Transformer Curiosity: Secondary Increases If Primary Phase Is Reversed (Huh?)

Thread Starter


Joined Jan 15, 2013
Good Day To All!
While salvaging components from a defective switch mode power supply, I tested the transformer (removed from the circuit board) and ended up puzzled.

When I connect the primary (120 vac), I can have a secondary of 12 vac. However, if I swap "Line" and "Neutral" on the primary, I get 16 vac across the secondary.

I am pretty confused as to why the secondary voltage changes when the primary phasing is reversed. At first I thought it might be because my test bench's isolation transformer is exactly that- the ground and neutral are not bonded. Next, I carefully tested with the iso trans bonded and got the same result. A search of the part number showed no data sheets to help me out. (Note that one of the secondary taps is common with one of the primary taps. Photos Attached)

Thinking age has caught up with my brain and I forgot something basic, I took a spare 40va transformer I have for my boiler and connected it to my isolation transformer. I can swap phases with no change in secondary voltage. (Phew! I'm not losing it...yet)

The only real experience I have with transformers is my former employment, but most of the transformers I installed or serviced were three phase and typically in the 480 vac to kV range. On those, one may swap phases A & C (and B if it was a Wye system) with no change in secondary voltage. I occasionally was called upon to swap out single phase transformers on unit heater controls or motor starters, or buck-boost transformers, but I never did any experimenting around with reversing primary phases.

What is causing a phase reversal to change the secondary voltage?

Thanks For The Education- I appreciate it!
Enjoy Today,



Joined Jul 18, 2013
If it came from a SMPS then it most likely has a ferrite core?
And also designed for a certain switching frequency, are you running it off of a 120v wall outlet (60Hz)?


Joined Jun 4, 2014
Weird. Try putting a load on the output, say a 330 ohms. You may be seeing a capacitive effect.
Yes it looks like a ferrite cored transformer intended for much higher frequencies than the mains. If a winding on such a transformer was connected to the mains I would expect smoke. If the terminals the mains was connected to don't actually have a winding connected between them then there would be no current and no smoke. If you then connect a meter somewhere else on the transformer you might read a voltage via the capacitance between windings inside the transformer. Now it makes sense that swapping the mains wires would change the voltage.

If the transformer has a ferrite core and not a laminated iron core please don't try to connect it to the mains any more.

Thread Starter


Joined Jan 15, 2013
Thank You All for your replies and information. I learned a lot here today.
After reading what each of you wrote, I studied the circuit board and some on-line wiring diagrams for switched mode power supplies and see now how the transformer is supplied with high frequency. It's really interesting.

Thanks Again to all! I appreciate your help.