Oscilloscope And Isolation transformer

[suryaputhra]

I have connected Oscilloscope to 24VAC isolation step down transformer. Scope reads and shows good sine wave with 24AC but if I remove ground clip scope reads and shows sine wave with 240AC, but if I remove probe tip and keep ground clip nothing reads. What is phenomenon.

[capt bullshot]

Capacitive coupling from transformer primary to secondary.
Swap the input wires of the transformer and watch what happens - at least the amplitude should change

[suryaputhra]

Capacitive coupling from transformer primary to secondary.
Swap the input wires of the transformer and watch what happens - at least the amplitude should change

Thanks for response can you elaborate. What I feel obscene of ground(disconnected at transformer) the scope referencing its own earth ground and showing 240AC.

[Gyro]

What capt bullshot, means is that there will be some capacitance between the primary and secondary windings (just from being wound close together).

One end of the primary winding will be physically closer to the secondary winding that the other. This will couple more voltage to the secondary winding through this stray capacitance if that end of the primary is connected to live (once your Scope ground clip is removed). If Neutral is connected to that end of the primary winding instead then there is less voltage across the stray capacitance.

This is not of any consequence normally because the stray capacitance is very small. You are only seeing it because your scope probe has a high impedance.

Just to be safe, unplug the transformer and make sure there is 'infinite' resistance between the primary and secondary windings.

[suryaputhra]

What capt bullshot, means is that there will be some capacitance between the primary and secondary windings (just from being wound close together).

One end of the primary winding will be physically closer to the secondary winding that the other. This will couple more voltage to the secondary winding through this stray capacitance if that end of the primary is connected to live (once your Scope ground clip is removed). If Neutral is connected to that end of the primary winding instead then there is less voltage across the stray capacitance.

This is not of any consequence normally because the stray capacitance is very small. You are only seeing it because your scope probe has a high impedance.

Just to be safe, unplug the transformer and make sure there is 'infinite' resistance between the primary and secondary windings.

Disconnected ground measured V RMS reads 80V so Vpk is (80 into square root of 2) so 80X1.4  = 112 so Vpp = 224V this is solved. But why 80V V RMS coming from 24V transformer when ground disconnected from 24V AC transformer.

[Gyro]

As I explained above, the voltage comes from stray capacitance between the primary and secondary windings. This capacitance is 'distributed' - some of the primary winding is at full Live potential, further along the winding it is less and less, until you reach the Neutral connection. So the coupled voltage into the floating secondary is a combination of all of these stray capacitances at different voltages between the primary and your floating secondary winding.

As I said previously, the stray capacitance is very small, so the input impedance  of your scope probe will be loading it too. The magnitude of the voltage you are seeing is arbitrary, based on the combination of all of these factors. If you reversed the connections to the primary, or put your scope probe on the other end of the secondary, you would see a different voltage.

Although interesting, try not to get too hung up on these measurements, everything goes away when you connect your ground clip and ground the secondary winding.