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Default Oscilloscope 'floating' measurements

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
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Default Oscilloscope 'floating' measurements

On Nov 30, 7:48*am, pfm 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.
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Default Oscilloscope 'floating' measurements

pfm 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


What are you trying to measure?
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Default Oscilloscope 'floating' measurements

pfm wrote in message
...
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


Damage maybe if some voltage rating of a cap or something is exceeded but
"cumulative " damage, to what?


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Default Oscilloscope 'floating' measurements

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" wrote in message
...
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




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Default Oscilloscope 'floating' measurements

Thanks all for replies. Please google for 'Fundamentals of Floating
Measurements and Isolated Input Oscilloscopes'. This is a Tektronix
application note. The situation to which I refer is is at the bottom
of page 4. I want to use an oscilloscope to check the PWM switching
chip on the primary side of an SMPS.

BobH
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Default Oscilloscope 'floating' measurements

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 bull****, just like MICROS~1 telling you
that software other than theirs will cause you problems.
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Default Oscilloscope 'floating' measurements

pfm 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


you can do it, but there is a potential shock hazard if your scope chassis
is at some strange voltage- but all depends on what you're measuring.

Floating your scope and probing around in a microwave oven so you don't
don't smoke your ground leads is probably a bad idea for example.

It's pretty hard to completely float stuff anyways. I once got a memorable
shock from touching one terminal of 16 kV capacitor that was just sitting
on a bench. I'm still not sure why it was able to conduct to the ground I
was standing on as effectively as it did, but it sure did.

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Default Oscilloscope 'floating' measurements

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

What are you trying to measure?


heh... Gullibility, perhaps?

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Default Oscilloscope 'floating' measurements




The claim is bull****, just like MICROS~1 telling you
that software other than theirs will cause you problems.

Has little or nothing to do with MS. We are talking safety here and a
potential lethal shock.

Has to do with the breakdown voltage of the scope case, the probes, internal
components, leads, and so on.

Tektronix needs to hire you, as you know more than they do.



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Default Oscilloscope 'floating' measurements

pfm 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 have used differential probes and battery operated scopes. That's the
only thing I would recommend.

Unless it's a specific service mans isolation transformer, most every
commercial isolation transformer is NOT isolated from ground. Ground
isolation is often what you after because neutral is tied to ground. Using
complete isolation through a service mans isolation transformer still
passes a low level capacitive ac line connection to the circuit, and
measuring high impedance circuits can throw you off. Using a battery
operated scope still connected to the charger will also pass capacitive ac
line coupling.

Greg
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Default Oscilloscope 'floating' measurements

pfm wrote:
Thanks all for replies. Please google for 'Fundamentals of Floating
Measurements and Isolated Input Oscilloscopes'. This is a Tektronix
application note. The situation to which I refer is is at the bottom
of page 4. I want to use an oscilloscope to check the PWM switching
chip on the primary side of an SMPS.

BobH


You do NOT want to do this. Leave the scope grounded and put an
isolation transformer on the device under test.
But you still DON'T want to hook the ground of the probe onto the
common node of the DUT. there's just too much common mode stuff
floating around.
You can get some information by using two probes and a dual-channel
scope with one channel inverted and summed. Hook the probe grounds
together, but to NOTHING else.
You have to turn down the scope gain until EACH channel can handle
all the common mode signals on the inputs. Often, that results in
a sensitivity too low to get much info from the difference trace.
And you'll see junk because the ground path is ill-defined.

The TEK A6902A isolated probe works well in this application.
You can hook the ground lead of the probe almost anywhere you want.

In a perfect world, you can get by doing dangerous things. But it
takes only one slip-up to smoke your probe, your scope, your DUT
and YOURSELF!!!
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Default Oscilloscope 'floating' measurements



"pfm" wrote in message
...
Thanks all for replies. Please google for 'Fundamentals of Floating
Measurements and Isolated Input Oscilloscopes'. This is a Tektronix
application note. The situation to which I refer is is at the bottom
of page 4. I want to use an oscilloscope to check the PWM switching
chip on the primary side of an SMPS.

BobH


I do this all the time - virtually every working day in fact. My current
scope - a 4 channel 100MHz Hitachi that I've had from new - is probably 25
years old now, so if it has been suffering "cumulative damage", it's bloody
slow to reach a point where it's a problem ...

My scope is normally grounded to the incoming line power ground connection.
The SMPS that I'm working on is connected via a variac, that is plugged into
a full 1 : 1 isolation transformer. To make measurements on the primary-side
drive waveforms etc, I first solder a stiff wire to the "-" tag of the main
filter cap. That is then used as the scope probe ground point. This
arrangement has never caused me a problem on any type of switcher from
simple single stage ones, right through to the multi-stage types that have a
PFC supply at the front end.

Arfa

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Default Oscilloscope 'floating' measurements

On Nov 30, 8:14*pm, "Arfa Daily" wrote:
"pfm" wrote in message

...

