Re: Oscilloscope grounding question...


"Default User"

How can you find out if a conductor is safe to ground?

For example, in a battery powered circuit since it is isolated from mains,
I should be able to ground it through the scopes ground lead and since it
is an isolated circuit, very little if any current should leak.

But what about various mains power supplies? How do you know if their
output is a ground that can be grounded safely?


** Well, you have to know or figure out just what you are dealing with.

I've been using a cheap meter I don't care that much about ($4) that does
current measurement. I have a cable that connects the mains ground pin
only to a wire and before I hook it up to the scope's ground, I've been
using the meter to test to see if any current flows through it to the
ground first.

Is there a better method?


** Yep.

Inspection should tell if you are dealing with an * isolated * supply - ie
one with a transformer inside it. It may be a switching supply or a iron
transformer. I you find it has neither - then BEWARE as the output is
NOT able to safely be connected to ground.

You could also use the ohms or continuity test ranges on your meter to see
if there is a direct ( few ohms or less) connection from one of the output
terminals to the AC safety ground of the supply.

If there is, then that terminal can be grounded to the scope but not any
others.

If there is none ( as with wall warts and the like) you can safely connect
the scope ground to any of the output terminals.



....... Phil


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