I am testing a transformer with a center tap on its secondary winding and I am pretty sure it is broken. Never the less I cannot explain to myself the reason behind some of the values I read with a digital multimeter.

The transformer was the only component inside a Yamaha PA-20 power (AC-AC step-down) adapter.

Input: 230V~ 50Hz 42W
Output: 17.5V-X2 0.94A

This is the schematic (labels on the connector are written in purple):


simulate this circuit – Schematic created using CircuitLab

Measurements of windings resistance values:

  • P1-L2: inf
  • P1-(A or B or C): inf
  • L2-(A or B or C): inf
  • A-C: inf
  • C-B: inf
  • A-B: 0 Ω

Given these results, I suspected a broken primary winding and a detached tap (C). If I give an input line voltage, though, I can measure these AC voltages on the output.

  • A-B: varies 0.000 - 0.015 V
  • A-C and C-B: varies between 3 V up to 5-6 V, with asymmetric values.

And this is the behaviour I cannot explain, given that the primary winding seems broken. Can you please shed some light on this?


After @Mattman944 suggestion, I have added resistive loads on the outputs. With a 10kΩ resistor between A and C and another 10kΩ resistor between C and B, all output voltage readings are near zero (AC, AB, CB).

The purpose of my original question was to understand the strange measurements I had with a broken primary. I did not think about adding a load on the output.

Thanks to @Transistor's answer, I was able to restore the transformer, something I originally did not expect to be able to do.

  • 1
    \$\begingroup\$ Modern DMMs have a very high input impedance. Parasitic capacitance can cause weak coupling. For your voltage measurements, put a load resistor in parallel with your meter. Any 1/4W between 2k and 10k should work. You can go lower if you have a power resistor. \$\endgroup\$
    – Mattman944
    Mar 16, 2020 at 8:33
  • \$\begingroup\$ And be sure to remove the resistor if you probe the primary! \$\endgroup\$
    – Mattman944
    Mar 16, 2020 at 8:45
  • \$\begingroup\$ Is there a winding table for it? Can you measure continuity between each winding? \$\endgroup\$
    – winny
    Mar 16, 2020 at 8:59
  • \$\begingroup\$ @Mattman944 I have added two 10kΩ resistors on the outputs and the voltage readings are now zero. I did not expect parasitic capacitance to affect my readings so much, and only when probing the centre tap. \$\endgroup\$
    – Mauro
    Mar 16, 2020 at 9:32
  • \$\begingroup\$ @winny I do not have a table for it. Not sure what you refer to with "continuity measurement", isn't that what I have done with my first set of measurements? \$\endgroup\$
    – Mauro
    Mar 16, 2020 at 9:33

1 Answer 1


Many of these transformers have an internal fuse on the primary winding. The terminals for these are sometimes visible on the winding terminals. These are usually designed to offer both over-current and over-temperature protection.

I have no explanation for the loss of the C, centre-tap, continuity on the secondary. It may also have a fuse but it would be a surprise that both failed at the same time.


simulate this circuit – Schematic created using CircuitLab

Figure 1. Possible internal arrangement.

  • \$\begingroup\$ Thank you very much for mentioning the fuse, I did not know about this. I have found it and it was indeed melted. While removing the outer layers of insulating tape and paper, I found another contact on the secondary. Maybe this transformer had originally two secondary windings. Using the other contact (D) I was able to restore the transformer. I now have two 18V outputs (AD, DB) and 36V on AB, as expected. \$\endgroup\$
    – Mauro
    Mar 16, 2020 at 10:22
  • \$\begingroup\$ My experience is the first step in xfrmr troubleshooting is a continuity and resistance test \$\endgroup\$ Mar 16, 2020 at 13:01
  • \$\begingroup\$ OP did that. See the question. \$\endgroup\$
    – Transistor
    Mar 16, 2020 at 13:03

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