Isolation Transformer for Scope - Page 1
Forum section: electronics / projects

author: jaipursky
author_rank: Newbie
timestamp: March 10, 2019, 12:45:56 am
content: 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?

author: ogden
author_rank: Super Contributor
timestamp: March 10, 2019, 12:55:59 am
content: Quote from: jaipursky on March 10, 2019, 12:45:56 amSure. 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.

author: stj
author_rank: Super Contributor
timestamp: March 10, 2019, 09:43:41 am
content: 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.

author: ArthurDent
author_rank: Super Contributor
timestamp: March 10, 2019, 02:07:36 pm
content: QuoteThe 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.

author: magic
author_rank: Super Contributor
timestamp: March 10, 2019, 09:16:01 pm
content: I have floated my scope sometimes...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 plugOne 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.

author: tggzzz
author_rank: Super Contributor
timestamp: March 10, 2019, 09:48:07 pm
content: Quote from: magic on March 10, 2019, 09:16:01 pmThat is a stupid, dangerous suggestion to make to a newcomer to the forum. We have no idea of their experience nor competence.Biting my tongue, I'll merely point out that I have walked into a busy road without looking.QuoteFor every problem there is a simple easywrongsolution.That endangers the scope (nobody cares), the scope user (probably the scope user cares, do you care?) and the UUT - especially if the UUT is an SMPS.Read and understand these threads:https://www.eevblog.com/forum/testgear/floating-scopes/https://www.eevblog.com/forum/testgear/ground-of-oscilloscope-always-connected-to-earth/See what's "necessary" to float a scope so that the user is not endangered:https://www.eevblog.com/forum/testgear/to-isolate-test-equipment-eg-oscilloscope-using-ups/msg2017633/#msg2017633but note that won't "save" the UUT.Read theprofessionalway to do approach safety with scopes and probes in the refs athttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/Examplefatalityfromhttps://groups.io/g/TekScopes/topic/7632418#10795withmy emphasisQuoteExcerpt from Tektronix' "Fundamentals of Floating Measurements and Isolated Input Oscilloscopes Application Note"https://info.tek.com/www-fundamentals-of-floating-measurements-and-isolated-input-oscilloscopes_ty.html,withmy emphasisQuoteFundamentally either use the (special) right probe for the job, or the (special) right scope for the job,or don't do the job.

author: Electro Detective
author_rank: Super Contributor
timestamp: March 10, 2019, 11:18:32 pm
content: Quote from: tggzzz on March 10, 2019, 09:48:07 pmThat about sums it up perfectlyFor those few 2019 Darwin Award Contenders that still won't buy:It can be done ***IF*** the isolation transformer is a TRUE isolation transformer (most times NOT, without a hack or two, or three...)and physically isolated from the surrounding area (rubber feet, wooden box, custom leather profilactic)Once confirmed that you have indeed a TRUE isolation transformer, properly zapped and SAFETY pass granted with an insulation tester at 250/500v,and good solid shake-proof internals and secured cable tied wiring,then, and only then, do you begin to learn the correct method to use it with a scope for that 'one off job' that some will do... regardless of the advice and cautions presented by experienced (and perhaps bitten) folks here,some that probably took out a scope or two, and luckily not an eye or two, and or a heartbeat skip or two.. finding out how 'isolation' can go south during a heated experimentand or fading patiencerepair troubleshoot.Ground/Earth cheaters are a no no especially if the scope has a metal housing and or touchable screws or metalwork, and the condition or insulation of the scope transformer is unknown or old,and maybe an aging capacitor or two tied to metal chassis, to keep interference levels low..and user risk levels highOne must also confirm just what's coming out of the wall socket to begin with, and verify all power cabling to the isolation transformer and scope are in good shape and proper polarity.The scope probe leads, clips and 10x switch and internals,are they REALLY ok ?or a drama about to unfold when moved about, even though everything else 'appears' perfect ?Even if you get it all spot on perfect, I would have PPE eye-wear, thin leather gloves, long sleeves shirt and rubber sole shoes/boots,i.e. I don't want to come into contact with anything!This way chances are better you only pay out for fried electronics and not on hospital/funeral fees if or when it all goes horribly wrong.I can keep going, but hopefully I've painted a fair picture of -Murphy Isolation-If you're not 110 percent sure and or experienced, with someone nearby just in case, DON"T DO IT !FWIW those experienced Youtubers and forum gurus posting their methods won't be there to bail you out...