Why do I measure AC voltage on my multimeter?

[Abraham900]

Hello.

I'm currently, just for fun, trying to measure the resistance on an item that is class II (double insulated, not connected to earth that is).

I connect the black probe on earth/ground from the electrical outlet and the red probe on a metal screw on this item. The multimeter cannot read the ohm value on the multimeter since it is telling a 2V+ voltage is present on that area. So I measure the voltage and read 19VAC. Why is that? Does it have to do anything with introducing earthing to a double insulated item?

If I then disconnect the charger from this item and measure between earth and the charger output, I measure 70VAC! (It is supposed to deliver 12V). Why?

Grateful for some answers!

 

[betwixt]

It probably is delivering 12V but the measurement has to be taken between the output connections, not Earth.

The reason you are seeing the voltage is because although double insulated, there is still capacitance between the primary and secondary sides of it's transformer. That capacitance allows a tiny current to flow and the isolated side adopts a voltage which could be anywhere between zero and peak of the line AC. In practice, it rarely is either but falls somewhere between the two. What is important is that the amount of current is extremely tiny and poses no harm if it should be touched. If you have a resistor, say 10K Ohms to simulate the body resistance of someone touching Earth and the charger, connect it across the meter and take the measurement again, you should notice the voltage has been safely conducted away.

Brian.

 

[Abraham900]

Thank you! Your answer was really good.

Have a pleasent day.