Another scope problem

[MikeK]

I'm working on a Fluke 8050A multimeter.  Scope is Rigol DS1052E, shouldn't matter though.  Scoping things works fine.  I also have a PIC micro in the Fluke and attached to a PICKit2.  Programming the PIC works fine.  If I try to scope something while the PICKit is attached the PIC goes haywire.  Unless I connect the scope probe ground to the same potential as the PICKit ground.  I'm not sure I understand this.  I'm guessing it has something to do with the PICKit ground being tied to USB/PC ground?

[ledtester]

The Fluke is powered by a transformer and thus is floating (as long as it isn't connected to anything else or it is probing another floating circuit).

The scope's ground probe is at the same potential as earth ground. So in a mains powered device you only want to attach it to earth ground in the device - otherwise you'll create a short.

Here's some more discussion viz. probing USB powered devices:

https://www.eevblog.com/forum/beginners/probing-usb-powered-circuit-should-i-be-scared/25/

If the PICKit is getting power from a laptop running off of batteries and there are no other electrical connections to earth ground, it should be safe to place the scope's ground probe anywhere in the PICKIt.

In general, though, I wouldn't connect your oscilloscope's ground probe to anything but the ground of the circuit you're testing. Also, understand how your circuit is powered and what relationship the power supply has to earth ground. A lot of the time the DC ground of an AC adaptor is either floating or the same as earth ground but some AC adaptors can work differently.

[MikeK]

The scope's ground probe is at the same potential as earth ground. So in a mains powered device you only want to attach it to earth ground in the device - otherwise you'll create a short.

I've watched Dave's video on the subject, about not "blowing up your scope".  Several times.  And he says that with a transformer-based device, where the secondary side is not referenced to earth, you can put the scope probe ground clip anywhere.  And indeed I can.  It doesn't create a short.

My problem only comes from having the scope *and* the PICKit attached.  I suspect this is because the two grounds (scope probe and PICKit GND) need to be connected to the same point and that doing otherwise does create a short.  No?

[bdunham7]

My problem only comes from having the scope *and* the PICKit attached.  I suspect this is because the two grounds (scope probe and PICKit GND) need to be connected to the same point and that doing otherwise does create a short.  No?

Yes, with an isolated DUT, you can select any point to serve as your 'ground' reference.  But you can't pick 2. 

[Doctorandus_P]

I assume your pickit is connected to a PC.

PC's should always be grounded, especially when they are a part of your electronics workbench setup.
If a (normal) PC is not grounded, then it has half the mains voltage on it's "gnd" because of the Y capacitors in the power supply. Because these filter capacitors are quite small and the currents are small enoughto not be dangerous, it can still feel unpleasant or wreak havoc in circuits that are designed for low voltages only.