Electronics > Projects, Designs, and Technical Stuff
Isolation Transformer for Scope
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jaipursky:
Simple question: planning to buy an isolation Transformer to troubleshoot hot side power supplies.
I plan to buy it as per scope's  power rating.

I see some suggestions online to buy the isolation Transformer for the Device under Test. Devices and vary in power consumption so it requires a really large transformer if we go this route.

Is there any specific scenario in which DUT needs to be isolated and not the scope?
ogden:

--- Quote from: jaipursky on March 10, 2019, 12:45:56 am ---Is there any specific scenario in which DUT needs to be isolated and not the scope?

--- End quote ---

Sure. Scenario where operator cares for his life. Isolated scope may work till your first error when you grab scope part which is connected to live AC mains and... die. Either isolate DUT or even better - use high voltage differential probes.
stj:
isolation transformers wont save your life, only your scope.
anybody who thinks an isolation transformer cant pass enough current to "get the job done" shouldnt be working with mains.
ArthurDent:

--- Quote --- "Is there any specific scenario in which DUT needs to be isolated and not the scope?"
--- End quote ---

The answer is every single scenario is one where the scope definitely should not be isolated. This question, for some unknown reason pops up on a weekly basis and if you search the forums you can find this question answered again, and again, and again... Generally the device under test shouldn't be isolated either. Search and you will find many references and videos on differential/isolated scope probes and cautions about doing any of these measurements if you don't know what you are actually doing.







magic:
I have floated my scope sometimes...  :box: :popcorn:

The really quick and dirty, cheap-ass solution is to just disconnect Protective Earth. The scope already contains a transformer which isolates its ground from mains, both line and neutral, but the ground is still connected to PE. I break this connection using a suitable adapter which goes onto the scope's mains plug ;)

One immediate consequence is that if the scope's transformer breaks down, mains could appear on the scope's ground. So I always attach all probes before applying power and operate the scope with a PVC stick, regardless of what DUT it is "grounded" to.

Never "ground" it to any high voltage with respect to mains. Same applies to any other unearthed, mains-powered device too. I don't know how many secondary-to-primary volts those transformers can withstand and don't wish to find out. For probing in rectified mains circuits like PSU primaries, it is prudent to use the same mains phase as the scope is connected to.

Finally, if I were to deal with all the hassle and hazard more than once per year, I would probably get myself proper differential probes.
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