Hello there,
I recall videos where Dave has commented that if you probe stuff that is earth grounded together with your scope, you can get short circuits. However I don't recall what the justification for the statement was.
Basically I want to probe an FPGA demo board, which is powered by a PC USB, and I don't want anything to blow up. Is this safe?
Thanks
This thread has suffered massive "mission creep", so I will initially address the actual question.
If your USB GND connection is returned to the chassis of the PC, & hence Mains Earth, ( as it probably will be on a desktop PC), if you get your probe connections wrong, & put the "earth (ground) clip" on the +5 volt supply, you will short circuit that supply, see a spark, & probably kill your USB port,
but it is most unlikely to damage your Oscilloscope.
With a laptop, the 5v supply will
normally be "floating" with respect to Earth, so there will be no damage to either the 'scope or the USB port.
That said,
some peripherals connected to the laptop may place an Earth connection on the USB port GND pin..Another possibility is that a desktop PC may have its USB GND connected to chassis, but have no Earth conductor in the power cord.
In this case, it is not unusual for the chassis to be at around half the Mains voltage w.r.t Earth, so connecting the probe correctly with the ground clip to GND may cause a tiny spark.
This will not damage your 'scope, as the current available is tiny.
As is usual with questions re probing circuitry, the thread went off at a bit of a tangent, with "much wailing & gnashing of teeth" about the dangers of probing Mains, etc.
Most Mains Electricity supplies are similar to the Australian MEN system, which, for historical & good Engineering reasons, returns one side (called the Neutral) of the Mains to Earth ( the "Earth" itself, with earth stakes), both at the Power Station, & at various points along the way, including at the entry point to your premises.
The single phase wiring usually used runs a "Protective Earth" (PE)conductor from that point to all the power sockets (GPOs in Oz speak).
If you measure across the Active & Neutral pins of the GPO with a (suitably rated) DMM, you will see
(in Oz),nominally 230v RMS.
Between Active & PE, it will be "near as dammit" the same.
Measuring between Neutral & PE you may see a volt or less.
(but not always!)*The big bugbear with people measuring Mains with an Oscilloscope, is that, if you get it "wrong", & put the "groundclip on the Active, you will see a blinding flash, & hopefully the Residual Current Device will trip, protecting the circuit.
If you don't have a RCD, it is up to the "old school" fuse in the "meter box" to blow, protecting both you & hopefully your 'scope.
Classic Oscilloscopes had their groundclip connected back to the chassis via the BNC connector, & hence to the earth conductor in the 'scope's power lead.
Unfortunately, some modern 'scopes run the Earth connection as a track on the main PCB.
This may burn out, so that you no longer have the protection which several generations of hardworking EEs built into the power distribution system.
Without that intact track, next time you make this same mistake, there will be no "blinding flash" & everything will work OK,
BUT!! the BNC connectors, & everything else normally returned to Earth in the instrument will be at Mains Active.
If you now touch any of that metal, you will get a dangerous shock.
If you have a RCD fitted, it should hopefully not be fatal.
If not, send us your address beforehand so we know where to send the flowers!
The same applies if you deliberately remove the power lead Earth.
*But, Wait!---There's more!.
It would seem that you are OK,if you put the groundclip on the Neutral,
BUT!! sometimes if there is a lot of current being drawn by a device on the same circuit, the Neutral may be elevated enough due to volt drop in the line, that it can still cause a spark, killing your PCB track.
This is why, people such as myself who worked on SMPS used an isolation transformer on the DUT,
( not the 'scope).
OK, you're pretty much convinced about the Mains thing---how about power supplies inside equipment?
Yep!, some of them can supply enough current to damage the PCB earth track in just the same way.