Floating the 'scope

[The Phantom]

On Sat, 06 Mar 2010 15:25:02 -0800, Fred Abse <excretatauris@invalid.invalid>
wrote:

On Sat, 06 Mar 2010 16:05:28 -0500, Paul E. Schoen wrote:


"Fred Abse" <excretatauris@invalid.invalid> wrote in message
newsan.2010.03.06.11.18.10.148083@invalid.invalid...
On Fri, 05 Mar 2010 19:05:54 -0800, George Herold wrote:

Why do they make 'scope inputs ground referenced to the third wire of
the AC plug? Today I smoked the 10 ohm resistor that was separating
The AC ground from the 'ground' of my circuit. I'd forgotten to float
the 'scope and connected -15V to the ground clip of the scope probe.



Why not invert one channel and switch to "add".

Then use both inputs differentially.

Most, if not all worthwhile instruments should do that.

Probe compensation needs to be accurate for best CMRR.

Or buy a differential probe?

This will not work for reading a voltage of 1 or 2 volts or less, which is
floating as much as 400 volts above ground. For instance, reading the gate
voltage of an SCR on a 480 VAC mains system. You must float the scope, and
I've done it safely and effectively, with a hand-held scope. There may be
some differential probes that can handle this; I don't know.


The OP was talking about 15 volts common mode, where the technique will
work perfectly well.


56 dB CMR with a common mode voltage of 480 RMS is beyond the
capability of even a 7A13. There are differential probes that will do it,
but they have 4 figure price tags.

The CT2593-1 described here:

http://www.caltestelectronics.com/www/Cat1GetSubCategory.asp?PN=General%20Purpose%20Differential%20Probes&ID=1.c.1&subcat=Differential%20Probes

is specified for 86 dB CMRR at 50 Hz, 66 dB at 20 kHz, and 1400 volt common mode
rated for only $331

I've never needed to go to that extreme. Most of what I do is IGBTs with a
DC bus of around 280V, and the (now occasional) SCR bridge at 200V RMS. A
7A13 will handle those, as will a fairly cheap differential probe (few
hundred bucks).

I hope the handheld you used was battery powered. I wouldn't like to stick
670-something volts peak across the PSU isolation of a floating
line-powered 'scope.

 

[The Phantom]

On Sat, 06 Mar 2010 15:25:01 -0800, Fred Abse <excretatauris@invalid.invalid>
wrote:

On Sat, 06 Mar 2010 14:39:01 -0600, The Phantom wrote:

As I asked John, why would anyone go to the trouble to get inside a piece
of equipment to disconnect the safety ground, when they can just use a
ground buster at the wall socket?

Most people that do electrical measurments and are in the habit of breaking the
ground would have ground busters on hand. They might have to go to the hardware
the first time to get one (or more), but never again.

To disconnect the ground inside an instrument like a scope or signal generator
would involve voiding the calibration. Just have a ground buster on hand.

The one can be done in five minutes for free, the other involves going out
to the hardware store and costs money?

 

[The Phantom]

On Sat, 06 Mar 2010 17:14:50 -0500, Jamie
<jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote:

The Phantom wrote:
On Sat, 06 Mar 2010 15:57:21 -0500, Jamie
jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote:


The Phantom wrote:


On Sat, 06 Mar 2010 12:26:16 -0500, Jamie
jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote:



John Larkin wrote:



On Fri, 5 Mar 2010 19:05:54 -0800 (PST), George Herold
ggherold@gmail.com> wrote:




Why do they make 'scope inputs ground referenced to the third wire of
the AC plug? Today I smoked the 10 ohm resistor that was separating
The AC ground from the 'ground' of my circuit. I'd forgotten to float
the 'scope and connected -15V to the ground clip of the scope probe.

George H.


That's a safety thing.

Our TPS2024 has four truly floating channels and floating trigger
input. You can hook a scope ground clip anywhere.

John


So does my cheap crappie OWEN 20 mhz hand held. That also includes
isolation between the DMM inputs.


