Learning Transformers - How to see Phase on Scope

[K1JOS]

I am learning about transformers and the phase changes between primary and secondary and buck/boost examples.  I would like to put these signals on my scope.  I have a 115vac primary xfrmr with several low voltage secondary's.  What is the preferred method to compare the phase changes on a scope?  Is the scope probe high enough impedance that I could put the probe tip on the primary hot and the probe ground on the neutral?  Should I instead use an isolation transformer to supply the transformer primary?

[AG6QR]

Be very careful here.  See one of the "how NOT to blow up your scope" videos or threads. 

The ground clip on your scope's probes is grounded through the scope's mains plug.  The neutral wire isn't quite exactly the same as ground, though they should be connected at the service entrance.  But if there's any current flowing through neutral, its potential isn't the same as ground potential, due to  V=IR and the fact that the neutral wires aren't zero resistance.  If there is a GFCI protecting the circuit, it should trip as soon as your scope probe provides an alternate path for some current to leak to ground.

If you connect the ground clip on a scope probe to a hot lead instead of neutral, something is going to go BANG!  If you're lucky, it'll only be your scope that dies.

An isolation transformer can help somewhat, but it comes with its own safety issues which you should understand.  As soon as you ground one point of the DUT using the scope probe ground, then the DUT should no longer be considered isolated.  If you connect the scope probe clip to some point on the device that has a voltage relative to the main chassis/frame, it's possible you could get a shock by touching the chassis with one hand while touching a grounded point (for example, a shield on a piece of test equipment) with the other.

I won't say it can't be done, but it does require extreme caution and understanding of the issues.

If the purpose is for learning about transformers, I'd recommend starting with a UL listed low voltage transformer, a "wall wart" or something of the type used for landscape lighting, irrigation sprinklers, or other small stuff.  These usually put out something like 12 to 24VAC, with a fairly modest current limit.  Use that as your source of AC, and do your experiments on a transformer that's starting with that low voltage, low energy AC.  The theory is the same at lower energy levels.

[K1JOS]

Perfect... many thanks.  Why don't they put a low current fuse on the scope probe ground lead?

[electr_peter]

Why don't they put a low current fuse on the scope probe ground lead?

• It will compromise high BW performance
• it is not needed with proper measurement setup and
• blown fuse can create very dangerous situation in some cases.