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Recently I've attempted to use an oscilloscope to analyze directly the output of a bridge rectifier and I've learned that this is impossible and dangerous unless using an isolation transformer. Also, along the way, I've learned many other things thanks to the wonderful help and support received on this site.

But I've an other question: Could I have any issue powering a circuit by means of a linear power supply (that internally has a transformer) directly connected to the AC mains power (TT system in Italy) and connecting the oscilloscope's probe to the same circuit?

In other words: Has any linear power supply intrinsically got isolated outputs or do I need to search for a particular linear power supply?

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    \$\begingroup\$ That would depend on your linear power supply. If it has an ungrounded mains plug then it can't be mains earth referenced. If it has a grounded mains plug then it could be, but is not necessarily, referenced to mains earth. If it is, then the black/0V lead is typically the point that is connected to mains earth. Which linear power supply you have? \$\endgroup\$
    – Justme
    Mar 6, 2021 at 19:11
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    \$\begingroup\$ M4Biz, Back in the day, linear DC supplies were usually isolated as the secondary wasn't galvanically tied to either primary source lead. Today, I wouldn't be so sure about it. There are new requirements related to balancing noise emissions and other considerations where there may be a capacitor tying the supposedly isolated output back to the primary side. Sometimes, two such connections. In these cases, you aren't strictly isolated and there can be a noticeable "shock" to you if you touch the output leads. For example, Lenovo and Microsoft both tie their outputs back to mains via caps. \$\endgroup\$
    – jonk
    Mar 6, 2021 at 19:40
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    \$\begingroup\$ M4Biz, So it is always a good idea to verify this when you get a power supply so that you know what it does. Leave it unplugged but turn on the power switch, and check both (+) and (-) to all of the mains prongs with an Ohmmeter. If you see anything that looks like a capacitor behavior when you do that, you know you've got a galvanic connection. Otherwise, perhaps it is isolated. Also, if you have the ability with your scope, you can often use the difference between two probes and ignore the grounding ring to get an isolated measurement. Or buy a scope that is isolated (battery type?) \$\endgroup\$
    – jonk
    Mar 6, 2021 at 19:45
  • \$\begingroup\$ @Justme. Hi I've this: tek.com/tektronix-and-keithley-dc-power-supplies/…. I think that it have isolated ouput . I've just contacted Keithley support for a confirmation. \$\endgroup\$
    – M4Biz
    Mar 8, 2021 at 16:08

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Linear supply has an isolation transformer, so the outputs are isolated from mains. But almost every lab PSU has also a ground connector, so you may or not define which output + or - is grounded or left floating.

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    \$\begingroup\$ Also consider using an isolating transformer - we avoid working directly with 230VAC where possible but where it’s unavoidable, an isolating transformer adds a limited amount of safety. In particular it means that you can make a ground connection somewhere in the circuit. As mentioned though, avoid working with high voltages where possible. \$\endgroup\$
    – Frog
    Mar 6, 2021 at 19:19
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    \$\begingroup\$ I think you mean "most linear power supplies". Non-isolated ones are rare, but do exist (mostly for high voltage applications) \$\endgroup\$
    – user16324
    Mar 6, 2021 at 19:33
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    \$\begingroup\$ Disconnect from mains and measure the resistance from each output with earth as your reference (frame or earth ground pin). It should be infinite. If it clears then power it up and measure voltage to earth. If it fails you have a problem, if it passes it is probably isolated. Also realize most bench or line powered test equipment, especially bench instruments are connected to earth ground. \$\endgroup\$
    – Gil
    Mar 6, 2021 at 19:39
  • \$\begingroup\$ Note also that usually, a lab supply states what is the safe AC/DC voltage you can pull the output over/under earth ground reference (e.g. you can't stack ten 60V units to reach 600V). That said, the isolation transformer is the most common safety measure to work on live AC circuits \$\endgroup\$
    – LuC
    Apr 21, 2023 at 6:36

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