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. Differential probes are one
answer, though quite expensive.

Comments please. Thanks

BobH

 

[mike]

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?

 

[N_Cook]

pfm <bobh@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

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

 

[Wild_Bill]

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" <bobh@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

 

[nesesu]

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.

 

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

BobH

 

[JeffM]

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.

 

[Allodoxaphobia]

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

What are you trying to measure?

heh... Gullibility, perhaps?

 

[Charles]

The claim is bullshit, 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.

 

[Cydrome Leader]

pfm <bobh@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

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.

 

[gregz]

pfm <bobh@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 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

 

[mike]

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!!!

 

[Arfa Daily]

"pfm" <bobh@pfmbox.com> wrote in message
news:4af829d8-c8ce-48a7-ba17-cd3d1fde5b68@t16g2000vba.googlegroups.com...

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

 

[hrhofmann@att.net]

On Nov 30, 8:14 pm, "Arfa Daily" <arfa.da...@ntlworld.com> wrote:

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

news:4af829d8-c8ce-48a7-ba17-cd3d1fde5b68@t16g2000vba.googlegroups.com...

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

You're doing just what I was going to recommend to the OP,,,totally
isolate the EUTor UUT or DUT, whatever you call the equipment being
tested. Then you can call"Ground" whaatever you want to connect to
the EUT that youwant to call ground, ie, the ground terminal of the
scope probe. This is what I did for the 44 years I was at Bell Labs
and never blew up anything, Tektronix or HP. Funny how HP never did
make scopes as well as Tektronix, and Tek never made Spectrum
Analyzers as well as HP.

 

[Phil Allison]

"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 bullshit artist.

They do point out it can KILL the operator.


.... Phil

 

[Phil Allison]

"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/18312/3AW_19134_1%20Fundamentals%20of%20Floating%20Measurements%20MR_Letter_2011.11.22.14.09.25_18312_EN.pdf

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

 

[Bob AZ]

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

 

[Jim Yanik]

gregz <zekor@comcast.net> wrote in
news:1367496343344369390.877635zekor-comcast.net@news.eternal-september.o
rg:

pfm <bobh@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 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

 

[Jeffrey Angus]

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"

 

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

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

PlainBill