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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?

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    \$\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\$
    – Ale..chenski
    Commented Nov 12, 2016 at 18:39
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    \$\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
    Commented Nov 12, 2016 at 18:42
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    \$\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\$
    – Spehro 'speff' Pefhany
    Commented Nov 12, 2016 at 18:53
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    \$\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
    Commented Nov 12, 2016 at 18:59
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    \$\begingroup\$ Good question. Problem was in tracing the cct. Good learnung exercise. See this cct \$\endgroup\$
    – Russell McMahon ♦
    Commented Nov 13, 2016 at 7:57

2 Answers 2

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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

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  • \$\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
    Commented Nov 12, 2016 at 18:48
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    \$\begingroup\$ @MikeTeX I say leave it; see my comment under the question itself. \$\endgroup\$
    – gbarry
    Commented Nov 12, 2016 at 19:01
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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

schematic

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.

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