I have recently cannibalized a 6 year old microwave oven. I was very surprised to find the following beast at the electrical entry of the oven:

photo of tiny transformer on small circuit board

The reason: this small 1:1 isolation transformer was powering a 900W oven, without any switching technology (everything in the circuit remains at the main 50Hz): the big internal high voltage transformer was connected to it. I have analysed the circuit and drawn the schematic:

schematic showing transformer with very few other compoonents

If I'm not wrong, the blue cubic component is a capacitor, and so are the two other small blue components (as indicated in the schematic). I guess that the resistor is a bleeder, and that the tank LC circuit is tuned to resonate at 50Hz (in order to block the current in the case where the secondary is opened). I would like to know if I am missing something, and if not, if this is a well known technique to reduce the size of the transformer connected to the mains supply. Also, what is the essential reason this thing was inserted here? Galvanic isolation?

  • 10
    \$\begingroup\$ Could you double-check your reverse engineering, and check winding connections? The circuit is more likely a common-mode filter, to prevent high-frequency noise from entering AC mains. \$\endgroup\$ Nov 12 '16 at 18:39
  • 1
    \$\begingroup\$ The AC line is connected to the fuse and to the node you labelled OUT1. As Richard already pointed out it's a common-mode choke for conducted EMI purposes. As you suspected there's no way a microwave oven is being powered by using it as a transformer. \$\endgroup\$
    – John D
    Nov 12 '16 at 18:42
  • 3
    \$\begingroup\$ If it was actually connected that way, the fuse would blow instantly. Rotate the choke 90° so the two coils are in series with the respective lines and with dots at the same end. \$\endgroup\$ Nov 12 '16 at 18:53
  • 7
    \$\begingroup\$ I recommend you not erase the question. Don't you think someone else will have the same question? Many Q-and-A's revolve around an initial misunderstanding. In your case you "knew" it was a transformer, and from there, it was just too easy to draw what you thought you were seeing. \$\endgroup\$
    – gbarry
    Nov 12 '16 at 18:59
  • 1
    \$\begingroup\$ Good question. Problem was in tracing the cct. Good learnung exercise. See this cct \$\endgroup\$
    – Russell McMahon
    Nov 13 '16 at 7:57

If you double-check your wiring diagram you will find that is NOT a transformer. That is a common mode choke to keep RFI from being transmitted out through the mains power cord.

There is no way (with currently-known technology) to make a mains-frequency transformer that small that handles as much current as a microwave oven draws. You have seen how big the transformer needs to be by your comparison with the size of the high-voltage transformer.

Ref: https://en.wikipedia.org/wiki/Choke_(electronics)#Common-mode_chokes

  • \$\begingroup\$ OK, it is too late to check again where were the wires connected: I noted this on the board as I cut the wires; I was sure this was correct, but this is apparently a mistake. Too late to erase this question too. \$\endgroup\$
    – MikeTeX
    Nov 12 '16 at 18:48
  • 3
    \$\begingroup\$ @MikeTeX I say leave it; see my comment under the question itself. \$\endgroup\$
    – gbarry
    Nov 12 '16 at 19:01

As an amplification of Richard Crowley's answer, I'd do this in a comment but there is no way to include a circuit diagram. Your circuit actually looks like


simulate this circuit – Schematic created using CircuitLab

as far as the "transformer" is concerned. As stated, it's a common-mode filter which keeps the microwave from driving noise onto the power lines.

Most likely, R1 is a PTC surge preventer, although this is not guaranteed.
R1 (the resistor clearly visible in the OP photo with a green multiplier band) will be across C3 and is likely intended to bleed any residual voltage from it after AC supply is removed.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.