"Charles" wrote in message
news

"David" wrote in message
...
I'm a bit confused about the ground clip on oscilloscope probes. I have
read that it can cause a ground loop when the measurement circuit is
grounded to the same ground as the oscilloscope itself. Is the ground
clip only to be used when there is an isolated ground then?

I have been taking measurements from 12V car batteries and DC power
supplies. Should the ground clip always be used in these cases?

If I am correct the ground clip should not be used to measure between two
voltages as one would do with a multimeter (referencing a different
voltage in the circuit instead of its ground). It confuses me why I
would not be able to use the ground clip to do that. An explanation/link
would be appreciated.

Line operated equipment and bench oscilloscopes often lead to ground
loops. A big spark erupts when the ground clip touches the chassis! Has
probably happened to most workers at least once. Often, the circuit
breaker trips and that is the end of the matter (other than a burnt clip
end and soiled underwear). The reason is that the clip is grounded
through the 3-wire power cord on the scope. If the equipment is not line
isolated, it is possible for the chassis to be hot with respect to ground.
The short circuit that results is called a ground loop.

Solutions:

1/ Isolation transformer for the equipment (best) or the scope (shaky).
2/ Use the differential mode on a dual channel scope (be careful about
common mode voltage ratings).
3/ Use a separate differential amplifier before the scope.
4/ Use a battery-operated scope (again, one must observe the scope's
voltage ratings).
5/ Make a floating measurement (use an adaptor to defeat the scope ground
... this is dangerous but is often done ... please AVOID this method).

Hope that helps.



Thanks Charles.

I'm guessing DC power supplies are usually isolated or is it typical for the
negative to be connected to the line ground? I will check this with a
meter.

If the DC supply IS isolated (or if I am using a 12V battery):

Can I connect the ground clip to any point of the circuit and measure the
voltage of another point with respect to the one the ground clip is
connected to (as I would with a multimeter) or is there some reason that the
ground clip should be connected only to a terminal?

I think I'll avoid the ground-defeating adaptors in hopes of having a better
understanding of ground loops some day (I have read about these in the past,
I'm not sure why I'm having such a difficult time getting the concept down).

I will investigate differential mode on a dual-channel scope. I thought
this would involve using two ground clips and two probes and show the
difference between the two probes but I may be way off on this.

Thanks again,

David