Is this a two channel scope? If so, what does an ohmmeter measure between the
shells of the two BNC inputs of the two channels?

Aren't the two BNC input grounds connected to the same internal ground
reference?


The 2 BNC's are isolated.


What's the model number of that scope?


http://www.saelig.com/PSHA/PSSA017.htm

HDS1022MN

and the 100 Mhz version

http://www.saelig.com/PSHA/PSSA023.htm

That last link has a better picture.

Where are the scope inputs?

I see 4 multimeter-like inputs along the bottom, colored red, red, black, red
from left to right. Are these also the inputs for the scope? I don't see any
other inputs. Are they on the side?

 

[The Phantom]

On Sat, 06 Mar 2010 15:59:17 -0800, Fred Abse <excretatauris@invalid.invalid>
wrote:

On Sat, 06 Mar 2010 17:37:01 -0600, The Phantom wrote:

On Sat, 06 Mar 2010 15:25:03 -0800, Fred Abse
excretatauris@invalid.invalid> wrote:

On Sat, 06 Mar 2010 14:26:01 -0600, The Phantom wrote:

On Sat, 06 Mar 2010 07:03:56 -0500, Hammy <spam@spam.com> wrote:

On 6 Mar 2010 03:09:01 -0600, The Phantom <phantom@aol.com> wrote:



Tektronix has probes designed to solve these problems:

http://www2.tek.com/cmswpt/psdetails.lotr?ct=PS&cs=psu&ci=13471&lc=EN

They are specifically intended for floating use with the TPS2000
series scopes, which, as John mentioned, have true isolated inputs.

Or, you can use these probes with an ordinary scope:

http://www2.tek.com/cmswpt/psdetails.lotr?ct=PS&cs=psu&ci=13415&lc=EN

Those are nice expensive but nice.

You should use an isolation transformer on any line powered device your
testing. You can usually find a 500VA one for under 100 bucks. I got a
TEMA 500VA one for 45 bucks from Newark. Well worth the money.

Its not really a good idea to float your scope.

A problem arises when you need to make measurements with a 2 or 4
channel scope at locations in the circuit which don't have a common
reference.

For example, suppose you need to look at the gate-source voltages of
the two top MOSFETs in an H-bridge configuration. Isolating the
equipment doesn't allow you to connect the ground clips of the two
scope probes to different points in the circuit that have a substantial
voltage difference.

You need to use floating differential probes, or a scope with true
isolated inputs.

7904, two 7A13s, job done.

TPS2024. No plugins. 4 channels. Job done, and battery operation for
ultimate portability.

No comparator / voltage slideback function.

That's what the vertical offset knob is for.

You only get one differential channel per 7A13. Each isolated channel on the
TPS2024 is a differential input when the ground clip can be connected anywhere,
so there are 4 differential channels.

 

[Hammy]

On 6 Mar 2010 14:26:01 -0600, The Phantom <phantom@aol.com> wrote:

A problem arises when you need to make measurements with a 2 or 4 channel scope
at locations in the circuit which don't have a common reference.

For example, suppose you need to look at the gate-source voltages of the two top
MOSFETs in an H-bridge configuration. Isolating the equipment doesn't allow you
to connect the ground clips of the two scope probes to different points in the
circuit that have a substantial voltage difference.

Point taken.

You need to use floating differential probes, or a scope with true isolated
inputs.

Yep a scope with true isolated inputs is the best solution.

 

[Hammy]

On 6 Mar 2010 15:10:02 -0600, The Phantom <phantom@aol.com> wrote:

Also available are these:

http://www.caltestelectronics.com/www/Cat1GetSubCategory.asp?PN=General%20Purpose%20Differential%20Probes&ID=1.c.1&subcat=Differential%20Probes


I think they may be made by the same folks that make the Yokogawa differential
probes.

I've used the 100MHz Yokogawa probes, and they are very handy.

The last time I went looking for one they were all in the thousands or
at least over a grand. That was a couple of years ago though.

