# Behaviour of a (broken?) transformer

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?

Thanks.

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.

• 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. Mar 16, 2020 at 8:33
• And be sure to remove the resistor if you probe the primary! Mar 16, 2020 at 8:45
• Is there a winding table for it? Can you measure continuity between each winding? Mar 16, 2020 at 8:59
• @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. Mar 16, 2020 at 9:32
• @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? Mar 16, 2020 at 9:33