Do I need an Isolation transformer?

[Koolaide]

Hi all. Got a new to me Oscope(and an old one to repair) coming and have been looking at YT and there are those that say an Isolation transformer is a must to protect scope and for safety.

Looking for opinions from the hive here.

How large for tube amps, is there a cheap but good alternative.
All replies appreciated.

[pompeiisneaks]

Hi all. Got a new to me Oscope(and an old one to repair) coming and have been looking at YT and there are those that say an Isolation transformer is a must to protect scope and for safety.

Looking for opinions from the hive here.

How large for tube amps, is there a cheap but good alternative.
All replies appreciated.

They are not 'mandatory' but you can definitely destroy a scope if you aren't careful. If you're just probing with one lead, then you're way less likely to cause issues, it's the second probe that tends to be problematic if I recall.

I've not used one yet, and so far, I've been fine.

/me crosses fingers

~Phil

[pdf64]

As long as you always remember that the probe screen is connected to the wall ground, then an isolation transformer isn’t necessary. I’ve never found it a problem in 40 years.

[sluckey]

As long as you always remember that the probe screen is connected to the wall ground, then an isolation transformer isn’t necessary. I’ve never found it a problem in 40 years.

They were certainly a necessity back in the '60s when most of the consumer electronics that would come across your bench had one side of the ac cord connected directly to the chassis. And the ac cord was two prong non polarized. Even the two prong polarized cords were likely to have had the wide blade trimmed so it would fit the non-polarized wall socket. The boss did not think it was funny when you blew up the VTVM, or sig gen, or sig tracer, or scope because of a $10 table radio!

[martin manning]

You can isolate the DUT, and that was the role of an isolated autotransformer. Tektronix specifically recommends that you do not isolate the scope.

[PaulD]

that was the role of an isolated autotransformer.

There is no such thing, an autotransformer is not an isolating transformer. The recommendation not to isolate the scope is a good one, the device under test can be isolated but if the scope is isolated the scope's chassis could potentially become live.

[martin manning]

There is no such thing, an autotransformer is not an isolating transformer.

I'm going to disagree there, a typical autotransformer (Variac) does not provide isolation.

[PaulD]

How is that disagreeing? That is precisely what I said - an autotransformer is NOT an isolating transformer!

[martin manning]

The double negative threw me. Autotransformers do not provide isolation, but you can isolate an autotransformer.

[Koolaide]

Thanks all for the replies.

What I understand is:
1-Lots of folks work without an IT (Isolation Transformer) and never have an issue.
2-Do not isolate the scope-only the DUT(device under test).
3-Most autotransformers (i.e. VARIAC) do not provide isolation. (mine does not)
4-Using multiple probes can be as safety issue and if used, must be grounded to the same point (potential)

Added research provided the following information:
A-In the US (and some other countries) the Neutral/Return is tied to ground, and therefore ground referenced and may be an issue if the IT output Neutral/Return is grounded.
B- Once the scope ground clip is attached to the DUT the DUT is now referenced to ground through the mains connection of the scope.

I have decided my choice is to use an IT for another layer of safety.

I have another related question .Am I correct in thinking if I want to use a bulb current limiter in conjunction with IT I would connect the current limiter to the IT then the DUT and not the other way around?

Please let me know if anything in this post is incorrect.

Thanks again for all the responses.

Peace
Jim

[PaulD]

1-Lot's of folks work without an IT (Isolation Transformer) and never have an issue.

Correct, as long as the correct safety procedures are followed there is no problem. It's also important to note that an isolation transformer is not a substitute for following the correct safety procedures, don't let it lul you into a false sense of security, you can still get a shock from something connected to an isolation transformer.

2-Do not isolate the scope-only the DUT(device under test).

Correct.

3-Most autotransformers(i.e. VARIAC) do not provide isolation.

ALL (not most) autotransformers are non-isolating, only transformers with separate primary and secondary windings provide isolation.

4-Using multiple probes can be as safety issue and if used, must be grounded to the same point (potential)

Correct, it is important to remember that the probe ground clips are connected together via mains ground so if they were connected to different potentials current would flow.

A-In the US (and some other countries) the Neutral/Return is tied to ground, and therefore ground referenced and may be an issue if the IT output Neutral/Return is grounded.
B- Once the scope ground clip is attached to the DUT the DUT is now referenced to ground through the mains connection of the scope.

Correct, although the Isolation transformer secondary doesn't actually have a neutral, it is floating. If either leg were referenced to ground it would no longer be providing isolation.

I have another related question .Am I correct in thinking if I want to use a bulb current limiter in conjunction with IT I would connect the current limiter to the IT then the DUT and not the other way around?

Yes that would be the sensible way of connecting them.