Isolation transformers

[Paul P]

I just picked up an oscilloscope for cheap and the seller gave me
a homemade isolation transformer with it saying that the scope
should be plugged into it if I work on anything without a transformer,
to protect the scope.

I've read information describing how an isolation transformer can
be useful to protect people from shocks but not about protecting
test equipment. Could someone please elaborate on this ?

Can a single transformer (of the appropriate rating) be used to
isolate me, my test equipment and the amp under test all at once
or should separate transformers be used for the different pieces ?

I borrowed the book "McGraw-Hill Electronic Testing Handbook,
Procedures and Techniques" from the library and as far as I can
tell there is not a single mention of isolation transformers ?

I'm currently putting together the pieces for a bench at which I'll
be playing with amps and am interested in anything that can
increase my chances of living through the experience.

Thanks.

[Tubebass]

I would just use the transformer on the transformerless amp. The scope can fend for itself.

[Enzo]

An isolation transformer is to isolate a piece of equipment. If you plug everything into it, they are all connected together again - not isolated.

Use the iso for the unit under test, not the scope. I can connect the ground lead of a scope to some hot point in a circuit, including a hot chassis, and if the scope is grounded, it will ground off that point. If I plug the scope into an iso, then the ground lead of the scope no longer goes to earth ground, so it wouldn't ground off the point in the circuit. However, the scope chassis and anything else connected to the scope would now be at the voltage of that hot point. If you connect the UUT to the iso, then the UUT is floating - not everything else. Your scope can remain grounded, your other test gear can remain grounded.

For amp work, about the only place I use an iso is for hot chassis amps. Usually those are the ones without a power transformer.

I also use an iso working on switching power supplies so I can reference the scope to -170VDC points.

I also work on hot chassis video monitors for the arcade industry, and they REQUIRE an iso.

[MKB]

Isolation trannies are also useful if you use a variac, as one side of the variac output is connected directly to the power line (variacs/autotransformers do NOT isolate from the power line).

[bob p]

For electrical safety purposes, I think that the textbook answer is to go one step further -- one isolation transformer for the amp being serviced, one isolation transformer for the 'scope. Maybe Enzo could comment on this, as he knows more than I do.

[Enzo]

I suppose it couldn't hurt, but I don't see the need. I like my scope nicely grounded, and as long as the UUT is isolated, I feel safe. I am sure there are times when the scope ought to float, but they don't come up often in my life.

[R ski]

Iso Oh Oh

Being an industrial technician had jolted me good many moons ago.

Your oscilloscope is probably a single end input.

This means that the shield clip is grounded. For most amp work
clipping the ground to amp chassis will not hurt you or the amp when probing
with the oscilloscope probe.

However, when you insert an isolation transformer between the
wall plug an the scope, the chassis ground of your scope is
floating from hydro ground. This can raise the ground clip to 115 VAC.
Not good. When touched and your are somewhat grounded BAAAAAMMMM

This is the jolt I experienced .

In industrial power circuitry virtual grounds cannot be clipped on
with a single ended scope, unless it runs from batteries.

[Enzo]

If your scope is on the iso, the chassis shouldn't float to anything in particular. It won't have 115VAC potential to earth ground since it is isolated from it. The safety issue is that the shield lead of your probe will connect the scope frame to whatever the UUT is at with respect to earth. If you reference the scope ground to -160VDC for example, the whole frame of the scope will sit at -160VDC. If you run the scope not isolated, but do lift the ground, then the chassis WILL float up to mains potential.

Running a scope on an iso is more or less equivalent to running it on batteries. If you reference your battery scope to some high voltage circuit node, the whole frame of the battery scope will be at that potential.

Frankly, is working on guitar amps, isolation is not usually an issue. Well tube amps anyway. I get out my iso when I work on the switching power supplies in GK amps for example.

[bob p]

I'm resurrecting an old thread because I have to troubleshoot the triac-fed power supply on an old Carver HiFi amp.

The troubleshooting instructions in the factory service manual specifically require that, "all primary waveforms must be viewed with an isolated, line triggered scope with no secondary ground reference."

To me, that sounds like "plug an isolation transformer into the wall and plug your scope into the isolation transformer."

The unit under test gets plugged into a variac, and triac function is observed at below normal operating voltages.

The service manual includes photos of the triac's conduction at idle, viewed at the MT2 lead (cathode).

This raises a couple of questions:

1) If you're attaching the probe tip to MT2 (cathode) where do you put the probe's ground lead?

2) What happens to the chassis potential of the scope?

For reference, here's a trace of the triac's normal activity at idle.