There seems to be a lot more out there now for just a couple of
hundred. Maybe its time to look around again.

 

[Jamie]

The Phantom wrote:

On Sat, 06 Mar 2010 17:14:50 -0500, Jamie
jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote:


The Phantom wrote:

On Sat, 06 Mar 2010 15:57:21 -0500, Jamie
jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote:



The Phantom wrote:



On Sat, 06 Mar 2010 12:26:16 -0500, Jamie
jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote:




John Larkin wrote:




On Fri, 5 Mar 2010 19:05:54 -0800 (PST), George Herold
ggherold@gmail.com> wrote:





Why do they make 'scope inputs ground referenced to the third wire of
the AC plug? Today I smoked the 10 ohm resistor that was separating
The AC ground from the 'ground' of my circuit. I'd forgotten to float
the 'scope and connected -15V to the ground clip of the scope probe.

George H.


That's a safety thing.

Our TPS2024 has four truly floating channels and floating trigger
input. You can hook a scope ground clip anywhere.

John


So does my cheap crappie OWEN 20 mhz hand held. That also includes
isolation between the DMM inputs.


Is this a two channel scope? If so, what does an ohmmeter measure between the
shells of the two BNC inputs of the two channels?

Aren't the two BNC input grounds connected to the same internal ground
reference?



The 2 BNC's are isolated.


What's the model number of that scope?



http://www.saelig.com/PSHA/PSSA017.htm

HDS1022MN

and the 100 Mhz version

http://www.saelig.com/PSHA/PSSA023.htm


That last link has a better picture.

Where are the scope inputs?

I see 4 multimeter-like inputs along the bottom, colored red, red, black, red
from left to right. Are these also the inputs for the scope? I don't see any
other inputs. Are they on the side?

yes, they are on the right bottom side. I don't know why they don't show

the picture of them.

Power jack on the top left, along with the serial port/USB and back
light switch on the left side.

 

[Michael A. Terrell]

"Paul E. Schoen" wrote:

"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in message
news:4B92D17E.A66A9652@earthlink.net...

The Phantom wrote:

As I asked John, why would anyone go to the trouble to get inside a piece
of
equipment to disconnect the safety ground, when they can just use a
ground
buster at the wall socket?


Most idiots just cut the ground pin off the power cord.

Sometimes when I have seen this, I complete the job by cutting off the plug.
Makes it safe again, but not very useful.

I've seen a lot of IEC power cords with the ground pin cut off.
Those were cut up for scrap.

--
Greed is the root of all eBay.

 

[Michael A. Terrell]

The Phantom wrote:

On Sat, 06 Mar 2010 15:25:01 -0800, Fred Abse <excretatauris@invalid.invalid
wrote:

On Sat, 06 Mar 2010 14:39:01 -0600, The Phantom wrote:

As I asked John, why would anyone go to the trouble to get inside a piece
of equipment to disconnect the safety ground, when they can just use a
ground buster at the wall socket?

Most people that do electrical measurments and are in the habit of breaking the
ground would have ground busters on hand. They might have to go to the hardware
the first time to get one (or more), but never again.

To disconnect the ground inside an instrument like a scope or signal generator
would involve voiding the calibration. Just have a ground buster on hand.

WTF is a ground buster? Are you referring to a two to three wire
ground adapter? If so, use the proper name.


--
Greed is the root of all eBay.

 

[Fred Abse]

On Sat, 06 Mar 2010 18:08:01 -0600, The Phantom wrote:

On Sat, 06 Mar 2010 15:25:01 -0800, Fred Abse
excretatauris@invalid.invalid> wrote:

On Sat, 06 Mar 2010 14:39:01 -0600, The Phantom wrote:

As I asked John, why would anyone go to the trouble to get inside a
piece of equipment to disconnect the safety ground, when they can just
use a ground buster at the wall socket?

Most people that do electrical measurments and are in the habit of
breaking the ground would have ground busters on hand. They might have to
go to the hardware the first time to get one (or more), but never again.

