They are often used for common mode choke applications.
Max.

 

You can make a transformer with a toroidal core. The primary and secondary will add up to 4 leads. Technically that's only 2 conductors, with 2 leads each.

 

When the conductors are connected in parallel there are two reasons.
It can be easier to wind the smaller conductors than a single conductor of the same cross sectional area.
The usual reason is to reduce AC loss due to skin effect. If separate conductors that are electrically insulated from each other are used the effective "skin" of the conductor is greater than it would be for a single conductor. The most sophisticated version of this is litzendraht ("litz") wire which is made of many individually-insulated strands of wire carefully arranged so that all individual conductors spend the same amount of "time" at the surface of the bundle. Litz wire is easy to wind, hard to terminate and expensive.

 

If wound in opposite directions then usually a common mode choke.
https://www.coilcraft.com/cmc/index...MIw5aj7fDT3QIVzrXACh3PzwvuEAAYASAAEgLdZPD_BwE
Max.

 

I was just referring to something like below. I guess we're not sure what the TS is asking about.

 

It is rare to find a common mode choke where the windings are not physically separated on the core unless the choke is intended for low voltage use. There is considerable art to winding CM chokes on toroids to minimize the capacitance between the input and output ends of the windings which degrades performance at high frequency. They sometimes use a "two steps forward one step back" method.

 

I was just referring to something like below. I guess we're not sure what the TS is asking about.

I like the fake leads in that picture.

 

I like the fake leads in that picture.

LOL, yeah, I should've spent a little more time finding a better picture.

 

Thx for great replies.

If wound in opposite directions then usually a common mode choke.
https://www.coilcraft.com/cmc/index...MIw5aj7fDT3QIVzrXACh3PzwvuEAAYASAAEgLdZPD_BwE
Max.

i'm a bit unclear on that. From your link, it looks like the mode has to do with the direction of current, not the direction of winding.

It is rare to find a common mode choke where the windings are not physically separated

In the cheapo ebay toroids i posted in the OP, what the likely arrangement? Is it like the image in this comment, where one coil is on half the donut, and the other coil is on the other half of the donut? Or do the two coils likely go around the donut together, in the same direction? Or in opposite directions? Or:

THX

 

Common mode and differential mode refer to noise.

A common mode filter is used to block high frequency noise that is present more or less equally on both sides of a two (or more) conductor connection. Big ones with two windings are very often used for keeping switch mode power supply noise off of AC mains. Because the low frequency current is equal and the two windings produces opposing magnetic fields, the core is not "saturated' by the the low frequency current. CM chokes are normally wound on very high permeability ferrite cores.

Differential mode chokes with two windings on a common core are rare because the magnetic fields add which can lead to core saturation. I've used them in a couple of designs. It is far more common to just use individual chokes.

It sort of looks like one or two of the inductors in the original photo are wound with two parallel strands. You do realize that what looks like four leads is actually two leads and two shadows.

 

use individual chokes.

How?

what looks like four leads is actually two leads and two shadows.

omg, thx for catching that!