I am experiencing weird behavior with a common mode choke and had a few questions. The circuit in question is this: enter image description here

I am using a desktop power supply at 28V and 200mA current. This is the first time I've powered this board. Immediately, the voltage dropped to 0.64, the current went to 200mA and the power supply indicated an overload.

I troubleshot this, down to the TCK-128 choke.

So I put this on a bread board. Just a 3 Ohm current limiting resistor and the choke. I get the same results.

First question is what is going on (What am I doing wrong)

As far as I can tell there is no pin indication on the choke. However, looking at the data sheet pin 1 can only be 1 of 2 pins.

enter image description here

As follow up questions, I just noticed the dot notation and reading about this seems to indicate that positive polarity should go on the dot.

If that is true, my above schematic may be wrong since 1 is battery ground and 2 is battery positive, where as 4 should be positive and 3 should be ground. This this a concern?



2 Answers 2


The choke needs to be rotated 90°. You are shorting out the power supply in the configuration in you're post. Using the proper symbol for the choke (essentially, a transformer) would make this obvious


simulate this circuit – Schematic created using CircuitLab


There is no PCB layout how the pins of your PCB are really connected.

But if the PCB layout matches the pin numbers, the inputs and outputs are connected to wrong pins.

Giving power input to pins 1 and 2 means the choke shorts the power supply and that is not how the part should be connected.

Pins 1 and 4 should be on one side, for example power supply input, and pins 2 and 3 should be on the other side, output to device.

  • \$\begingroup\$ Ah, if 1 and 4 are inputs, that's definitely going to be a problem. Thanks! \$\endgroup\$
    – jgauthier
    Nov 29, 2022 at 20:14
  • 1
    \$\begingroup\$ Inputs could be 2 and 3. Or 1 and 4. As long as you pass the supply and return current through the coil in correct direction. \$\endgroup\$
    – Justme
    Nov 29, 2022 at 20:29

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