Common mode choke as a regular inductor (small signals), #inductance? 1

[njiruk]

Hi,

For a specific audio application (see my previous thread

http://www.eng-tips.com/viewthread.cfm?qid=341157),

I'm considering to use a common mode choke as a regular inductor (this one:

http://uk.farnell.com/multicomp/mct25x15x15c-502nu/choke-common-mode-5mh-6a/dp/1864501)

as a single inductor, placing both windings in series.

I have two questions:
-Besides of the very low saturation aspect (which is not a problem for my as I can use very small signals), I don't see a reason for any other artifacts, or are there?
-Can the total inductance be roughly estimated in advance? Considering twice the windings this would give 2^2=4 times the single coil inductance. But both coils are not so close to each other, so this might lower the factor significantly I guess...?

Thanks!{urijn

 

[ijl]

Total inductance is roughly the sum of the individual inductances, because the coils are not magnetically coupled. (Toroids seldom are.)

 

[benta]

Total inductance will be around 4 times that of a single coil, as the two coils are on the same toroid and are magnetically well coupled.

Benta.

 

[LiteYear]

Total inductance is either twice that of a single coil, if you connect the windings such that they wind in the same direction, or close to nothing (just the leakage inductance) if you wire them in the opposite direction.

At least in theory. In practice, weird things can happen. Hope that makes it all crystal clear for you!

 

[njiruk]

OK, thanks,
I guess benta's answer is the most meaningful...
I'll post again if I find out the contrary!{

 

[Skogsgurra]

Coupled they are. Square it is!
That's a fact - not an opinion.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[LiteYear]

Mea cupla! Quite right, because the toroid loops the magnetic path, inductance is proportional to N^2, not N as in a solenoid. The point about wiring in the correct direction still applies, but I agree, the factor is squared.

 

[ijl]

OK. Agree. Sorry. I did not notice that the windings were on the same core. I thought about two separate components.

 

[Skogsgurra]

Can't be common-mode if they aren't coupled. In a common-mode choke, the MMFs are opposite to cancel normal mode current/flux.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[OperaHouse]

These also make a great transformer for high voltage isolation. I'v used some of the very small ones to avoid making custom transformers. Got maximum power transfer at about 40khz.

 

[Skogsgurra]

That's an interesting application. Now in my Book-of-Tips.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[zappedagain]

I'm surprised no one mentioned loss. Chokes are typically designed to be lossy (low-Q) so they don't resonate like a nice (high-Q) inductor can.

Z

 

[njiruk]

Lossy doesn't necessarily mean low Q as for a RLC circuit for example: Q=sqrt(L/C)*1/R
For an equal resonant frequency (f0=1/(2Pi LC), Q increases when L/C increases, because the losses increase more slowly (also see my previous post...)
!{

 

[benta]

One more thing to keep in mind: these kind of chokes are not optimised for linearity (inductance changes with current), so keep your signal levels low.

Cheers,

Benta.