Oscilloscope 'floating' measurements

[pfm]

On Dec 1, 11:38 am, Jeffrey Angus <grendel...@aim.com> wrote:

On 12/1/2011 9:43 AM, Jim Yanik wrote:

Proper practice is to put the Device Under Test on the isolation
transformer,not the scope. leave the scope grounded.

I don't know why this is so hard for people to understand.

Test equipment is grounded. Period.

If your probe can't handle the offset voltage, you're using
the wrong probe.

If you're trying to measure something normally connected
directly across the utility power line, then use an isolation
transformer and you can ground any point in the device being
tested.

Use a solid ground jumper. Do NOT rely on the ground path
back through the scope probe.

Jeff-1.0
wa6fwi

--
"Everything from Crackers to Coffins"

Thanks. 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.

 

[Phil Allison]

"pfm = bullshit artist "

Thanks. 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.


** Nothing like that is in the app note YOU cited.

In fact there is no mention of the idea at all making the app note seem
deliberately misleading.




.... Phil

 

[Wild_Bill]

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
..............


"pfm" <bobh@pfmbox.com> wrote in message
news:6d76c5ed-e319-41e7-a457-a362352a4912@w15g2000yqc.googlegroups.com...

Thanks. 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.

 

[Phil Allison]

<stratus46@yahoo.com>

Agreed on the dangerous aspect but I have had issues with ground loops
between an audio generator and the scope when testing audio
amplifiers. I carry a ground lifter in the scope probe bag and use it
when needed - not very often.


** 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.


.... Phil

 

On Nov 30, 9:51 am, "Wild_Bill" <wb_wildb...@XSPAMyahoo.com> wrote:

It's not clear what you were trying to say.. "THIS? is a dangerous
practice".

Floating a scope plugged into a utility line power source (and omitting the
earth ground connection) is hazardous because the chassis (possibly the case
and other exposed hardware) of the scope will be at the same potential as
the probe ground clip while testing.
Also, it's possible to exceed the scope input's safe limits.

When using a proper isolation transformer, the equipment/device being tested
is commonly plugged into the I-T, not the scope (some exceptions apply,
in-place industrial equipment which can't be powered by a servicing I-T, for
example).

Some I-Ts have a pass-thru earth ground connection, and others may bond the
line Neutral or one side of the line power (which isn't any type I've ever
used, just read about here).

When servicing gear, I want the DUT device under test to have no paths at
all, to the utility service/line power Hot(s) or Neutral connections.
The DUT is then as safe as if it were powered by battery (except for only a
miniscule uA leakage at the I-T transformer).

FWIW..
In the U.S. the AC line Neutral is often bonded to earth ground at the
service panel).

Common Variacs or autotransformers do not provide line isolation, and
shouldn't be considered safety equipment.

A 2 channel/input scope with a Differential Mode selection and proper
isolation of the channel circuits can generally be used for safe testing,
but only if this procedure is included in the scope's operating manual.

--
Cheers,
WB
.............

"pfm" <b...@pfmbox.com> wrote in message

news:1a9c5db9-48fa-4129-a645-0f7982489ce1@v29g2000yqv.googlegroups.com...



Many advocate the use of an isolating transformer. The oscilloscope
probe and ground are then used like multimeter leads. Tektronix and
others maintain this is a dangerous practice and may result in
cumulative damage to the oscilloscope. Differential probes are one
answer, though quite expensive.

Comments please. Thanks

BobH

Agreed on the dangerous aspect but I have had issues with ground loops
between an audio generator and the scope when testing audio
amplifiers. I carry a ground lifter in the scope probe bag and use it
when needed - not very often. I NEVER connect the scope ground lead to
any elevated supplies when using a ground lifter. There are other
better and safe ways to measure high Voltage circuits.

G˛

 

[pfm]

On Nov 30, 2:24 pm, JeffM <jef...@email.com> wrote:

pfm wrote:
Many advocate the use of an isolating transformer.
The oscilloscope probe and ground
are then used like multimeter leads.

...which will get you exactly where you want to be.