I've been doing electrical measurements for nearly fifty years, and I've
never had one.

Tek actually used to make one. Anything over about 15 - 20 volts float and
it tripped the supply, I've only ever seen one, in the hands of someone
else.

To disconnect the ground inside an instrument like a scope or signal
generator would involve voiding the calibration. Just have a ground
buster on hand.

Some people are animals. Calibration? Wossat?

Some of the things I've seen done to instruments, you wouldn't believe.

--
"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more
durable; the other is a cheaper thing, but the moths get into it."
(Stephen Leacock)

 

[Fred Abse]

On Sat, 06 Mar 2010 18:03:01 -0600, The Phantom wrote:

The CT2593-1 described here:

http://www.caltestelectronics.com/www/Cat1GetSubCategory.asp?PN=General%20Purpose%20Differential%20Probes&ID=1.c.1&subcat=Differential%20Probes

is specified for 86 dB CMRR at 50 Hz, 66 dB at 20 kHz, and 1400 volt
common mode rated for only $331

Bandwidth / risetime isn't enough for what I need. The CT3686 might cut it.

--
"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more
durable; the other is a cheaper thing, but the moths get into it."
(Stephen Leacock)

 

[Fred Abse]

On Sat, 06 Mar 2010 18:31:01 -0600, The Phantom wrote:

On Sat, 06 Mar 2010 15:59:17 -0800, Fred Abse
excretatauris@invalid.invalid> wrote:

On Sat, 06 Mar 2010 17:37:01 -0600, The Phantom wrote:

On Sat, 06 Mar 2010 15:25:03 -0800, Fred Abse
excretatauris@invalid.invalid> wrote:

On Sat, 06 Mar 2010 14:26:01 -0600, The Phantom wrote:

On Sat, 06 Mar 2010 07:03:56 -0500, Hammy <spam@spam.com> wrote:

On 6 Mar 2010 03:09:01 -0600, The Phantom <phantom@aol.com> wrote:



Tektronix has probes designed to solve these problems:

http://www2.tek.com/cmswpt/psdetails.lotr?ct=PS&cs=psu&ci=13471&lc=EN

They are specifically intended for floating use with the TPS2000
series scopes, which, as John mentioned, have true isolated inputs.

Or, you can use these probes with an ordinary scope:

http://www2.tek.com/cmswpt/psdetails.lotr?ct=PS&cs=psu&ci=13415&lc=EN

Those are nice expensive but nice.

You should use an isolation transformer on any line powered device
your testing. You can usually find a 500VA one for under 100 bucks. I
got a TEMA 500VA one for 45 bucks from Newark. Well worth the money.

Its not really a good idea to float your scope.

A problem arises when you need to make measurements with a 2 or 4
channel scope at locations in the circuit which don't have a common
reference.

For example, suppose you need to look at the gate-source voltages of
the two top MOSFETs in an H-bridge configuration. Isolating the
equipment doesn't allow you to connect the ground clips of the two
scope probes to different points in the circuit that have a
substantial voltage difference.

You need to use floating differential probes, or a scope with true
isolated inputs.

7904, two 7A13s, job done.

TPS2024. No plugins. 4 channels. Job done, and battery operation for
ultimate portability.

No comparator / voltage slideback function.

That's what the vertical offset knob is for.

How many screens will it window?

7A13 will measure a 1mV portion of a 10V signal.


Does it have a calibrated comparison voltage? Useful for things like
measuring staircase steps.

You only get one differential channel per 7A13. Each isolated channel on
the TPS2024 is a differential input when the ground clip can be connected
anywhere, so there are 4 differential channels.

I've never needed 4 differential channels, but if I did, I'd use two
mainframes and four 7a13s (Yes, I do have four)

I've used a 7a13 for vertical deflection on a couple of occasions. You can
put amplifiers in timebase slots. Full differential X-Y.


