Does anybody know what this could be? It is wound 1:1, and each winding has 17.9ohms and ~20mH. I've removed the ferrite (It was brittle and reacted to a magnet) and it was a typical E I.

In a different thread (are common and differential mode chokes interchangeable) it was suggested that coupled inductors (pulse transformer) are used for this task.

Bottom view with pinout

It's used in a powered bus, as seen in the diagram below. I drew it as a differential choke because that's how I understand it should work. It blocks a superimposed ~20kHz signal from entering the power section. Application Schematic


2 Answers 2


Based on where you put the dots indicating the winding direction, and the connected circuitry, It's a differential choke, to block the pulses from being absorbed by the capacitor

If that capacitor before the 7805 is small, and those dots were not determined by observation, it could be a common-mode choke. And they're just relying on headroom and power-supply rejection in the 7805 to prevent the 20Khz from being eaten up. its purpose then would be to prevent common-mode interference from flowing on the power lines, but I can't see if in or out.

However looking at the bobbin pictured it appears that the core window area is smaller than the core cross-section, this design suggests that it was used differential mode, common-mode chokes usually have the window larger than the core, as in common-mode the only source of amp-turns is unbalanced current. but high differential mode currents may flow without magnetizing the core, so lots of copper is needed to carry these currents .

In differential mode the DC current magnetizes the core, so more core bulk is needed with respect to coil bulk to avoid saturation.

  • \$\begingroup\$ That's what we thought, do you know if these are available off the shelf? I couldn't find any in element14, mouser or digikey... \$\endgroup\$ Jan 25, 2018 at 9:45
  • \$\begingroup\$ Just to confirm, your answer isn't based just on the "schematic" right? Because I chose the differential choke symbol since it seemed to be what it is, but it isn't documented anywhere (this schematic is just me tracing a circuit) \$\endgroup\$ Jan 25, 2018 at 9:48
  • \$\begingroup\$ Off the shelf, Digikey groups them under "inductors, arrays and signal transformers" Mouser groups them as "pulse transformers." \$\endgroup\$ Jan 29, 2018 at 8:31
  • \$\begingroup\$ somewhat based on the schematic, see edits. \$\endgroup\$ Jan 29, 2018 at 8:41
  • \$\begingroup\$ I've just unwound the "transformer" and it's two wires wound together in the same direction. On the pcb they're connected to the two outermost pins for one section and on the innermost for the other, I believe that the "dots" would be differential like drawn. I've also measured that they're using a gapped core, with a 0.1mm gap and an Al ~= 350. I've just ordered some coilformers and gapped cores and I'll try to wind a "differential choke" in a similar manner. I bought a few pulse transformers and unfortunately they didn't work as expected :/ \$\endgroup\$ Jan 29, 2018 at 9:14

This is indeed a common mode choke.

It basically consists of 2 identical coils sharing the same core so that their magnetic fields are coupled. The ferrite core enhances the magnetic coupling and increases inductance which makes the coil's impedance higher for high frequencies. That means it becomes more effective as a filter.

Here it is used to keep the "decoder" signals away from the 7805 voltage regulator. The common mode choke forms a very high impedance for the decoder signals so they cannot reach the voltage regulator. Those signals would be shorted anyway by the capacitor at the input of the voltage regulator.

It would be possible to use two inductors to do the same task but to reach the same amount of suppression (of the decoder signal) those inductors would have to be larger and more expensive than this single common mode choke. A common mode choke is simply more effective. And of course it lets the DC for the voltage regulator pass through unharmed.

Common mode chokes are most commonly found in mains power supplies, there they prevent the switching noise from the electronics from reaching the mains and so polluting the mains.


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