"Wild_Bill" wrote in message
...
According to this part, it seemed possible that the OP (or possibly other
readers) may be inclined to do something risky.. even though earlier
replies recommended proper safe procedures (and the reasons for them)..
*** No question about the scope being grounded, no one suggested
otherwise. Tektronix maintain that floating measurements, taken in a
DUT powered by an isolating transformer, is bad practice. ***
Using a scope in the conventional/safe manner (earth grounded power cord),
on a Hot circuit (primary side of a SMPS) without proper isolation IS bad
practice, no question.
I know you knew that, Arfa.
So, that just takes the subject back to using a proper dual-trace model
with suitable input capacity, and a Differential Mode or Add feature
described in the scope's operation manual.. *or* using a proper isolation
xfmr between the utility power source and the DUT (device under test).
My opinion is that using a proper I-T is the best overall solution.
BTW, it hasn't been mentioned yet, so I'll throw it in here..
A properly isolated DUT is safe in terms of there is no path to the
utility power source for the test equipment or the operator/user to
accidently come in contact with.
That doesn't mean that the operator is fully safe from electrical shocks
or electrocution, if one is careless.
The 'one hand rule' is safe practice.
There can be high voltage potentials and sources of current within the DUT
to present hazards for the tech/equipment operator.
Safe practices are still required, even though the threat of having
line-sourced current pass thru the operator to earth ground is eliminated,
the DUT when powered may still be capable of injuring the operator/tech.
Safety information is covered in the Repair FAQ
http://www.repairfaq.org/
--
Cheers,
WB
.............
"Arfa Daily" wrote in message
...
"Wild_Bill" wrote in message
...
It's not clear what you're after (echo).
For most technicians, the term "floating" in connection with scope use,
refers to 'floating the scope' which means:
Read This Part separation of the scope power cord's 3rd pin from earth
ground.
I didn't look at Tek's cautions/recommendations, but I know scope
manufacturers are familiar with the (nearly always) ill-advised practice
of Floating A Scope.
Are you unfamiliar with proper and safe practices, or unfamiliar with
using a scope, or unable to express what it is you want ask about?
All of the above?
I'm fairly certain that all of Tek's recommended procedures are safe..
maybe you're just not understanding what's presented in the literature..
or maybe you're trying to apply the wrong procedure for your intended
purpose.
As far as various types of electronic gear/equipment go, it's very
common that there are floating voltages present inside them.
No one was placed here to try to understand what you meant, even though
you didn't say it.
Maybe you were thinking of sci.telepathic.electronics.
--
Cheers,
WB
.............
The OP was very clear in what he wanted to do i.e. make scope
measurements on the primary side of a non isolated switcher. The only
'problem' was that he misinterpreted what had been said in a Tek
application note, which he now understands correctly for what it was. I
have seen no indication that he is incompetent at using a scope or
understanding safety procedures anywhere in his post or various replies.
He has merely asked for what people on here who are involved in this type
of work, recommend in terms of safe practice from both a personal injury
and equipment damage point of view. There has been a consensus expressed,
and as far as I can see, he accepts and fully understands what he has
been advised.
Arfa
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.