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

Phil Allison wrote:
"Phil Allison"
** Typical ground loops can be eliminated by fitting a resistor between
the scope chassis ground and safety ground of about 10 to 22 ohms. For
obvious safety reasons this resistor must be protected by shunt diodes in
each polarity.

A simple way is to use a 25 amp rated bridge rectifier and connect the
resistor across the AC input terminals while shorting the " + " and " - "
terminals - giving two diode drops in each direction with current
capacity enough to trip a 20 amp supply breaker if necessary without
damage.

The bridge and resistor can go inside the scope.


** Just to spell it out, the surge current rating of a 25 amp bridge
rectifier is several hundred amps - adequate to survive repeated dead
shorts across the AC supply with no damage.

So, if the earth clip of your probe contacts mains active - the only result
will be a tripped breaker, just as it would be without the above
modification.

One of us is smokin' something.
You're saying that you can hook the ground clip of your probe to the mains
active, blow the mains breaker and still have no damage to the probe
or scope?????
Set up a webcam so you can record the experiment. I'd like to see it.


Also, diode bridges fail short when overloaded so in this case would fail
safe.



... Phil

"mike"
Phil Allison wrote:
"Phil Allison"
** Typical ground loops can be eliminated by fitting a resistor between
the scope chassis ground and safety ground of about 10 to 22 ohms. For
obvious safety reasons this resistor must be protected by shunt diodes
in each polarity.

A simple way is to use a 25 amp rated bridge rectifier and connect the
resistor across the AC input terminals while shorting the " + " and " -
" terminals - giving two diode drops in each direction with current
capacity enough to trip a 20 amp supply breaker if necessary without
damage.

The bridge and resistor can go inside the scope.


** Just to spell it out, the surge current rating of a 25 amp bridge
rectifier is several hundred amps - adequate to survive repeated dead
shorts across the AC supply with no damage.

So, if the earth clip of your probe contacts mains active - the only
result will be a tripped breaker, just as it would be without the above
modification.

One of us is smokin' something.


** Must be you.


You're saying that you can hook the ground clip of your probe to the mains
active, blow the mains breaker and still have no damage to the probe
or scope?????


** I am saying the 25 amp bridge will protect the resistor over an over.

Whether the earth clip lead survives is moot since it is not related to the
mod.



.... Phil

"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.

"Mark Zacharias" wrote in
.com:


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.



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

"Jim Yanik" wrote in message
4...
"Mark Zacharias" wrote in
.com:


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.



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


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.

On Dec 4, 3:04*pm, "Mark Zacharias"
wrote:
"Jim Yanik" wrote in message

4...



"Mark Zacharias" wrote in
s.com:

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.

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

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.

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 bull****
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