I have a PSU that plugs into 220-240V AC mains (L&N, no ground) and outputs 9V DC to a single barrel plug.

On the barrel plug side, just about 10cm before it, there's a little box on the cable that I opened and it reveals a small board with 2 toroidal transformers in series with a 1:1 ratio each. The +9V connects to the primary of the first transformer, directly into the second transformer and then to the +9V lead of the barrel plug. The ground lead is connected analogous to that on the secondaries.

My knowledge is pretty basic I'm afraid. It seems like these transformers are connected in a way that I would think they are two inductors. Okay, that should be an impedance to possible AC but the PSU is rectified and has a cap on the other end already. And why two in series? This is a bit puzzling to me.

So what exactly happens here? Can anyone please clarify?

Thank you!

Edit: Photos as requested; I labelled the joints and colored those that weren’t gooped according to the winding colors.


PCB back

I checked with a multimeter for continuity and it is as it seems, A-E-F-B and C-G-H-D are connected, nothing between those groups. Unloaded, Any of AEFB measured against any CGHD is 9.42VDC

Edit: I took some time to carefully crack the housing of the wall wart part to take pictures of that side as well. It is a switching PSU, yes. enter image description here

  • 3
    \$\begingroup\$ Probably you're asking about a common mode filter but to be sure, make a clear photo and mark the components you're asking about. \$\endgroup\$ Apr 7, 2021 at 11:30
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    \$\begingroup\$ Could also be a "coupled inductor" switch-mode supply. We won't know for sure until the edit button is clicked and a photo added. \$\endgroup\$
    – rdtsc
    Apr 7, 2021 at 12:23
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    \$\begingroup\$ Flauscheloni - Welcome :-) I have removed your additional questions, from the "P.S." in your question above. Here on Stack Exchange, it is important that questions stick to one point, to allow readers to find a specific answer in future, and so that answers can clearly address one point. If desired, please ask them as separate questions (preferably start with just one). You will need to include full details of your project, why you want to "clean up" the o/p & want isolation etc. You can copy your original text from here. Thanks. \$\endgroup\$
    – SamGibson
    Apr 7, 2021 at 13:24
  • 1
    \$\begingroup\$ Well, there is more than meets the eye, certainly. \$\endgroup\$ Apr 7, 2021 at 20:21

3 Answers 3


Transformers don't work on DC, so they're most likely common mode chokes.

enter image description here

A common mode choke is a transformer, but turned 90° so the current flows through both coils, in opposite directions, so the magnetic flux cancels, which does not saturate the core so it can be small and cheap, and presents a low impedance to differential current.

However, to common mode current, which flows in both wires in the same direction, the transformer coils are in the correct direction to create a high impedance.

  • 1
    \$\begingroup\$ @user263983 It depends on the winding direction around the core. If you look closely at 3 and 4 you see that both coils have an inverse winding sense, so parallel schematic current flow results in antiparallel flux \$\endgroup\$
    – tobalt
    Apr 7, 2021 at 13:35
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    \$\begingroup\$ On second thought, I should have chosen another illustration lol. But, well, at least it made you think about the details ;) \$\endgroup\$
    – bobflux
    Apr 7, 2021 at 18:53
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    \$\begingroup\$ @user263983 The first green arrow is the direction of the current, the second green arrow is the direction of the magnetic flux \$\endgroup\$
    – Christian
    Apr 7, 2021 at 19:37

I am going to expand on some of the answers above, and I am new here so go easy on me :)

Power comes in as AC. You mentioned it gets rectified and goes through a filter capacitor. This gives you a smoothed out rectified ac voltage (Calling it DC depends on how smooth it is and what your requirements are). Below shows how the voltage waveform changes through that process for low frequency AC. AC waveform bing rectified and filtered.

So far so good. But what happens when there is high frequency AC? If you have a transformer based power supply, then there could be external noise from outside sources (Like a motor on the same circuit), or if you have a switched mode power supply it could generate its own high frequency noise. All wires can act like antennas (they pick up ambient radio waves as AC) and noise can be introduced though other means like motors or other noisy devices.

Some electronics are particularly sensitive to high frequency noise, like this person's LVDS data sending/receiving circuit. Capacitors are good at filtering low frequency, and the size and type of the capacitor affects the frequencies it can attenuate or reduce. High frequencies can pass right through capacitors, and this property is often taken advantage of in radio frequency amplifying circuits. To reduce high frequency, inductors are often used.

There are two common types of high frequency noise that the two inductive toroidal filters in your image address, common mode noise and differential mode noise.

enter image description here

Differential Mode Noise is when Vcc alternates, or Vcc and ground alternate opposite of each other. Think of it as noise relative to ground. Common Mode Noise on the other hand is when ground and Vcc alternate together. The whole circuit is being shifted up and down. The two modes of the filters in your picture are there to smooth out these types of noise that get past the capacitor, and can interfere with sensitive electronics. I am not an expert on these types of noise someone can correct me if I'm wrong :)

  • \$\begingroup\$ Welcome to EE.SE, Ben. I think the first part of your answer might not be relevant as the PSU could be switched mode. The OP doesn't seem to have opened it or mention the specification in the question. I think you have mixed up common and differential. \$\endgroup\$
    – Transistor
    Apr 7, 2021 at 20:50
  • \$\begingroup\$ Thank you Transistor! You are correct, I did have them reversed. I fixed the image and my description. I will add some info about switched mode PSU, since switched mode PSU generate relatively HF noise, and that is another reason to use those filters! \$\endgroup\$
    – Ben Lusk
    Apr 7, 2021 at 21:00
  • \$\begingroup\$ Good, but I recommend that you keep the focus on what was asked in the question. You can always add supplementary information below a horizontal line ----. \$\endgroup\$
    – Transistor
    Apr 7, 2021 at 21:02

there's a little box on the cable that I opened and it reveals a small board with 2 toroidal transformers in series with a 1:1 ratio each

If there's two toroidal devices, you might have a common-mode choke and a differential-mode choke in series together: -

enter image description here

Picture from this site.


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