Tektronix and others maintain this is a dangerous practice

...to the bottom line of Tektronix, Inc.

and may result in cumulative damage to the oscilloscope.

...of which they couldn't provide a convincing example.

Differential probes are one answer, though quite expensive.

See "bottom line", above.
The claim is bullshit, just like MICROS~1 telling you
that software other than theirs will cause you problems.

Thanks, I think you are right.

 

[Phil Allison]

"pfm = FUCKWIT bullshit artist "

Thanks. 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.

** Nothing like that is in the app note YOU cited.

In fact there is no mention of the idea at all making the app note seem
deliberately misleading.

Try this:
http://www.tek.com/Measurement/App_Notes/Technical_Briefs/tds3000-float/eng/safety.html


** How about you try FUCKING YOUR MOTHER ??

Or is that a superfluous question ??

You fucking illiterate TROLLING asshole.

FOAD


.... Phil

 

[pfm]

On Nov 30, 5:34 pm, Allodoxaphobia <knock_yourself_...@example.net>
wrote:

On Wed, 30 Nov 2011 09:10:06 -0800, mike wrote:

What are you trying to measure?

heh...  Gullibility, perhaps?

You could be right

 

[pfm]

On Dec 1, 6:25 pm, "Phil Allison" <phi...@tpg.com.au> wrote:

"pfm =  bullshit artist "

Thanks. 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.

** Nothing like that is in the app note YOU cited.

 In fact there is no mention of the idea at all making the app note seem
deliberately misleading.

...  Phil

Try this: http://www.tek.com/Measurement/App_Notes/Technical_Briefs/tds3000-float/eng/safety.html

 

[Jeffrey Angus]

On 12/2/2011 7:10 AM, pfm wrote:

On Dec 1, 6:25 pm, "Phil Allison"<phi...@tpg.com.au> wrote:
"pfm = bullshit artist"

Thanks. 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.

** Nothing like that is in the app note YOU cited.

In fact there is no mention of the idea at all making the app note seem
deliberately misleading.

... Phil

Try this: http://www.tek.com/Measurement/App_Notes/Technical_Briefs/tds3000-float/eng/safety.html

Phil's still right.

Yes they show an isolation transformer. But they show it isolating
the scope, NOT the device being tested.

There's no mention of using an isolation transformer to float the
device being tested.

Tek's app note is based on NOT being able to isolate the device
being tested.

Jeff-1.0

--
"Everything from Crackers to Coffins"

 

[mike]

Phil Allison wrote:
snipped


profanity and name calling = what you do when you have no
logical argument, but still need to hear yourself speak.

 

[pfm]

On Dec 2, 8:52 am, "Phil Allison" <phi...@tpg.com.au> wrote:

"pfm = FUCKWIT  bullshit artist "



Thanks. 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.

** Nothing like that is in the app note YOU cited.

In fact there is no mention of the idea at all making the app note seem
deliberately misleading.

Try this:http://www.tek.com/Measurement/App_Notes/Technical_Briefs/tds3000-flo...

** How about you try  FUCKING  YOUR   MOTHER  ??

    Or is that a superfluous question  ??

    You fucking illiterate TROLLING  asshole.

    FOAD

...  Phil

You are right, my interpretation was wrong. I should not have made it
sound a definitive statement.

 

[pfm]

On Dec 2, 9:41 am, Jeffrey Angus <grendel...@aim.com> wrote:

On 12/2/2011 7:10 AM, pfm wrote:

On Dec 1, 6:25 pm, "Phil Allison"<phi...@tpg.com.au>  wrote:
"pfm =  bullshit artist"

Thanks. 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.

** Nothing like that is in the app note YOU cited.

  In fact there is no mention of the idea at all making the app note seem
deliberately misleading.

...  Phil

Try this:http://www.tek.com/Measurement/App_Notes/Technical_Briefs/tds3000-flo...

Phil's still right.

Yes they show an isolation transformer. But they show it isolating
the scope, NOT the device being tested.

There's no mention of using an isolation transformer to float the
device being tested.