--
"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more
durable; the other is a cheaper thing, but the moths get into it."
(Stephen Leacock)

 

On Sun, 07 Mar 2010 01:38:48 -0800, Fred Abse
<excretatauris@invalid.invalid> wrote:

On Sat, 06 Mar 2010 18:31:01 -0600, The Phantom wrote:

On Sat, 06 Mar 2010 15:59:17 -0800, Fred Abse
excretatauris@invalid.invalid> wrote:

On Sat, 06 Mar 2010 17:37:01 -0600, The Phantom wrote:

On Sat, 06 Mar 2010 15:25:03 -0800, Fred Abse
excretatauris@invalid.invalid> wrote:

On Sat, 06 Mar 2010 14:26:01 -0600, The Phantom wrote:

On Sat, 06 Mar 2010 07:03:56 -0500, Hammy <spam@spam.com> wrote:

On 6 Mar 2010 03:09:01 -0600, The Phantom <phantom@aol.com> wrote:



Tektronix has probes designed to solve these problems:

http://www2.tek.com/cmswpt/psdetails.lotr?ct=PS&cs=psu&ci=13471&lc=EN

They are specifically intended for floating use with the TPS2000
series scopes, which, as John mentioned, have true isolated inputs.

Or, you can use these probes with an ordinary scope:

http://www2.tek.com/cmswpt/psdetails.lotr?ct=PS&cs=psu&ci=13415&lc=EN

Those are nice expensive but nice.

You should use an isolation transformer on any line powered device
your testing. You can usually find a 500VA one for under 100 bucks. I
got a TEMA 500VA one for 45 bucks from Newark. Well worth the money.

Its not really a good idea to float your scope.

A problem arises when you need to make measurements with a 2 or 4
channel scope at locations in the circuit which don't have a common
reference.

For example, suppose you need to look at the gate-source voltages of
the two top MOSFETs in an H-bridge configuration. Isolating the
equipment doesn't allow you to connect the ground clips of the two
scope probes to different points in the circuit that have a
substantial voltage difference.

You need to use floating differential probes, or a scope with true
isolated inputs.

7904, two 7A13s, job done.

TPS2024. No plugins. 4 channels. Job done, and battery operation for
ultimate portability.

No comparator / voltage slideback function.

That's what the vertical offset knob is for.

How many screens will it window?

I'll have to check that on Monday, but I've never needed more.

7A13 will measure a 1mV portion of a 10V signal.

10 volt offset is the max the 7A13 can do. The TPS2024 can do way
more than that for large signals.

Does it have a calibrated comparison voltage? Useful for things like
measuring staircase steps.

The value of the offset is displayed as a 3 digit number on screen.
The deflection factor is also calibrated.

You only get one differential channel per 7A13. Each isolated channel on
the TPS2024 is a differential input when the ground clip can be connected
anywhere, so there are 4 differential channels.

I've never needed 4 differential channels, but if I did, I'd use two
mainframes and four 7a13s (Yes, I do have four)

When using two mainframes, how do you see all four traces on the same
screen?

I've used a 7a13 for vertical deflection on a couple of occasions.

Isn't vertical deflection the usual mode? Maybe you mean that you
used a 7A13 for horizontal deflection. Isn't that what you'd have to
do for X-Y mode?

You can
put amplifiers in timebase slots. Full differential X-Y.

Most modern scopes have an X-Y mode; so does the TPS2024. Since all
the channels on the TPS2024 are effectively differential, its X-Y mode
is a differential X-Y mode

The TPS2024 can do math functions on differential signals, too.

 

[George Herold]

On Mar 5, 11:16 pm, John Tserkezis
<j...@techniciansyndrome.org.invalid> wrote:

On 6/03/2010 2:49 PM, Phil Allison wrote:

"George Hairoil"
I've been floating 'scopes for.... say since the early '80's.  I
haven't been biten by it yet.  (Though I've made plenty of other
mistakes.)  What are the obvious safety reasons?
** My god you are stupid.

 Welcome to an entirely new class of stupid, Phil.

