Say you got a transformer from a old stereo amplifier and you only know it can output 12volts maximum after rectification. You can’t find any data sheets for it since it is a custom built transformer for that amp line. So when you want to reuse it for a project such as a power supply you need to know if it is in phase or not since there are more then one pair of secondary taps.
To do this I would advise to use a one to one transformer so you can be isolated from earth ground and don’t have to worry about blowing your equipment up. However not everyone has a one to one transformer laying around so here is a quick way. It’s kinda dirty but it works.
To see if the transformer is in phase or out of phase a dual channel oscilloscope is used. One channel is connected to the mains voltage input and the second channel is connect to the output mains. However if you were to do this with out any isolation you will blow something up such as the oscope. Reason being the earth ground isn’t 100% earth ground but it is tied to the neutral line from the fuse panel or circuit breaker panel.
The easiest way to get around this is to not use the earth ground on the oscope. I’ve seen people use kapton tape and wrap it around the earth ground pin but I wouldn’t advise it. Since modern oscopes use IEC power jacks all you need is a typical IEC cable that you don’t really care about and do some surgery to it.
Go to a home improvement store and buy a replacement plug. I would go ahead and get a three prong unit for the sake you don’t mix up the polarities of the AC line to your scope.
Just wire up the new plug but leave the earth ground disconnected and cut off from the circuit.
North America – All you need to worry about is single phase.
Most of the EU – Once again I would look at the single phase.
After making a custom semi isolation cable you can test and see what set of secondary tap wires are in phase. You’ll know they’re in phase when the sine waves match.
When they’re out of phase they will not match and look something like this.
Reason why we want a in phase shift is it would be in sync. A out phase shift would make the transformer work harder and cause break down thus damaging the transformer.