Variac question

[Jeanguypataterubberboot]

Hi there. This question may be pretty basic but I don't know much about variacs.
Here is the scenario:

I plug in the variac and set it to 0V. I have a voltmeter across the output and also the test probe of my oscilloscope to one of the outputs leads.(I disconnected the ground from the scope test lead so as to not create a short). On the meter I see 0 Volts. On the oscilloscope I see a full 118V sine wave. When I crank up the variac to 120 volts I get 118 volts on the meter and a flat 0volts on the scope. Can someone explain this? I am getting conflicting readings.

Thanks for the help.

 

[kpatz]

Leaving the ground on the scope floating is going to give some erratic readings, as you can see.

Anyway, this is what I think is happening. If you think of voltage, it's always a difference in potential between 2 points. The + and - terminals of a battery, the live and neutral connections on a household outlet, or the 2 outputs on the variac. Now consider how the variac works, a bit like a voltage divider/transformer combo. The coil is connected between the live and neutral of your outlet. One output is connected to one end of the coil (probably the side plugged into the neutral, assuming a 3 prong or polarized plug), and the other to a wiper that slides across the windings of the coil. When you turn the variac down to 0V, both outputs are on the same side of the coil, and thus at the same potential, so you get 0V across the outputs. As you turn the dial up, the wiper moves across the coil, tapping a higher voltage as you turn it up, until at max it's at the same level as the input (or maybe it goes higher, depending on the design of the variac and where the inputs are connected).

Since your multimeter was connected across both outputs of the variac, it was able to read the voltage correctly. But your scope was only connected to one terminal, presumably the one connected to the wiper. So what was it reading? A voltage, a difference in potential, between the wiper terminal of the variac, and... ??? a floating ground, which was probably picking up live (120V) line voltage which both the scope and variac were plugged into, or maybe the EM field coming from the variac coil. In essence, it was a little like "almost" connecting the scope's ground to the other side of the variac coil, so it was measuring the opposite side of the voltage divider, so you got full voltage with the variac at 0 and no voltage when it was turned all the way up.

This voltage, although it's 120V, there should be a very high resistance in between, so there's no shock hazard (hopefully!) If you connected the scope's ground to anything, the reading would be different, or disappear. The scope's high input impedance allowed it to detect this voltage.

 

[Jeanguypataterubberboot]

Thank you very much for the detailed explanation. That's what I needed.