# Oscilliscope and Ground

Discussion in 'Electronic Basics' started by [email protected], Mar 10, 2005.

1. ### Guest

I've been trying to learn how to use an oscilliscope. But I get a
little confused about when and when not to use the ground clip on the
probe.

I tried taking measurements from a 9-volt DC power supply that I built.
It is basically a two-prong power cord into a transformer that knocks
the voltage down to about 12-volts. Then it goes through a bridge
rectifier to make it pulsating DC, and then through some capacitors to
smooth the signal and a voltage regulator, and then I have my positive
and negative output connectors.

I tried measuring the AC voltage before the transformer. I just used
the probe without the ground clip and it gave me an accurate reading.
Why didn't I need to use the ground clip? And if I did, where would I
connect it to? Would ground in this situation be the Earth? My power
cord plug doesn't have a ground.

Then I tried measuring the DC voltage at the outputs. Since I didn't
need the ground for measuring the AC voltage, I didn't use it when
measuring the DC voltage. I just touched the probe to my positive
output. But this gave me an AC waveform. Why would it give me an AC
waveform? Then I touched the probe to the positive output and the
ground clip to the negative output and that displayed the 9V DC voltage
I was looking for.

I figured I would need to use the ground clip when measuring my DC
voltage output, but why didn't I need it for measuring the AC voltage?

- Clint

2. ### John PopelishGuest

The ground clip on the scope probe should have a direct connection to
the case and safety ground prong of the scope. (check this with an ohm
meter with the scope unplugged.) So it is safe to connect the probe
ground only to nodes that are either isolated from ground (any node in
your low voltage circuits that have been isolated from ground by the
small supply transformer) or to nodes that already have an an earth
ground connection by some other route.

If you connect it to any node that has some stiff voltage with respect
to earth ground, then the connection will constitute a short circuit
across that voltage and cause large current to pass through the probe
shield and scope ground conductor, possibly destroying the probe and
possibly injuring you. If in doubt, make an AC and DC voltage
measurement between any node you are thinking of grounding (with the
probe clip) and the clip. If more than a fraction of a volt shows up
(there can be slight ground noise differences between points that have
a connection to ground), investigate further before making the direct
connection.

The purpose of the clip is to provide a high frequency path for the
signal across the scope front end amplifier, back to ground, without
having to go out the scope safety ground prong, around the
distribution wiring and back up the power cord of the equipment being
tested. Or, for isolated equipment, to provide a ground reference
point for the scope, with which to measure some voltage.

3. ### Rick FoxGuest

Correct, the Earth is the ground. Even though your plug doesn't have
the ground connection, the 'live' or 'hot' side of the supply is still
referenced to it. The oscilloscope is also connected to the Earth
ground through its plug, so you already have a ground connection.
Trying to connect the 'scope's ground in this situation to anything
other than the Earth ground (which it already is anyway) would be
hazardous. You did it right.
The reason you need the 'scope's ground lead on the other side of the
transformer is that there is no return path for the current otherwise,
no 'zero-reference'. Without it, the 'scope will measure AC voltage
induced onto the wires with reference to Earth (you can touch just a
large piece of metal for the same effect). Not much use.
A last tip - never 'float the 'scope' as you may see advised in some
places. That means removing its own Earth connection from the plug. It
can be extremely hazardous, putting the whole body of the scope at
mains potential.

Rick.

4. ### Kitchen ManGuest

Excellent experiment. Even without a ground prong on your oscope power
cord (how old is that thing?) the scope will sense your building's earth
ground through its neutral. Thus, they are common grounded, and you can
measure a good ac signal. The DC side is isolated from that common
ground via the transformer, so you need that second reference to make a
measurement. You saw an AC signal on the output with the ungrounded
probe, because that's the voltage the scope could see with reference to
the building earth ground. Hope that makes sense.

5. ### mikeGuest

Ground is a DEFINITION, not a PLACE. People often (mis)use (IMHO) the
term ground to mean earth. But that only makes sense if you're sitting
on the dirt.
Where there's distance, there's impedance. Your measurement problem
gets much more difficult as the frequency increases. And there's also
mutual coupling of all kinds...but I digress.

The best you can hope for is to measure one specific point relative to
some other specific point. You put the probe ground clip on the point
you've
defined as the reference using the shortest possible wire. This is
often zero volts nominal relative to system ground...which may also be
nominally the same as building wiring ground...which may also be
nominally the same as earth ground at the dirt outside. I say nominally
because if there's current and distance, there's always differential
voltage. IF that voltage gets really big, you blow out the ground wire
on your probe and (maybe) electrocute yourself. If it's not so big, you
get measurement errors.

