Common Mode Transformer Effect

Discussion in 'General Electronics Chat' started by AnalogKid, Apr 22, 2015.

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  1. AnalogKid

    Thread Starter AAC Fanatic!

    Aug 1, 2013
    Since your question does not involve AC mains - as clearly stated in your post and shown on the schematic - here is an answer.

    First, there is no input or output to your filter section. It is symmetrical, and presents the same impedance to signals going through it in either direction.

    Second, don't think of T1 as a transformer. In this application, it is not. It is a common mode choke, and exhibits a large common mode inductance that is intended to act as the series element in a balanced lowpass filter. As such, it presents an impedance to both of the input power lines, and this can cause problems if something (like a control signal) on each side of the filter needs to be at the same potential. Everything is fine at DC, but noise in the power return can appear as a additional voltage on signal lines.

    Since you are not using a switching regulator or transformer, two common sources of common mode noise, are you sure you need T1 in the first place? I would replace T1 wit a single inductor in the + power input line only, and tie your GND (U3 pin 4, etc.) directly to P1 pins 3 and 4 (- power), and see if that gets you the right combination of filtering and reference potential.

  2. DickCappels


    Aug 21, 2008
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