Tek's app note is based on NOT being able to isolate the device
being tested.

Jeff-1.0

--
"Everything from Crackers to Coffins"

You are right, thank you.

 

[Phil Allison]

"mike"


** You snipped all the facts out and then say there are none.

What sort of FUCKING TENTH WIT MORON does that ??

One like YOU of course.

FOAD.

 

[Phil Allison]

"Jeffrey Angus"

Phil's still right.

** Course he is....

Yes they show an isolation transformer. But they show it isolating
the scope, NOT the device being tested.

There's no mention of using an isolation transformer to float the
device being tested.

Tek's app note is based on NOT being able to isolate the device
being tested.

** Tek are very remiss in not mentioning the MOST COMMON method of using a
scope on non mains isolated circuitry - which is to install a 1:1 mains
isolation tranny and then ground any part of the circuit you need to.

What this achieves is ELIMINATING the need to make floating measurements.

Similarly, even with equipment that has mains isolation by virtue of a 50/60
Hz transformer - one can temporarily disconnect the secondary side from
supply ground and float the circuitry. This again allows you to ground any
part of the circuit you desire to facilitate scope measurements.




.... Phil

 

[Arfa Daily]

"Wild_Bill" <wb_wildbill@XSPAMyahoo.com> wrote in message
news:VTXBq.119491$1x3.2710@en-nntp-16.dc1.easynews.com...

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]

"Arfa Daily"

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.

** The OP reminds me of the very old joke about the difference between a
computer and a drummer.

Nothing.

Both require new information to be punched into them ...



.... Phil

 

[Phil Allison]

"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.

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



.... Phil

 

[Jim Yanik]

"Arfa Daily" <arfa.daily@ntlworld.com> wrote in
news:9sfCq.209438$Vv5.193415@newsfe05.ams2:

"Wild_Bill" <wb_wildbill@XSPAMyahoo.com> wrote in message
news:VTXBq.119491$1x3.2710@en-nntp-16.dc1.easynews.com...
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

Heh,I've seen the results of people connecting scope grounds to what they -
thought- was an OK place to connect. line V can really do a lot of
destruction.
I've also replaced a LOT of scope power cords that had the ground pin cut
off. I used to include a copy of TEK's publication on safe isolated scope
measurements with each of those scopes;my own personal policy,not TEKs.
I don't know if it ever did any good....


--
Jim Yanik
jyanik
at
localnet
dot com

 

[josephkk]

On Wed, 30 Nov 2011 08:53:36 -0800 (PST), nesesu
<neil_sutcliffe@telus.net> wrote:

On Nov 30, 7:48 am, pfm <b...@pfmbox.com> wrote:
Many advocate the use of an isolating transformer. The oscilloscope
probe and ground are then used like multimeter leads. Tektronix and
others maintain this is a dangerous practice and may result in
cumulative damage to the oscilloscope. Differential probes are one
answer, though quite expensive.

Comments please. Thanks

BobH

I can see no reason that using an isolation transformer properly can
have any more detrimental effects on a probe or the 'scope itself than
testing equipment that is self isolated [transformer operated].
When using an isolation transformer, the UUT [Unit Under Test] must
have the circuit 'common' bonded to the 'scope common [and ground]
usually by the probe 'pig tail' or even a separate test lead joining
them. Indeed, if the probe tip is touched to an unbonded UUT, it can
damage the 'scope or probe, regardless of wether it is powered through
an external isolation transformer or through an internal transformer
or SMPS.
On the other hand, connecting an 'scope that has been 'floated' from
ground by disconnecting the power cord ground pin CAN be severely
damaged by connecting it to a non-isolated line operated UUT. The
usual reason is excessive voltage applied between the 'scope common
and the 'scope supply neutral or line. These usually turn out to be
transient pulses that are unexpected.

Neil S.

Tek has historical basis for its warnings, the cases of 'scopes used to be
metal and connected to probe ground. If every exposed (metal) part of the
scope becomes live there is a definite shock hazard involved.

?-)