 At a place I used to work at, the cro on my bench was floated by the
previous "tech", he just forgot to tell anyone about it.  And forgot to
label it too.  No clues at all unless you measured it, or took it apart
to actually look.

 Made worse by the fact it was a dual trace cro, and could have done the
job without floating it.

 I can only presume these idiots have been doing for so long, they stop
caring about the fact you hadn't had to do it since two trace (or more)
CROs appeared on the market.
 Hey, I don't care if they want to kill themselves, but their idiocy
will continue to kill others well into the future.

 I'm starting to think that being dropped as a baby isn't enough to
explain this level of stupid.

Sorry I'm not trying to be stupid. I certainly wouldn't float the
scope if I was looking at high voltages or AC line signals. Most
often I float the scope because I've got some interference that looks
like it might be a ground loop. I want to see if it's through the
scope. I have a single AC line cord with the third wire clipped.
It's got green tape on both ends of the plug. It stays in my wire
drawer. If it somehow makes it's way onto the production floor I can
easily retrieve it (The green tape helps.)

George H.

 

[George Herold]

On Mar 6, 2:36 am, John Tserkezis
<j...@techniciansyndrome.org.invalid> wrote:

On 6/03/2010 5:31 PM, Paul E. Schoen wrote:

Handheld and portable scopes often have floating inputs, and it is

 Even if a portable scope is double insulated, that's not always safe.

 If you clip the "ground" of the scope probe to a live line, that shield
and any metal bits all the way to the scope are also live.

 If the ground clip is exposed, it'll bite (I've frequently seen it, and
on some probes, that's the way it's built) , if the insulation is
damaged, it'll bite.

 Also note that none of the cable insulations I've seen are rated for
high voltage, only the centre conductor is good for the specified
voltage, not the outer to the outside world.

 Your only avenue is to completely isolate yourself from ground, which
is going to prove to be a bit difficult, considering the capacitance you
can't really avoid..
 Forget about wearing an antistatic strap.  I've worked in some
environment where it's mandatory, so that's going to be a bit of a
challenge.

 Or use an earth leakage breaker, and have the thing cut out on you a
dozen times a day even under normal use.

 There are too many things that can go wrong.   There are no procedures
to ensure all the avenues are covered and enforced to prevent shocks
from this method.

Oh, connecting to line voltages is something I thankfully have to do
rarely, and when I do it's with great care. (I do own a suicide plug
that is a one piece AC to banana... scary...)

George H.

 

[George Herold]

On Mar 6, 7:03 am, Hammy <s...@spam.com> wrote:

On 6 Mar 2010 03:09:01 -0600, The Phantom <phan...@aol.com> wrote:



Tektronix has probes designed to solve these problems:

http://www2.tek.com/cmswpt/psdetails.lotr?ct=PS&cs=psu&ci=13471&lc=EN

They are specifically intended for floating use with the TPS2000 series scopes,
which, as John mentioned, have true isolated inputs.

Or, you can use these probes with an ordinary scope:

http://www2.tek.com/cmswpt/psdetails.lotr?ct=PS&cs=psu&ci=13415&lc=EN

Those are nice expensive but nice.

You should use an isolation transformer on any line powered device
your testing. You can usually find a 500VA one for under 100 bucks. I
got a TEMA 500VA one for 45 bucks from Newark. Well worth the money.

 Its not really a good idea to float your scope.

Thanks Hammy, you're right we should buy an isolation transformer. Is
it OK to power the 'scope with the isolation tranny? Or does this
defeat the whole purpose again?

"> Its not really a good idea to float your scope."

What about floating it to get rid of ground loops?

George H.

 

[George Herold]

On Mar 6, 12:48 pm, Hammy <s...@spam.com> wrote:

On Sun, 7 Mar 2010 03:01:46 +1100, "Phil Allison" <phi...@tpg.com.au
wrote:







"Hammy"

You should use an isolation transformer on any line powered device
your testing.

