Common Mode Choke question

[[email protected]]

Hi everybody,

I have a 1mH, 10A common mode choke. Can I use this as an inductor with
the same specs?
If yes, how? Like (a) connecting one winding and leaving the other open
or (b) paralleling both.

Thanks,
Mohit.

 

[Phil Allison]

I have a 1mH, 10A common mode choke. Can I use this as an inductor with
the same specs?

** No. It is a essentially low current inductor of 1 mH.

A genuine 1mH, 10 amp inductor will have a laminated iron core and be about
the size of your fist.




....... Phil

 

[[email protected]]

Thanks for the reply. But I don't understand. How is it a low current
inductor? The choke uses a toroid and is about 25mm(dia)x20mm.

Please be kind,
Mohit.

 

[Phil Allison]

<[email protected]

Thanks for the reply. But I don't understand. How is it a low current
inductor? The choke uses a toroid and is about 25mm(dia)x20mm.

** Go look up "core saturation" on Google.

A " common mode " choke avoids the problem by having equal and opposite
currents in two coils.

A genuine, high current choke must be of reasonable core size and have an
air gap in the magnetic circuit.




......... Phil

 

[John Popelish]

Hi everybody,

I have a 1mH, 10A common mode choke. Can I use this as an inductor with
the same specs?
If yes, how? Like (a) connecting one winding and leaving the other open
or (b) paralleling both.

Sorry, no. A common mode choke is wound such that the load
current (the one rated at 10 amperes) trough one coil
magnetizes the core in one direction, while that same load
current, returning back out the other winding, magnetizes
the core in the other direction. So the fields of these two
currents cancel, preventing the core from saturating, and
producing very little inductance.

The device shows 1 mH only for current that goes through one
or both windings in the same direction, so that the fields
do not cancel. In line filter applications (where these
devices are usually used) this common mode (same direction
in both sides)current is very much smaller than the load
current (that goes in one winding and out the other).

For instance, the common mode current may be a result of
voltage that is in common on both line wires, like signals
picked up from local AM stations by the line wiring acting
as an antenna, or generated by diode switching or switching
regulator operation on the load side.

The core of a common mode inductor is generally a solid,
high permeability ferrite (with no air gap) to maximize the
winding coupling and inductance, since the common mode
signals are generally very much smaller than the load
current. They are really a form of transformer with two
equal windings.

Normal mode inductors (two lead devices) generally are made
with distributed gap powdered iron material that takes a lot
more turns to produce the same inductance, but are
saturation resistant, because of the nonmagnetic gaps in the
flux path that store energy when a magnetic field is forced
through them.

 

[Greg Neill]

Sorry, no. A common mode choke is wound such that the load
current (the one rated at 10 amperes) trough one coil
magnetizes the core in one direction, while that same load
current, returning back out the other winding, magnetizes
the core in the other direction. So the fields of these two
currents cancel, preventing the core from saturating, and
producing very little inductance.

If you can get at all the leads there should be
no reason why a suitable cross connection couldn't
effectively parallel the windings.

* *
1----+ +----2
) (
( )
) (
( )
) (
3----+ +----4

In common-mode configuration the input connections
are to terminals 1 and 2, with the load connections
being 3 and 4. Current entering at 1 will return
via terminal 2, so the fields oppose and cancel.

Connect 1 & 2 together and 3 & 4 together to form
a two terminal device and you should end up with
the fields reinforcing rather than canceling.

 

[John Popelish]

If you can get at all the leads there should be
no reason why a suitable cross connection couldn't
effectively parallel the windings.

* *
1----+ +----2
) (
( )
) (
( )
) (
3----+ +----4

In common-mode configuration the input connections
are to terminals 1 and 2, with the load connections
being 3 and 4. Current entering at 1 will return
via terminal 2, so the fields oppose and cancel.

Connect 1 & 2 together and 3 & 4 together to form
a two terminal device and you should end up with
the fields reinforcing rather than canceling.

Yes, the zero current inductance will be about 1 mH, and the
windings will handle about 20 amperes. Unfortunately, the
core will saturate with something like a fraction of an ampere.

You could also connect 2 to 3, and between 1 and 4 you would
have about 4 mH zero current inductance, and the windings
would carry about 10 amperes. but the saturation current
would be about half of the first case.