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My name is Nick. I’m a idiotic prideful 17 year old who didn’t think he needed to watch a youtube video before using his dad's oscilloscope. What makes this post more ironic is that I purchased an “oscilloscope tutor kit” which is just a PCB and tutorials to teach me how to use an oscilloscope. Where my trouble came in was it didn’t come with a power supply and it says “9V AC" on the PCB next to the barrel jack.

I found a very old AC bench power supply in my barn and checked it with a multimeter until the voltage read 9V RMS. I cut the end off of an extension cable and soldered that to a barrel jack that fit the socket. This is my first problem - I soldered the ground of the connector to the neutral wire not knowing there was a difference between neutral and ground. After that, having only used a multimeter before, I connected the ground of the oscilloscope lead directly to the grounded outer part of the barrel connector which tripped a GFCI and killed the power in my basement. Being the idiot I am, I reset the GFCI and did it again to the same result. The probe was in 10x mode if that helps at all.

Since it was only 9V RMS and I was in 10x mode on the lead, did I still harm and or destroy my dad's oscilloscope? I am very sorry for the length of this post and how basic it is but I need help.

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    \$\begingroup\$ Probably not , there’s only a few % different if you are drawing max current on a breaker thru neutral . Neutral is earth grounded further away either thru copper plumbing and at the distribution transformer. Just fix it. Besides the 9V transformer is isolated if it was on a barrel plug. The GFCI detected the current in balance and tripped. No drama. Fault detected. \$\endgroup\$
    – Tony Stewart EE75
    May 4, 2021 at 3:49
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    \$\begingroup\$ Ah, you reminded me that not too many years ago, I used my scope to display my home's mains 220VAC, hoping to see a 50Hz, 220V rms sine wave. But my home's MCB (Main Circuit Breaker) immediately tripped! Then I knew that my scope probe/ground is connected to the mains ground. Later I used a handheld scope, and successfully display the AC wave. I do also later use my scope to display the secondary, floating, side of my 220VAC to 12VAC transformer though. \$\endgroup\$
    – tlfong01
    May 4, 2021 at 4:01
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    \$\begingroup\$ Does the scope work? Then you're good. The fault current would have passed through the safety ground of the O-scope. Unless it's a piss poor design, all the current should have been sent to ground and none to anything sensitive inside. It should survive no problem. Especially since a GFCI trips at like 10mA. Circuit breakers trip at 15A or more. You'd have known the difference, cuz if you tripped a breaker doing that you would have also seen one heck of a spark. This would have scared the daylights out of you, and you'd have been shaking too hard to try it a second time. \$\endgroup\$
    – Kyle B
    May 4, 2021 at 4:30
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    \$\begingroup\$ Ah, Dave Jones of EEVblog is my friend! EEVblog #279​ - How NOT To Blow Up Your Oscilloscope! - 2012may16 739,817 views, 1495 comments \$\endgroup\$
    – tlfong01
    May 4, 2021 at 5:09
  • \$\begingroup\$ @nicksmith: A Youtube video is the last place to start. A better place is a guide from a well known oscilloscope manufacturer. I doubt you would find anything directly addressing your particular case, though. Your problem was more with the power supply for your “oscilloscope tutor kit." You rigged up a substitute for a part you don't understand (9VAC power supply) using another thing you didn't understand (a Variac.) Neither has anything to do with the oscilloscope. \$\endgroup\$
    – JRE
    May 4, 2021 at 9:23

1 Answer 1

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What I gather from your post is that you connected one of the leads of an auto transformer, which is not isolated, to the oscilloscope ground. The fact that the probe was in x10 mode or even the voltage at which you set the transformer doesn't matter. You created a short circuit through the oscilloscope's ground.

It is imposible to know if it was or was not damaged only from what you said. If it turns on and you can still measure with that channel it is probably fine. If it is damaged, you may or may not be able to have it fixed.

I would definitely recommend talking with your dad about what happened. He can help you determine if there is damage or not, and he deserves to know. Worst case, you made a mistake and have to pay for it. We all do, but at least you can make things right again.

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    \$\begingroup\$ This answer not only answers the OP's question but it is good in creating a bonding relationship between parents and son!+1. \$\endgroup\$
    – Miss Mulan
    May 4, 2021 at 4:04
  • \$\begingroup\$ I think one of us misinterpreted the situation. It was an “ old ac bench power supply “ meaning it was just a step down transformer (isolated) so there were no live voltages on either output . ! But apparently you convinced (7) so far.IMHO It is NOT the correct technical answer but sage advice otherwise. \$\endgroup\$
    – Tony Stewart EE75
    May 4, 2021 at 14:52
  • \$\begingroup\$ It does sound to me like the short created was between neutral and earth, which will indeed trip the GFCI, (with little other side effect in normal circumstances) \$\endgroup\$
    – Rodney
    May 4, 2021 at 14:54
  • \$\begingroup\$ @Rodney With no neutral current , not a short, the imbalance line neutral causes the trip. it does not sense ground current at all \$\endgroup\$
    – Tony Stewart EE75
    May 4, 2021 at 14:55
  • \$\begingroup\$ yes specifically what happens is that the neutral will not be at exactly 0V due to other loads on the circuit. This is normal, but even a tiny variance from 0V will be enough to create the 30mA or more of mismatch that is required should you short this directly to ground. \$\endgroup\$
    – Rodney
    May 4, 2021 at 14:57

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