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Common Mode Choke Effect

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Newbie level 5
Oct 15, 2009
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Hello all,

Wondered if someone could help me with this issue; please see attached PDF file containing part of my simple circuit (part of an LED flasher). the question I have is regarding the effect of common mode transformer I use in the circuit.
See circuit (section of an LED flasher), P1 feeds a DC supply to the circuit, S1 or S2 when connected to the ‘-‘ side should change the flash mode.

D2, D3 and R4 and R5 form the input conditioning and protection for the flash pattern change circuit which eventually feed a couple of pins on a PIC.

Now, when T1 Common Mode transformer is in circuit which separates the nets PGND from VSS (as shown in the attached circuit) the change flash pattern inputs (S1 or S2) don’t work, they seem very noisy causing random flash patterns. It is obvious that this is the effect of T1 CM transformer, as it practically separates the grounds (PGND – VSS). If T1 is by passed, then it all works as normal.
I have a rough idea of how common mode transformer work, I use it to block noise from the right hand side of the circuit getting in to power supply (conducted EMC) however, using T1 renders the flash patter input useless.
My question is; is there an easy way around this problem, obviously the grounds or ‘-‘ (PGND and VSS) are not identical once passed through T1.
Some may say connect S1 or S2 to VSS rather than PGND but logistically that is not possible as the flash patters will be remotely selected and the VSS cannot be taken to the remote panel to be used that way. The normal way of operation is if ‘-‘ is connected to S1 or S2 then the flash pattern should change, when disconnected, it will go back to the original pattern.

I have had this issue before and now it has become a challenge and I just wanted to see if there are any solutions to this.

Any help would be appreciated.



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