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I just experienced a big bang caused apparently by a short through my oscilloscope's ground lead, and wanted to see if you could shed light on the why this happened.

Yes I have watched Dave's video on the subject of scope ground leads, and was trying to be very careful. But obviously not careful enough. :-/

I was diagnosing a washing machine control board. I wanted to view the tachometer output pin's status while in operation to see if it was working properly.

The tachometer (optical sensor) is powered by 13VDC, provided by an on-board power supply.

I connected the oscilloscope ground clip to the 13VDC ground. This is indicated in the schematic below. I connected the oscilloscope lead to the tachometer input pin. I did not disconnect the connector from the control board, but instead piggybacked the scope probe and ground clip into the back of the connector.

When I plugged in the machine's 120VAC power cord, BANG went the washing machine's on-board power supply. The bang was definitely more powerful than I would expect from 13VDC, and let the magic smoke out of several SMD components around the PSU. The oscilloscope was not damaged.

I simply cannot figure out why this connection created a short circuit. Even if the 13VDC ground was isolated from the chassis ground, why would it be at a high potential difference from chassis ground? That would seem dangerous.

I tested the electrical outlet to ensure it's wired correctly.

Thanks for shedding any light on this.

Schematic: (full manual here) enter image description here

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    \$\begingroup\$ Are you sure the bang was caused by oscilloscope probing? Are you sure you couldn't cause some accidental short? \$\endgroup\$
    – Chupacabras
    Commented Apr 1, 2017 at 18:56
  • \$\begingroup\$ I was very careful and nothing else was unplugged or loose which could have caused this. Even if I had mistakenly connected the ground lead one pin left or right, it would have only been a 13vdc direct short if the chassis ground was connected to the 13vdc ground. I am quite confident this did not happen however. The amount of damage was way more than a tiny 13vdc power supply could inflict on itself. \$\endgroup\$
    – Ryan Griggs
    Commented Apr 1, 2017 at 19:00
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    \$\begingroup\$ One hint to be extra sure in the future: you don't want any current to flow through your scopes gnd. This would happen if there is a potential where you want to attach it. You can check what with a multimeter. Ac and DC. \$\endgroup\$
    – PlasmaHH
    Commented Apr 1, 2017 at 19:45
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    \$\begingroup\$ Great jumping cats! Went flipping through that service manual, and it appears that nothing in there is isolated from the mains. Clipping your ground to just about anything inside would cause a short circuit with everything the mains can provide available to provide current for the bang. \$\endgroup\$
    – JRE
    Commented Apr 1, 2017 at 20:09
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    \$\begingroup\$ @RyanGriggs Fuses on the ground lead would mean the voltage in the even of a short can rise to lethal voltages, and if you then touch the bnc connector jack of your oscilloscope by accident (for example when you try to adjust the settings because you get weird results), you can get shocked by the bare metal \$\endgroup\$
    – Ferrybig
    Commented Apr 2, 2017 at 15:44

2 Answers 2

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Note that the 'ground' symbol on that 13V rail is NOT the same as the main safety ground symbol by the power cord.

Look at the whole wiring diagram and notice the two triacs a little lower down, if you are designing this thing you can save a couple of opto triacs by referencing the 13V line to the incoming live....

Get paranoid, a multimeter check would have saved you some SMPSU repair work.

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Always check the voltage between the ground scope lead and the circuit ground before making the connection. If there is a non zero value, try reversing the AC plug on the scope OR on the circuit feed. If both are three prong plus and thus you can't reverse either one, then you will need a 1:1 isolation transformer. Good luck!

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