Isolation Transformer Process

[lowell]

Thanks Enzo. I'm starting to get it. So Neutral in most guitar amps is still carrying all the current. Earth doesn't carry current until a fault or short to Earth happens. Therefore the 3rd prong became a standard so that neutral no longer in any way is physically connected to the chassis. Correct?

[Enzo]

That is not how I would have said it, but yes, that is essentially it.

The difference between common and ground is important.

Imagine a battery circuit, like a transistor radio, or for that matter, a wireless guitar belt pack. Inside those things we have a circuit common, what we all tend to call "ground", even though it has no earth connection. We really should train ourselves to use the term common instead of ground. Now if I wanted to, I could drive a grounding stake into the earth and run a wire to it. That is what they do in your electrical panel in your home.

I could take my radio and solder that wire to its common. Now the radio common is also really ground. Whether it works better or not is irrelevant, it is now earthed. But it is one point in a circuit. I could just as easily have connected the battery + terminal to my earth wire instead. Now common is not ground, +9v is now grounded in the radio. and what was common, well still is common, is now -9 with respect to ground.


What if I had a 9v battery and three LEDs and a resistor all in a series loop. The three LEDs all light up. It is just that simple circuit on my table top. The idea of ground there is not of much value. But I could ground any spot in that circuit, pick one, and have it grounded. We usually ground one end of a power supply, but I could ground one of the spots between LEDs if I felt like it. It makes that point "ground" but has no effect on the operation of my circuit.


And just so our iso and SMPS. In the stock situation the SMPS is connected via the mains wires to earth at one point. That means I cannot connect any other points also to earth. If I connect neutral to earth (as in your wall) and also -165 to earth, then I have a dead short across the -165v supply. But once plugged into the iso, the mains voltage is supplied by the isolated winding instead of the wall wires,l and so is very much like an AC "battery". Now the circuit has no earth connection, just like my LEDs or radio. And now I am free to arbitrarily connect any one point in the circuit to earth for my convenience.

[Enzo]

That is not how I would have said it, but yes, that is essentially it.

The difference between common and ground is important.

Imagine a battery circuit, like a transistor radio, or for that matter, a wireless guitar belt pack. Inside those things we have a circuit common, what we all tend to call "ground", even though it has no earth connection. We really should train ourselves to use the term common instead of ground. Now if I wanted to, I could drive a grounding stake into the earth and run a wire to it. That is what they do in your electrical panel in your home.

I could take my radio and solder that wire to its common. Now the radio common is also really ground. Whether it works better or not is irrelevant, it is now earthed. But it is one point in a circuit. I could just as easily have connected the battery + terminal to my earth wire instead. Now common is not ground, +9v is now grounded in the radio. and what was common, well still is common, is now -9 with respect to ground.


What if I had a 9v battery and three LEDs and a resistor all in a series loop. The three LEDs all light up. It is just that simple circuit on my table top. The idea of ground there is not of much value. But I could ground any spot in that circuit, pick one, and have it grounded. We usually ground one end of a power supply, but I could ground one of the spots between LEDs if I felt like it. It makes that point "ground" but has no effect on the operation of my circuit.


And just so our iso and SMPS. In the stock situation the SMPS is connected via the mains wires to earth at one point. That means I cannot connect any other points also to earth. If I connect neutral to earth (as in your wall) and also -165 to earth, then I have a dead short across the -165v supply. But once plugged into the iso, the mains voltage is supplied by the isolated winding instead of the wall wires,l and so is very much like an AC "battery". Now the circuit has no earth connection, just like my LEDs or radio. And now I am free to arbitrarily connect any one point in the circuit to earth for my convenience.

[Jazz P Bass]

Here we go with 'ground' again.

A word about the third wire.

The Neutral wire in a properly wired house is also bonded to the third 'safety ground ' wire AT THE Mains Panel.

The idea is that if something (an amp chassis) faults in the worse way & the chassis is Hot, that voltage on the chassis will blow the breaker at the mains panel, by returning through the third wire.

[dbdoc]

I just watched the ToddFun video indirectly linked in post 4 and thought he did a good job of demonstrating and how to prove that the secondary of an iso xformer is not referenced to earth ground and therefore the DUT connected to the iso also is isolated.

With or without an iso, I have always been a fan of using the differential/subtractive mode on a dual trace scope to measure anything that does not have earth ground as common. Even relatively cheap 20M dual trace scopes have the invert and add buttons to give you a single trace using the positive tips of the 2 scope probes. The probe ground pin is not used unless the DUT actually has an earth ground connection to connect to. Some of the SMPS I have worked on do have a grounded power cord, with earth ground as common for the secondary side. My iso has a grounded power cord connected only to the iso chasis and the ground pin of the iso outlets. So if the DUT has a grounded power cord, it is connected to earth ground when connected to the iso.