Filtering noise on power inputs

[Simon]

Well what I am trying to deal with is what will be conducted up the loom to the vehicle fuse board and battery transmitting to all in sundry in the process. As it is if the can cable is not twisted it will induce enough noise to cause problems.

Is there any value in splitting the inductors in two and putting half on the negative?

Sent from my Moto G (4) using Tapatalk

[T3sl4co1l]

Yes, filtering the negatives addresses common mode.

Technically, for N leads, N-1 of them need filtering, and the remaining one then happens to be "ground" both inside and out.  That works particularly well when the "ground" is a Faraday cage around the device.  If it's not shielded, then filtering all N still helps.

Tim

[Simon]

So I also have a higher power device that would be nice to filter. The maker have given me a nice big toroid that I can get 200uH of inductance on. Now he is talking just normal pi filter but if I do two windings one for neg and one for pos on the same core would this help ? obviously they need to be coupled in a non common mode choke way as I am trying to kill off the current spikes not help them through. I don't have room for two of these so it's two coils on one core or just a pi filter.

[T3sl4co1l]

Is the toroid ferrite or powdered iron?

Tim

[Simon]

Ferrite I think, it's certainly very solid, how can I tell the difference, it'a painted green and teal coloured on side, I don't know if that is some sort of colour coding for the material.

[T3sl4co1l]

Check the listings:
http://www.micrometals.com/materials_index.html
(Also compare to pictures of the core types, so you can check if your color perception is right.  The green and blue are usually somewhat pastel, which may be throwing off your perception.)

Sounds like #52 powdered iron.

I don't know of any bi-color ferrites, but it could be whatever, and painted whatever...

Tim

[Simon]

It's a toroid (going to make some nice beefy 40A 47uH inductors with them) Yes pastel colour green and a spray of pastelly dark green or light teal on one face. They were supplied by the guy that supplied the load I am making the filter for so I'd trust his judgement as he preferred I wind my own with the materials he sent than use off the shelf stuff.

[Simon]

I seem to remember them being sent to me with 52 written on one of them so I assume that was the material type.

[T3sl4co1l]

Should be okay then.

If you have enough, you can use one for each connection, independently, and not worry about coupling or anything.

The permeability is rather low to use as common mode chokes, so that's out.

Differential mode chokes are good, but they don't filter common mode very well (gee, y'think?..), which is often the bigger offender.

Here's a T300-26D with some wire on it:



#52 is only slightly better than #26.  Both are about as good for inductors as lead is for making bells...
That means, this particular circuit has to run at a fairly low frequency, and use a lot of extra inductance, to avoid burning up the core.  But for noise filtering, you won't have to worry about heating, and the loss can be an advantage, honestly (free dampening!).

Tim

[Simon]

I think the fundamental operation of this device is 20KHz or there abouts. It already has a lot of input capacitance and I'll be adding more so it should not be too harsh, this device may actually be quieter than the SMPS in the ECU.