Staggered 50uH and 250uH inductor design for LISN

[T3sl4co1l]

Designing around say 10 or 15A RMS seems reasonable, and having a saturation current more than twice that means you don't have to worry about a crest factor of the same proportion (i.e. 2:1)*.  More would be nice; that can be done by using more core and gap (= higher saturation current), or less wire (or using the same wire but just rating it more conservatively, in either case giving a lower maximum RMS current rating).

It doesn't seem very useful to just go and waste (RMS) current capacity.  So I think I would use 10-15A wire, and a 30A+ saturation rating (at inductance -30% or better), and just make sure to check that peak current isn't going into the concern range.  Maybe get a mains frequency current transformer, or Hall effect probe, and verify that load current is within these ratings, when in use.

*I forget if crest factor is defined in comparison to square wave (RMS = average = peak), or sine (peak = sqrt(2) RMS).  If the latter, then that'd be a ~1.4 crest factor, whatever.

The 250uH choke, I mean, it's literally five times bigger than the 50, so if you want the same rating, wire and fill factor, you're going to have to use a stack of five cores.  You can vary the gap and wire, but it's probably better to keep the gap nominal, whatever nominal is driven by.

(Which, because your winding factor is a bit low, what with all that insulation there -- a somewhat higher mu_e may be desirable, maybe 30 is ok, maybe 45-60 is better, give it a try and see what seems to fit best.)

Regarding winding area, keep in mind that you'll be lucky to fill more than half the area with wire -- that includes area wasted to the gaps between round wires, tape, bobbin, etc.  A bit better if you're not using a bobbin.  And that's just enameled wire -- a winding factor more like 0.3 might be reasonable with jacketed wire.  In any case, do some figures with turns/layer and number of layers, see if you can pack it in there with a little free space so it's not totally crammed in there.  That's your maximum turns count, and if you're heart set on using 3 and 1 core stacks for the two chokes, expect to fully load the triple stack, and only 60% load the other.

Which means, sure, you could fit more turns on the 50uH's core, it's got 40% wasted space; but so what?  If you increase turns, you increase inductance and decrease Isat; if you increase gap to compensate, L is still higher (for same Isat).  Why bother, y'know?

Tim

[coppercone2]

i thought about this, but the fact that the LISN needs to be on a specific metal work bench with a ground strap thats not really suited for much else made me think its ok just to go with big air cores?

I am not sure if its worth the effort to design since its only saving some shelf space because the actual test setup is wasteful of space, you are not really supposed to have other stuff on the bench anyway I think



They draw it pretty specifically



It looks like it might be useful to reduce size if you make a few if you want to daisy chain devices and do multiple device test with multiple lisns but that test looks really expensive. I don't think a LISN embedded in a test equipment wall is very useful (main appeal of size for me). I don't think its even safe to put it on a rack, so mine ended up being a oddball geometry.

[sixtimesseven]

i thought about this, but the fact that the LISN needs to be on a specific metal work bench with a ground strap thats not really suited for much else made me think its ok just to go with big air cores?

Well the 50uH air cores are manageable, 60x70mm or so I think was calculated.  But the 250uH aircores would be big. Somwhere in the range of 300x70mm or so per coil.
And you probably want them either place orthogonal to each other or far apart to make sure they do not couple. See the design Dave opened.

The magnet cores are pretty cheap overall. You also save some copper and a lot of space. Also the field is contained in the fer-rite end you do not have to worry about the effects of the housing. Ok, air core is probably lighter.

If you want to test 1:1 like in a lab with the bench, size does not matter that much (You still have to store the thing unless you have a dedicated emc bench).
But I want mine mobile. I know of several cases where I had to troubleshoot devices and industrial machines which we suspected had massive em issues. There I want a portable SA and a portable 1-Phase and later 3-Phase 32A or even better 63A capable LISN whith a handle on them. It won't be ideal but to get the machines out you would need a crane and sometimes a hole in the building. Building single Phase is the test run for me.

[sixtimesseven]

Finally got everything.

I used the: B66329G0000X127 cores with 13 turns of 1.5mm wire, a resin printed bobbin and cardboard as spacer (0.5mm, toilet roll cardboard fits perfectly).
The windings are spaced apart as far as possible. But the second iteration will have spacers on the bobbin itself. So work in progress.
L: ~55uH @ 100khz, SRF: ~33MHz

As a side note, 405nm elgoo mars resin seems perfect for the job I found. It can produce very precise (and potentially small) bobbins. It is plenty strong even with 1mm wall thickness. In particular if used with a "plug" while winding for reinforcement. First I worried about temperature but it seems to withstand 450degC from the soldering iron without burn and is pretty hard to set on fire( 45-60s in a propane flame will result in sustained, slow burn with thick black smoke though).