Electronics > Beginners

What should isolation transformer read like on DMM

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lordvader88:
Well I'm not planning on touching the transformer and their's folks with lots of experience that recommend doing it that way as well as floating the scope by un-grounding it as-well. And yes that can be dangerous , but thats it.

Gregg:
You need to step back and look at the whole picture in terms of circuits and what is forming a complete circuit.  Your mains input has the neutral and earth ground connected at some point in the circuit; the wall outlet you plug the transformer into can be considered a source of power.
The secondary wires of the isolation transformer can be considered an isolated source of power; however there is inductive and possibly capacitive coupling inherent in the transformer.  Good isolation transformers have the primary and secondary wound on separate coils to minimize the obvious problem of capacitive coupling. 
What you need to do is consider what potential power may or may not be available at any part of your complete circuit and what the ramifications of probing or putting your bare fingers into the circuit before doing anything rash. When in doubt, draw it out on a sheet of paper; remember to draw the complete circuit, not just the part you are about to commit to a Darwin Award or magic smoke.

BravoV:
Rather than risking your life, your scope and also your equipment. Why not consider buying a HV differential probes to your scope, its not very expensive these days, and for sure its much safer than using an isolation transformer. Here an example on how convenient to use it to measure mains rated lines/voltage related, the scope was measuring the mains lines 220 VAC at the same socket where it was powered and plugged.


lordvader88:
I'm no fun at all, I don't even like discharging caps with a screwdriver, since it might damage the cap, or the PCB.

Generally I turn off the DUT if I want to move a probe wire, just in-case I short +2 points. I'm cautious for sure.

I'll be alright for now, but yeah I'll get some diff-probes as well someday. I have a current limiter, auto-transformer, and now isolation transformer, and lot's to learn.

Gregg:

--- Quote from: lordvader88 on August 25, 2018, 02:19:39 am ---I'm no fun at all, I don't even like discharging caps with a screwdriver, since it might damage the cap, or the PCB.

Generally I turn off the DUT if I want to move a probe wire, just in-case I short +2 points. I'm cautious for sure.

I'll be alright for now, but yeah I'll get some diff-probes as well someday. I have a current limiter, auto-transformer, and now isolation transformer, and lot's to learn.

--- End quote ---
I used to be a lot more cavalier when I was younger.  The rule is that your first mistake could be your very last.  Here is a picture of me 12 years ago taking a voltage drop reading in a 3000 amp 480 volt transfer switch with a 2000 amp load on it.  Things were getting hot as noted by the 0.1288 volt drop across a bolted bus connection.  I was wearing 1000 volt gloves, a cotton long sleeve jacket and a face shield.  These days after all of the arc flash safety over-reactions the amount of personal protection required to meet OSHA guidelines would be prohibitively bulky.  The biggest part of staying safe is staying smart and keeping distractions away. 

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