Leakage inductance on toroids

An ordinary helically-wound toroid is basically a perfect transformer in series with a 1-turn solenoid. There are winding tricks to reduce this by an order of magnitude or thereabouts--the simplest one is to split each winding into two sections on opposite sides of the core, wound with the same current direction and opposite helicity. (I've never done this or I'd be more specific, but I've seen references.)

Cheers,

Phil Hobbs

The single turn in free space doesn't have stray inductance, save that of the hook-up and test leads.

A solenoid, infinite or otherwise, has a winding structure that has a magnetic field path twice it's physical length. The magnetic field path of a single turn in the solenoid does not, unless the permeability of an introduced core is sufficiently convincing (has a high enough permeability as compared to free space).

A single layer solenoid also has winding return leads that add to the conductor shape. In a toroid, this effect can be minimized. With only an air core, however, the effect is the same for both shapes.

Reduced leakage configuration will attempt to have all of the flux embrace all of the turns through:

interleaving of turns interleaving of layers minimizing of turns minimizing of un-interleaved turns and layers. minimizing length of flux paths that are not within the loops.

Current transformers work like any other tansformer. The secondary is effectively a short, so that inductance added to the low-turns count primary tracking is close to the leakage term that would exist if the core and secondary were not present.

RL

When the perfect transformer isn't perfect, and the core material is free space, the part doesn't get simpler - the single solenoid turn is still there, in series with the rest, reflecting worst-case effective leakage inductance of similarly shaped parts with cores and secondaries.

RL

- not 'worst-case'; a minimum.

RL

I don't mean that winding tricks can improve the core behaviour, or increase its effective permeability. But the solenoidal field (oriented 90 degrees from the normal toroidal part) can be reduced a lot by winding tricks. This can help pickup a lot.

Cheers

Phil

I think not...

-- Thanks, Fred.

I should say, its inductance. Making the assumption that, in terms of actual volts (which may not be measurable with real probes, in the same way that a single thermocouple junction is unmeasurable with probes), most of the voltage generated is where the wire passes through the toroid, so that the rest of the loop doesn't matter as far as transformer action and you would therefore wish to minimize all that loopiness, becoming undesirable stray inductance instead.

Wait, what does this paragraph have to do with mine? And it doesn't help that you cut the paragraph mid sentence, which doesn't even make sense. I mean, yes magnetic field path is what induces voltage, but length is a non sequitur, transformers work on Phi, not H.

Well, lead inductance is negligible if the turns are much more numerous than the wire's 20nH/inch inductance thing. This does happen to be a case where turns are few, so lead length may have a noticable effect, but that doesn't look like a good generalization.

As for some answers,

Indeed. But hard to interleave one turn. Next,

Ditto,

Got it,

Yup,

Okay, now slow down a moment. This one's harder. (Actually, it seems to me this is all of the above rolled into one, but nevermind that...)

Now, by "loops", you mean the loops of primary and secondary wire, correct? In the secondary case, it's just one loop, which goes around the core where most of the flux is, so we know it will transform at least. What's left is all that pesky flux around the outside, but like the solenoid, the external field is fairly small so this flux is also small (ignoring the noticable fringing from the small primary turn count).

Yeah, but what happens at the secondary doesn't do anything to the intrinsic leakage, which is a geometry thing. If I put a CT around a piece of 40AWG wire, straight down the middle, would I see shitty HF bandwidth, or what?

Tim

It's quite simple. Make half the winding, using up to half the core (say from left to right across the bottom) then pass the wire through to the opposite side (mirror image) from where you started, and make the second half of the winding (again from left to right, but across the top).