Products > Test Equipment
Floating Scopes
SeanB:
+1 for the Wacky German, support Dave and get his new differential probe set, which gets rid of many of the pitfalls of floating either the DUT or the scope. You can of course use an isolation transformer with the probe set, as a belt and braces approach, and this also gives extra protection in case something in the DUT goes F00F during your test work, you have less fuses and semiconductors to replace as the fault current is limited by the transformer inductance.
I have a ferroresonant transformer which can also serve as an isolation transformer ( not all ferroresonant units are isolated, and many have an earthed side of the mains, typically the designated neutral of the output side, connected to the PE conductor and the case as well as a continuation of the incoming protective conductor) and this can then be fed to the Variac to provide variable voltage. As the ferroresonant unit is 2kVA, and the variac is similar in rating, this provides very little in the form of overcurrent protection, aside from the 10A fuses they each have in them. I use care in using them, being aware of the danger associated with voltages that are above the SELV level of 50V peak. Much better to probe with a lower voltage applied to the control side to find faults, and no mains applied, you lose a lot less of the output of the foundry that way.
Shock:
--- Quote from: Electro Detective on June 25, 2017, 07:02:29 am ---and a BIG thanks for nothing to the "JUST DON'T DO IT" brigade :--
when all that a responsible technician, advanced hobbyist or EE requires is some simple 'how to and how not to' information,
so they can decide if their equipment and skill set is up to the task (or not) for a particular probe and measurement scenario,
taking ALL possible safety precautions,
including 'think before you probe' :-// ::) :-+ :-/O
Otherwise most uninformed/misinformed/Darwin Dumbass wannabees will try something NQR anyway, and BANG ! :o :horse:
--- End quote ---
Unfortunately explaining how and why over and over seems to add fuel to some who think the ground/earth lifting is a "hack" or "work around". "Don't do it, it's dangerous" is a perfectly adequate response considering the amount of people repeatedly chiming in saying it's acceptable practice or just be careful.
Fungus:
If I'm an outsider reading a thread that argues this both ways I'll be trying to figure out:
* What problem is solved by floating a 'scope?
and
* When might I want to do it?
The answers are "none" and "never" so I don't think those lines of thought should be encouraged by saying, "It's perfectly OK so long as ${LONG_LIST_OF_RULES}".
The only correct answers are:
a) Get a differential probe
or
b) Use AC coupling.
If you're doing (b) and there's a DC offset then tough. Oscilloscopes are for looking at the shapes of waves, you can see ripple just fine even without the correct DC offset. Connect your multimeter in parallel if you want that number.
mtdoc:
I've seen very few people on this forum, in this or the multiple past threads on this topic, say that floating a scope is ever ok. The vast majority of posters always say "don't do it". That is as it should be. The more important thing IMO, is the explanation and discussion about why it is not ok to do it. There are many people who will not heed safety advice unless they can be made to understand *WHY* a certain practice is unsafe. "Teach a man to fish, etc..'' That is what this forum provides: experienced engineers and hobbyists who can go beyond the glib "don't do it" or " it's ok as long as you.." type responses seen so many other places on the web. It's only through discussion that this teaching/learning happens.
The use and miss-use of isolation transformers is another closely related and perhaps more complicated topic. The multiple discussions around that have also provided great learning opportunities.
tautech:
--- Quote from: mtdoc on June 25, 2017, 10:44:46 pm ---I've seen very few people on this forum, in this or the multiple past threads on this topic, say that floating a scope is ever ok. The vast majority of posters always say "don't do it". That is as it should be. The more important thing IMO, is the explanation and discussion about why it is not ok to do it. There are many people who will not heed safety advice unless they can be made to understand *WHY* a certain practice is unsafe.
--- End quote ---
I touched on *WHY* with the 8 questions in reply #41, answer all those with something that covers ALL floating usages and we'll have something to move forward with.
If you can. :popcorn:
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