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Oscilloscope 'floating' measurements

There's no question that as technicians repairing consumer electronics, we have to be able to use a regular isolation transformer for the UUT and sometimes even connecting the scope ground to the "hot" ground. This always makes me nervous, and I always look carefully for any other ground path so as to avoid unpleasant surprises. I'll even check the safety of the ground connection using a 1/8 ampere fuse if I have any doubt.

Tektronix has a couple interests here which work against us. Obviously they want to sell more test equipment, but also they cannot control who uses their equipment and the pool of potential lawsuits is very large. Even being an unknowledgeable or incompetent technician does not protect Tek from lawsuits. Damn lawyers - a rant for another day...

Arfa (Geoff) is very correct that a power transformer in the scope will never suffer any cumulative damage from proper us of an isolation transformer.

I'm pretty sure the earliest Tek model only had a two-lead power cord, and their power transformers have held up fairly well the past 50 years.

Mark Z.

Reply to
Mark Zacharias
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"Mark Zacharias" wrote in news:4edb7cf1$0$29354$ snipped-for-privacy@newsreader.readnews.com:

As far back as I can remember,TEK scopes all used 3-wire power cords. the earliest used a round twist-lock connector with a center ground pin.

BTW,the invert/add function on dual-input scopes makes a pretty poor differential amp. you'll see a lot of artifacts that aren't really there.

--
Jim Yanik
jyanik
at
localnet
dot com
Reply to
Jim Yanik

My mistake, then. Nevertheless many early scopes had 2-wire power, as I'm sure you know. I had an instructor in tech school who was an old-timer and he said the best thing to do with that third wire was cut it off.

Seriously.

Mark Z.

Reply to
Mark Zacharias

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e.

est

Don't slip with that knife, you might accidentally cut something else off that you may someday need..... Many years ago I had a physics instructor at RCA Institutes who once who told the class "there is 6 million volts of electricity in a lightning strike". As he tried to skate right past this bullshit comment I raised my hand and asked "how did they determine that"? His jaw dropped, his face went pale, (he obviously didn't have a clue) and he replied, "well someone measured it with a Voltohmyst", what did you think"? Everyone had a good laugh, except me that is. Mr. Daras was a real asshole but there were many other good teachers there that made up for it. I've found this discussion very interesting because I've done it all three ways. I've busted ground on both the scope and the DUT, just the scope, and other times just the DUT. My intention every time was to isolate building ground on a line operated device from scope ground. And although each method accomplishes that, after reading this thread one can readily appreciate that busting ground on the test equipment is never a good idea. As someone earlier said, ground is put there for a reason. And although interrupting it may protect the equipment, it doesn't do a thing to protect the operator. Any line operated electronic equipment I work on on my bench is plugged into a variac which in turn is plugged into a 1: 1 isolation transformer. For a little extra measure of safety I also check for a difference of potential between my test equipment and DUT grounds. If you isolate them there should be none. Lenny

Reply to
klem kedidelhopper

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