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Question about common mode choke

Posted: Tue Jul 03, 2018 8:53 am
by RhythMick
https://uk.rs-online.com/web/p/leaded-i ... s/4890425/

Am I right in thinking that a common mode choke such as this can be wired in two ways?

The normal way (according to the datasheet) is "both dots on the left". Used for noise rejection, and to prevent noise from the load being passed back into the PSU. Happy to be corrected. I believe in such mode the choke provides almost no inductance and therefore almost useless for power smoothing in a filament supply.

My question is, can such a choke be wired "top dot left, bottom dot right" so that it DOES provide inductance and in conjunction with a cap would provide some smoothing and ripple reduction?

Re: Question about common mode choke

Posted: Tue Jul 03, 2018 10:03 am
by Nick
In the normal way the current in the two coils cancel creating zero flux at steady state. Imbalances in the two lines are seen by the inductance and so with the addition of a C will filter noise. Wiring them out of phase will prevent the currents canceling, will create a DC flux at steady state and cause what inductance there is to collapse because the ferrite core will saturate. Chokes for ripple reduction need to be able to handle standing DC flux, so will have an air gap.

Re: Question about common mode choke

Posted: Tue Jul 03, 2018 12:00 pm
by RhythMick
Ok thanks. The linked item specifies 20mH at max 3A. For filtering calculations should I use the 20mH?

Re: Question about common mode choke

Posted: Tue Jul 03, 2018 12:10 pm
by RhythMick
Re-read what you posted. So in common mode, the inductance will (in conjunction with C) cancel noise but won't provide ripple reduction.

Ok thanks.

Re: Question about common mode choke

Posted: Tue Jul 03, 2018 2:00 pm
by Nick
Yes.