Using a scope

[Gregory Chan]

> 
> At 09:29 AM 6/24/97 -0400, Frederic Breitwieser wrote:
> >My scope has two seperate leads, one with an alligator clip, one with a 
> >sharp point (mine's kinda flattened out actually), and putting across 
> >anything automotive so far hasn't resulted in anything exploding, smoking, 
> >or making sizzling sounds.  I do have a 100ma fuse in series with the 
> >ground lead, which has popped on occasion.  Maybe my scope's input not be 
> >grounded?  Or maybe its high impedence?  Maybe its just broken?  Its 
> >certainly an old scope....
> 
> unless you've got an old Dumont or such that has a "Sync" instead
> of a trigger (i.e. all vacuum tubes .... and still old even for
> those days), the scope ground lead is connected to the AC line
> ground -- check it with your ohmmeter.  There's normally
> no problem since bench power supplies (and your car) are
> isolated from the AC mains (unless the bench supply has
> one of the outputs grounded or some other piece of equipment
> hooked into the circuit is connected to the AC ground).
> 
> The 100 mA fuse should *never* blow -- after all, the
> scope's input Z = 1 megohm (not counting the shield/ground
> being connected to the chassis).
> 
> >>At 11:42 AM 6/23/97 -0400, Frederic Breitwieser wrote:
> >>
> >>>Fear not Mr. Mechanical <smile>, most 'scopes have a high enough impedance 
> >>>(loading capacity) that you can snoop around circuits without causing
> >>>issues, and you can directly attach your 'scope to the injector, and view
> >>>the pulses directly.
> >>
> >>Fred .... remember that the scope probe ground clip is attached
> >>directly to the scope chassis and then to the AC ground.  Care
> >>needs to be taken where it's attached in a circuit -- it's
> >>NOT high Z
> 
> 
That's where the isolation transformer comes in or you can break off 
the ground tab on the mains plug so there is no return path. This is 
not recommended by the electrical authorities though.

gchan at compserv.senecac.on.ca