This might have been answered, sorry if so. Please direct me to that discussion if it answers the question below.

I don't get how isolation transformers work. Everywhere I read, I read the same description, i.e., in this kind of transformer the earth reference is removed and so, now it's all of a sudden safe. But how can a ground reference be removed and there still be voltage? After all, voltage is a difference of two potentials. And now they removed one of those potentials? Arghhh, doesn't make sense. So, how does an isolation transformer work?

 

Hi, well if you think about it, the primary windings of the transformer are connected to the live and neutral wires. The transformer creates a proportional electrical signal from the mains onto its secondary side, depending on its magnetic properties and number of coil windings.

The secondary side is isolated from the primary (mains power) side because there is no physical connection between them, just an electromagnetic field.

 

The output of the isolation transformer has a voltage across its windings but the windings are not referenced or connected to the mains input ground. Thus there is no potential between either of the output windings to ground but there is between the winding outputs.

 

Don't get too hung up on "Ground" reference. If that were true, how does the electrical system on an airplane work? Not everything uses Ground as a zero reference. The reason for using an isolation transformer is because normal power coming into a home IS ground referenced. If you are touching anything grounded and touch an energized wire that uses ground as a zero reference, you will be shocked or worse. The isolation transformer removes only the Ground element of the hazard.

 

The transformer removes Earth reference, so there is no conductive path between the secondary wires and the floor you´re standing on. You can call any point in the circuit a Ground or common if you wanted to, but I think mains wiring ground should be refered to as Earth for clarity.

 

The transformer removes Earth reference, so there is no conductive path between the secondary wires and the floor you´re standing on. You can call any point in the circuit a Ground or common if you wanted to, but I think mains wiring ground should be refered to as Earth for clarity.

Don't get too hung up on "Ground" reference. If that were true, how does the electrical system on an airplane work? Not everything uses Ground as a zero reference. The reason for using an isolation transformer is because normal power coming into a home IS ground referenced. If you are touching anything grounded and touch an energized wire that uses ground as a zero reference, you will be shocked or worse. The isolation transformer removes only the Ground element of the hazard.

But if the any ground-like reference is removed, what kind of reference is left for the positive voltage on the coil? I.e., what is now the + and what is the -? I.e., if I wanted to measure that voltage, what would I connect a "voltmeter" to?

The output of the isolation transformer has a voltage across its windings but the windings are not referenced or connected to the mains input ground. Thus there is no potential between either of the output windings to ground but there is between the winding outputs.

Are you saying that one end of the coil is a common reference and the other is a +? This is important, please confirm.

But even so, say I touch the + of the coil and a grounded object. Won't there still be a difference of potentials?

 

Transformers are used on AC, alternating current. Plus and minus are used in DC, direct current.

With a DC voltage, or AC secondary voltage and isolation, there is NO return to ground, so no potential.

Maybe you should read this from our Ebook : http://www.allaboutcircuits.com/vol_2/index.html

 

Are you saying that one end of the coil is a common reference and the other is a +? This is important, please confirm.

That would be "one end is a common reference and the other is the phase" in case of a transformer.

Take this picture for example. It shows three times the same simple circuit, two 12V batteries in series with voltages with respect to ground. The only difference is which of the three points you decide to call ground - common reference.

In mains wiring, the common reference is almost allways the Earth potential, because that is what is important from the point of safety. But if a device is connected through an isolation transformer there is no conductive path to earth, so it is deemed safe* unless you connect something that creates that path like a grounded oscilloscope.
*safe meaning not as dangerous as mains voltage, but it still can be lethal if you don´t know what exactly you´re doing.

 

Ok, sorry, by "minus" I meant the point around which AC alternates. Isn't ground the reference point around which AC alternates? I guess it's technically the neutral wire. But neutral is usually ground, right? So, what sense does it make to remove the ground reference if the neutral is still there?

Is there a picture somewhere illustrating how it works? I couldn't find anything yesterday.

What I'm confused by is how can something providing voltage be safe if any voltage (isolated or not) in relation to an object with a 0 potential (say my body -- close to 0 say) will create a shock. I.e., if I touch the live wire of the isolation transformer, will I be shocked?

 

Ok, sorry, by "minus" I meant the point around which AC alternates. Isn't ground the reference point around which AC alternates? I guess it's technically the neutral wire. But neutral is usually ground, right? So, what sense does it make to remove the ground reference if the neutral is still there?

Is there a picture somewhere illustrating how it works? I couldn't find anything yesterday.

What I'm confused by is how can something providing voltage be safe if any voltage (isolated or not) in relation to an object with a 0 potential (say my body -- close to 0 say) will create a shock. I.e., if I touch the live wire of the isolation transformer, will I be shocked?

Good visuals found here.

Your body is grounded (earthed) itself and because the secondary side is not referenced to earth (no connection to earth) you are okay to touch the secondary side with one hand. Though, if you touched both the leads of the secondary side simultaneously you will get a shock because you created a parallel loop with your body. If you touch one lead on the secondary side with one hand, all you are doing is setting up the earth ground reference to the point that you touched. Since your body is at earth ground and you are touching a point that you made earthed, you are not at risk.

 

This is just one diagram I found. Try Google for a plethora of explanations. Most just repeat what has already been posted.

 

Ok, sorry, by "minus" I meant the point around which AC alternates. Isn't ground the reference point around which AC alternates?

No. Ground is just a point selected as an arbitrary reference.

I guess it's technically the neutral wire. But neutral is usually ground, right? So, what sense does it make to remove the ground reference if the neutral is still there?

Neutral is usually connected to ground which it's why it's dangerous to touch a live wire when you are connected to earth ground. If you remote that ground reference than there is no potential between the live wire and ground.

What I'm confused by is how can something providing voltage be safe if any voltage (isolated or not) in relation to an object with a 0 potential (say my body -- close to 0 say) will create a shock. I.e., if I touch the live wire of the isolation transformer, will I be shocked?

No. Potential is relative to someplace else, so when you say your body is at zero potential it's relative to earth ground. If you touch a single live wire of an isolation transformer output that does not have any outputs connected to earth ground then there is no potential between the output and ground, there is no path for the current, and you will not be shocked.

 

Thank you so much for the visuals! It helped!

Thanks everybody!

 

The transformer removes Earth reference, so there is no conductive path between the secondary wires and the floor you´re standing on. You can call any point in the circuit a Ground or common if you wanted to, but I think mains wiring ground should be refered to as Earth for clarity.

From a practical standpoint it means you can hook your scope ground or any other plugged in peace of test gear and not have fireworks or have the case of the instrument become electrically hot.