# I'm now confusing working principle of transformer isolation.

#### Dong-gyu Jang

Joined Jun 26, 2015
115
Hello everyone and be happy Christmas!

The article said utilization of isolation transformer in Fig. 2 breaks path from the ground secondary winding thus current is not flowing to shield (ground) of the cable of the voltage probe measuring V1. Firstly looking made me agree this statement as primary and secondary winding are separated in conduction.

However, I also can imagine that some current can flow from TP2 through ground to circuit of primary winding side and be coupled to secondary winding in the same way of how transformer works; return path is established.

I actually don't know strict reason why this current can not enter into primary side or coupling is prohibited for only this current.

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#### crutschow

Joined Mar 14, 2008
25,432
The primary and secondary are coupled only by the magnetic field and are galvanically isolated.
No current can flow directly from the primary to the secondary.
The ground connection you show is a single one-way path, there is no "return path".
Look at the primary and secondary currents. Each must have its own separate forward and return path.

#### Dong-gyu Jang

Joined Jun 26, 2015
115
The primary and secondary are coupled only by the magnetic field and are galvanically isolated.
No current can flow directly from the primary to the secondary.
The ground connection you show is a single one-way path, there is no "return path".
Look at the primary and secondary currents. Each must have its own separate forward and return path.
Hello.

As you said, direct current flow in form of conduction is of course not possible. However, conduction is not only way of transfering signal. The transformer itself is using magnetic coupling to transfer signal (in this case it is rather called power) from primary to secondary side. Thus I meant this coupling may become one part of the return path.

Could you give me the strict reason why this coupling is not possible for return signal coming from secondary through ground to primary while normal power coupling is done with no problem? Why the return signal is only segregatly not chosen for magnetic coupling?

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#### crutschow

Joined Mar 14, 2008
25,432
For magnetic coupling to occur between primary and secondary the primary had to generate a magnetic field
To generate a magnetic field, current must flow in the primary winding.
The only way for current to flow in the primary is for current to go from one primary winding connection, through the winding, and out the other winding connection.
This winding current cannot flow with just one wire from the primary to the secondary, thus there can be no current flow, either AC or DC from the primary to the secondary.

#### Dong-gyu Jang

Joined Jun 26, 2015
115
For magnetic coupling to occur between primary and secondary the primary had to generate a magnetic field
To generate a magnetic field, current must flow in the primary winding.
The only way for current to flow in the primary is for current to go from one primary winding connection, through the winding, and out the other winding connection.
This winding current cannot flow with just one wire from the primary to the secondary, thus there can be no current flow, either AC or DC from the primary to the secondary.
Oh..I see! So in order for return signal current to have magnetic coupling to transfer itself from primary to secondary side, obviously the current must flow primary winding first. However, as closed loop for this current is not made for primary side, the current can not flow -> no magnetic coupling.

Is my summary good to your comment?

#### crutschow

Joined Mar 14, 2008
25,432
Oh..I see! So in order for return signal current to have magnetic coupling to transfer itself from primary to secondary side, obviously the current must flow primary winding first. However, as closed loop for this current is not made for primary side, the current can not flow -> no magnetic coupling.

Is my summary good to your comment?
I believe that covers it.

#### KL7AJ

Joined Nov 4, 2008
2,225
Hello everyone and be happy Christmas!

View attachment 97105

The article said utilization of isolation transformer in Fig. 2 breaks path from the ground secondary winding thus current is not flowing to shield (ground) of the cable of the voltage probe measuring V1. Firstly looking made me agree this statement as primary and secondary winding are separated in conduction.

However, I also can imagine that some current can flow from TP2 through ground to circuit of primary winding side and be coupled to secondary winding in the same way of how transformer works; return path is established.

I actually don't know strict reason why this current can not enter into primary side or coupling is prohibited for only this current.

Why not just use the scope in the differential mode? That way there will be no question.

Merry Christmas and Happy New Year! How is the ICP running?\
Eric

#### Dong-gyu Jang

Joined Jun 26, 2015
115
Why not just use the scope in the differential mode? That way there will be no question.

Merry Christmas and Happy New Year! How is the ICP running?\
Eric
Hello.

The question is just to understand theory rather than my personal use.

The ICP runs well except that some portion of return current chooses other ground path than cable shield so radiation and hard impedance matching are problems. I'm now thinking to break ground loop.

#### KL7AJ

Joined Nov 4, 2008
2,225
Hello.

The question is just to understand theory rather than my personal use.

The ICP runs well except that some portion of return current chooses other ground path than cable shield so radiation and hard impedance matching are problems. I'm now thinking to break ground loop.
Dong-gyu...long time no hear! (all my fault). I'm really curious how the ICP is working. I would really love to work on one of these again!
Eric