connects to something big (e.g. the metal computer enclosure). :)

Tim

Keep in mind the only kind of grounding that matters here is RF grounding. Galvanic grounding is completely irrelevant in this discussion.

If there is truly no connection to your enclosure, what was even the point of using it? If strength, a plastic or fiberglass enclosure probably would've been cheaper...

When using a metallic enclosure, (shielded) cables can be grounded to it, at the point of entry. EMI washes over the enclosure, shielding huge amounts (>40dB) of outside noise.

What you do inside, is up to you. The PCB can be grounded (RF or otherwise), maybe your PCB(s) is/are rather noisy so there's internal ground loops to beware of too, whatever.

Preferably, connectors can penetrate the enclosure through only one side, and they can all be received by one PCB which is also grounded there, keeping the ground loops between cables/signals away from internal loops. If not, maybe ferrite beads would be useful inside as well.

Speaking of plastic, even metallized plastic does well, ca. 40dB. Commercially available ones tend to be prohibitively expensive for some reason, though. Proper metal enclosures, the sky's the limit in dB, of course.

Tim

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Consider this is being done by a group of volunteers on a very tight budget . The mechanical guys made a plexi early prototype. Now they are using sh eet metal that I believe they were quoted $100 for but actually built for f ree as a donation.

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What cables? The power cord is the only cable.

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Two boards, one power board with a pair of switchers and a motor controller and a controller board with the logic and analog circuitry. I am concerne d about noise from the power board impacting the control board. The guy de signing the power module seems to think the ARM MCU or FPGA will be the noi se makers. I can handle the digital chips. That's no problem. Switcher n oise can be brutal.

The enclosure has large openings on two sides for the plumbing. So I hope you aren't thinking this enclosure will contain any RFI.

Yes, you are thinking of the metal case being an enclosed EMI container. T hink of it more like an orange crate keeping the internal guts in but allow ing water to flow freely. Actually, we have to pass a "drip" test where th ey pour water over it and anything that gets in has to flow right back out. This group just isn't thinking of confining RFI. That will be someone el se's problem. In the end it may be a fatal problem, but I can't get anyone to consider it, the lead just wants to punt to down the road.

An open cable end (not at some big metal box) is an antinode of a voltage waveform, and a node of a current waveform. A grounded cable end is an antinode of a current waveform and a node of a voltage waveform. So, there's some ambiguity in what 'a node' means.

For ferrite beads with loss, an effective filter means you want to avoid current waveform nodes. It isn't critical exactly where the bead is placed, because mostly they're oversized (cuz that's cheap).