Thanks all for replies. Please google for 'Fundamentals of Floating
Measurements and Isolated Input Oscilloscopes'. This is a Tektronix
application note. The situation to which I refer is is at the bottom
of page 4. *I want to use an oscilloscope to check the PWM switching
chip on the primary side of an SMPS.


BobH


I do this all the time - virtually every working day in fact. My current
scope - a 4 channel 100MHz Hitachi that I've had from new - is probably 25
years old now, so if it has been suffering "cumulative damage", it's bloody
slow to reach a point where it's a problem ...

My scope is normally grounded to the incoming line power ground connection.

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Default Oscilloscope 'floating' measurements

Differential probes are one answer, though quite expensive.

Comments please. Thanks

BobH


BobH

We used our Tek scopes in the A-B mode to do this all the time.

If you don't have the use of a dual channel scope then an isolation
transformer is in order.

You do need to let us know what signals or what you are measuring.

Bob AZ



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Default Oscilloscope 'floating' measurements


"pfm"

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.



** Tek say no such thing - you bull**** artist.

They do point out it can KILL the operator.


.... Phil



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"pfm"

Thanks all for replies. Please google for 'Fundamentals of Floating
Measurements and Isolated Input Oscilloscopes'. This is a Tektronix
application note. The situation to which I refer is is at the bottom
of page 4.


** The warning is about the potential for a FATAL ELECTRIC SHOCK !

http://www2.tek.com/cmsreplive/tirep...18312_EN.p df


I want to use an oscilloscope to check the PWM switching
chip on the primary side of an SMPS.



** The Iso Tranny goes with the SMPS - you fool.



.... Phil



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Default Oscilloscope 'floating' measurements

gregz wrote in

rg:

pfm 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 have used differential probes and battery operated scopes. That's
the only thing I would recommend.



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


Unless it's a specific service mans isolation transformer, most every
commercial isolation transformer is NOT isolated from ground. Ground
isolation is often what you after because neutral is tied to ground.
Using complete isolation through a service mans isolation transformer
still passes a low level capacitive ac line connection to the circuit,
and measuring high impedance circuits can throw you off. Using a
battery operated scope still connected to the charger will also pass
capacitive ac line coupling.

Greg


TEK makes (or used to make) special probes/amps that isolate the scope from
the device under test(DUT).
they had one that used fiber optics,and another that actually disconnected
the ground for up to 40V,and monitored the ground potential to reconnect
the ground if the potential went above 40V.

A6902,IIRC.

TEK also makes differential amps that can be used with single ended
scopes,but you need a separate probe power supply if it's not a TDS model
scope.

--
Jim Yanik
jyanik
at
localnet
dot com
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Default Oscilloscope 'floating' measurements

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"
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Default Oscilloscope 'floating' measurements

On Dec 1, 11:38*am, Jeffrey Angus 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.


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On Wed, 30 Nov 2011 07:48:46 -0800 (PST), pfm 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


Don't isolate the scope; instead power the unit under test through a
isolation transformer.

PlainBill
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"pfm = bull**** 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





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Default Oscilloscope 'floating' measurements

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" wrote in message
...

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.

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On Nov 30, 9:51*am, "Wild_Bill" 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" wrote in message

...



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.


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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





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On Nov 30, 2:24*pm, JeffM 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 bull****, just like MICROS~1 telling you
that software other than theirs will cause you problems.


Thanks, I think you are right.
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On Nov 30, 5:34*pm, Allodoxaphobia
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
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On Dec 1, 6:25*pm, "Phil Allison" wrote:
"pfm = *bull**** 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_N...ng/safety.html
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On 12/2/2011 7:10 AM, pfm wrote:
On Dec 1, 6:25 pm, "Phil wrote:
"pfm = bull**** 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_N...ng/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"
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Default out of logic

Phil Allison wrote:
snipped


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



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On Dec 2, 9:41*am, Jeffrey Angus wrote:
On 12/2/2011 7:10 AM, pfm wrote:

On Dec 1, 6:25 pm, "Phil *wrote:
"pfm = *bull**** 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_N...fs/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.
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"mike"


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

What sort of ****ING TENTH WIT MORON does that ??

One like YOU of course.

FOAD.


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"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



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Default Oscilloscope 'floating' measurements



"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

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Default Oscilloscope 'floating' measurements


"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






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"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


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Default Oscilloscope 'floating' measurements

"Arfa Daily" wrote in
:



"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



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
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On Wed, 30 Nov 2011 08:53:36 -0800 (PST), nesesu
wrote:

On Nov 30, 7:48*am, pfm 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.

?-)
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On Fri, 2 Dec 2011 04:21:43 -0800 (PST), pfm wrote:

On Nov 30, 2:24*pm, JeffM 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 bull****, just like MICROS~1 telling you
that software other than theirs will cause you problems.


Thanks, I think you are right.


Just the same, i always prefer floating the UUT. Sometimes this is not
possible though, commonly with higher power equipment (10 kVA).

?-)
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On Thu, 1 Dec 2011 11:16:20 -0800 (PST), pfm wrote:

On Dec 1, 11:38*am, Jeffrey Angus 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.


Well they did want to sell expensive differential probes after all.

?-)
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