** Another recipe for dead technicians.

The horrendous fallacy in the thinking is way beyond the likes of retards
like Hammy.

.....  Phil

Really and I thought that an isolation transformer removes the earth
ground reference making it safe to connect your probe on say the
primary side of a DC/DC converter? That is what kills you isn't it you
are connected to ground and the line voltage is referenced to ground?
There still exist the high voltage but it no longer has an earth
ground reference. So it's not to say you can't still kill yourself
particularly if you don't follow the one hand rule but it reduces the
risk and it makes it safe to hook your probe up to anywhere reasonable
without sparks flying.

If this isn't the case then I guess I've been pretty fortunate over
the past ten years. Because following that line of thought I've yet to
fry a scope or myself.

Ideally you would want a wide BW active HV differential probe but they
are usually in the thousands of dollars so an ISO tranny will at least
provide some additional safety and be cheaper on your test equipment.
You should probably still use an ISO tranny with the diff probes for
safety anyways.

Some people prefer to hook there scope to the iso tranny because the
tranny will effect the accuaracy or alter the measurement I think
that's stupid. If you need to make measurements like that on a regular
basis in a professional environment tell your employer to buy you the
right dam equipment like the diff probes.

And over to you Philly.

Oh I almost forget the obligatory profanities

You fucking ASD fuckwit!!!!!!!!!!- Hide quoted text -

- Show quoted text -

"> And over to you Philly.

Oh I almost forget the obligatory profanities

You fucking ASD fuckwit!!!!!!!!!!"

Hee Hee,

Thanks Hammy, Hopefully Phil will also tell me if it's OK to run the
scope through an iso-tranny.

George H.

 

[George Herold]

On Mar 6, 3:18 pm, Fred Abse <excretatau...@invalid.invalid> wrote:

On Fri, 05 Mar 2010 19:05:54 -0800, George Herold wrote:
Why do they make 'scope inputs ground referenced to the third wire of
the AC plug?  Today I smoked the 10 ohm resistor that was separating
The AC ground from the 'ground' of my circuit.  I'd forgotten to float
the 'scope and connected -15V to the ground clip of the scope probe.

Why not invert one channel and switch to "add".

Then use both inputs differentially.

Most, if not all worthwhile instruments should do that.

Probe compensation needs to be accurate for best CMRR.

Or buy a differential probe?

--
"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more
durable; the other is a cheaper thing, but the moths get into it."
                                             (Stephen Leacock)

Yup, that's what I should have done, But I'd have had to find the
matching 'scope probe (They tend to wander around and become
separated.) And the floating AC cable was lying there behind the
equipment. (I'd just been using it.)

George H.

 

[George Herold]

On Mar 6, 5:04 pm, "Michael A. Terrell" <mike.terr...@earthlink.net>
wrote:

The Phantom wrote:

As I asked John, why would anyone go to the trouble to get inside a piece of
equipment to disconnect the safety ground, when they can just use a ground
buster at the wall socket?

  Most idiots just cut the ground pin off the power cord.

--
Greed is the root of all eBay.

Yup, I'm just like most idiots! (But I put green tape on both ends of
the cord.) The ground buster was before the universal AC input.

George H.

 

[Michael A. Terrell]

George Herold wrote:

On Mar 6, 5:04 pm, "Michael A. Terrell" <mike.terr...@earthlink.net
wrote:
The Phantom wrote:

As I asked John, why would anyone go to the trouble to get inside a piece of
equipment to disconnect the safety ground, when they can just use a ground
buster at the wall socket?

Most idiots just cut the ground pin off the power cord.

--
Greed is the root of all eBay.

Yup, I'm just like most idiots! (But I put green tape on both ends of
the cord.) The ground buster was before the universal AC input.

Good for you. That would get you fired any place I've ever worked.
If you needed isolation, you plugged whatever you were working on into
an isolation transformer. If OSHA found a cord like that, the company
would be fined and it could run into thousands of dollars.


--
Greed is the root of all eBay.