Is it possible to power a circuit in such a way that makes it impossible to fry my oscilloscope with a ground loop?

I've heard using a battery is a good way to prevent this. Is there a other way I can eliminate the threat of ground loops without having to use a battery? I'd like to be able to use mains power. Perhaps with an optical isolator?

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    \$\begingroup\$ Isolation Transformer \$\endgroup\$ – brhans Jul 25 '18 at 21:10
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    \$\begingroup\$ (1) What kind of power supply (voltage, current, DC or AC) does the device under test require? (2) Optical isolators can't deliver any appreciable amount of power. Optical isolators are for signal. \$\endgroup\$ – Nick Alexeev Jul 25 '18 at 21:31
  • \$\begingroup\$ @NickAlexeev I am not sure, I don't have any particular device in mind. Probably DC 0-30V. It would be nice if I could power a regular bench power supply in a way that would isolate it. \$\endgroup\$ – 888 Jul 25 '18 at 21:35
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    \$\begingroup\$ All regular bench supplies are already isolated from mains AC. Thousands of people use oscilloscopes with full success. Where your concern is coming from? \$\endgroup\$ – Ale..chenski Jul 25 '18 at 22:08
  • \$\begingroup\$ @AliChen Ah, I didn't know bench power supplies were already isolated. I guess I have nothing to worry about then :) \$\endgroup\$ – 888 Jul 25 '18 at 22:12

0 to 30V DC. It would be nice if I could power a regular bench power supply in a way that would isolate it.

In a typical bench power supply, the outputs are isolated from the mains AC. The isolation rating are around 500V, typically. Both positive (red) and negative (black) outputs are floating with respect to mains AC. Green terminal is directly connected to the earth ground.

A regular bench power supply should take care of the isolation needs which you have described.

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    \$\begingroup\$ You should mention some typically small (~ 100 uA) leakage into mains, so a bench AC-DC supply, when not grounded to scope ground, may produce scary AC voltages on high-impedance scopes and scare novices (and sometimes break functionality of test boards :-) \$\endgroup\$ – Ale..chenski Jul 25 '18 at 22:20
  • \$\begingroup\$ Even for medical-grade power supplies the acceptable leakage can be up to 500 uA, mddionline.com/leakage-current-standards-simplified \$\endgroup\$ – Ale..chenski Jul 25 '18 at 22:24
  • \$\begingroup\$ Interesting. My power supply has a piece of metal connecting the - and GND connectors. Does this mean I don't have a floating ground output on mine? \$\endgroup\$ – 888 Jul 28 '18 at 22:32
  • \$\begingroup\$ @888 Post a link to the power supply manual, please. \$\endgroup\$ – Nick Alexeev Jul 28 '18 at 22:51
  • \$\begingroup\$ @NickAlexeev I couldn't find a pdf of the manual on the internet, but it is the Dr. Meter PS305DM \$\endgroup\$ – 888 Jul 29 '18 at 4:32

If you test a power supply with a live ground such as an offline supply and return , and there is no earth ground in the circuit but you know it is low voltage output, you would use earth ground for two probes and invert channel 2 and Add Ch1&Ch2 to make a differential measurement.

If the supply is transformer isolated and floating you can use probe earth ground on either output if you know the output is < scope Vmax such as 200V.

If you are expecting a live ground fault and have the scope plugged into another outlet without a ground fault then connecting the probe ground may bridge the live ground creating a current fault possibly in the scope earth path to line plug. Thus isolation of grounds might protect the scope ground path but will not protect you, so my advice is use the same outlet as the supply under test. If you are unfamiliar with the site, AND suspect a live ground fault then check device ground with the probe ungrounded.

However understand that if the supply return 0Vdc is floating , it is common to see stray voltages say up to 1/2 of the line voltage if it has a leakage resistance to line equal to the probe resistance to ground. However the current is limited in uA due to resistance. So using the probe earth ground is useful to absorb these low currents and get low noise results.


I recall in mid ‘70’s while working in an Aerospace electronics R&D lab next to a machine shop, I got a healthy shock of maybe 10mA and high voltage connecting a long coax cable from one side of the lab to an instrument on the other side. As it turned out there was a live ground fault for some reason that I never found out. But it did not cause any failure, and it was reported to the Electrician to repair.

  • \$\begingroup\$ Basically a floating AC-DC PSU, with some powered test board or device, is like touching the hot AC mains wire via a 200-300 kOhm resistor. \$\endgroup\$ – Ale..chenski Jul 25 '18 at 22:41
  • \$\begingroup\$ The coupling for a linear transformer can be > 10Meg (x pF, don't care at 50,60Hz) with a line filter will leak up to 500uA to ground from 2 Y caps. But a switched DC-DC SMPS transformer must be low capacitance coupling to floating secondary tends to be much lower impedance at 50k to 100 kHz causing concerns with laptop users with tingling or slight burning sensations on case corners. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jul 25 '18 at 23:09

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