Now, if you're measuring low frequencies and don't need extreme accuracy
and can stand some noise on your reading, you can increase the length of
the wire on the ground clip to make probing more convenient. For rough
measurements, you can even clip a wire from scope chassis ground to the
device chassis ground and skip the probe ground altogether...again
assuming they're both at the same nominal potential (ground). This is
NOT always the case. If you care what the waveshape looks like, use the
probe ground wire.

You almost always want a wire from somewhere in the scope (ground)
system to someplace in the (ground) system in the device under test.
It's possible to make measurements where this is not the case, but it's
usually an unsafe measurement condition.

Even if you do have the ground clip on the proper reference node, there
will also be multiple parasitic return paths that can disturb the
measurement. It's not uncommon to use common mode measurement
techniques where you put one probe on the signal, another on the
reference (ground) and subtract the two. In this case, you can safely
measure differentials that are not ground referenced.

If you don't have a defined common (ground) return path for the signal,
the laws of physics will define one for you...but you have no idea where
that is or how it affects your measurement.

Good scope probes are not just a coax with a resistor. They're complex
networks that often include coaxial wire with a resistive center conductor.
Depending on vintage, your scope vertical input will probably say
something like 1Meg, 10pf to maybe 45pf. If you put a 1X probe onto the
scope, the input capacitance at the tip gets HUGE. It can cause
also makes the grounding problem much worse because you have to charge
all that capacitance.

You almost always want to use a 10X probe that's properly compensated
to the scope input capacitance.

mike

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6. ### Kitchen ManGuest

It appears we are in violent agreement. Terms, nevertheless, persist,
and a building's earth ground might just be called that because it is
sitting in the dirt. Something to consider.
Don't you think you're adding a bit more complexity to the issue than it
deserves? The man's question was pretty simple. If you recall,

"I figured I would need to use the ground clip when measuring my DC
voltage output, but why didn't I need it for measuring the AC voltage?"

8. ### Larry BrasfieldGuest

Rich did, and so would I for the OP. His questions
indicated that he does not yet know enough to make
sound judgments as to when that safety precaution
should be set aside and what additional precautions
to take once the o'scope is floated.

I agree that 'never' is a strong statement, and that
with appropriate caveats and precautions, floating
an o'scope can be done safely. But I would advise
the OP to walk away with 'never' for now and look
into it sometime later when he may have the need
to float his o'scope. There are usually better ways
to solve the problem anyway.

9. ### Robert MonsenGuest

No, no, Fred was asking who the hell would tell somebody to float a
scope, not who would advise somebody not to do this.

On the other hand, I've heard about people who planned to do this.
However, oddly, I've never gotten a report back from them on how it
went...

--
Regards,
Robert Monsen

"Your Highness, I have no need of this hypothesis."
- Pierre Laplace (1749-1827), to Napoleon,
on why his works on celestial mechanics make no mention of God.

10. ### Fred AbseGuest

I know a few people who have (and some who still do), including people
working on big industrial equipment where the thinnest piece of metal
between them and the power station is a hundred amp fuse. You can't tell
some people anything. Sadly, they're the ones that survive. It's the
people who are scrupulously careful and have one fleeting moment of
aberration who have to be scraped off the floor.

11. ### mikeGuest

Communication is always difficult in a half-duplex medium like the
internet. My experience has shown that people often ask simple
questions about complex issues because they don't know enough to ask the
question that gets them the answer they really need. Then they get
many fragments of information that range from correct to wrong to
downright dangerous.
I tend to overkill the answer so that there's a consistent message in
one place.
Since I'm always right ;-) this is best for all ;-) ;-)

Scope probing is a very complex issue and deserves treatment in a basics
newsgroup.

mike

--
Return address is VALID but some sites block emails
..
Wanted, PCMCIA SCSI Card for HP m820 CDRW.
FS 500MHz Tek DSOscilloscope TDS540 Make Offer
Wanted, 12.1" LCD for Gateway Solo 5300. Samsung LT121SU-121
Bunch of stuff For Sale and Wanted at the link below.
MAKE THE OBVIOUS CHANGES TO THE LINK
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12. ### Rick FoxGuest

I gave this advice mainly because when I bought my first 'scope (off
ebay) it had been floated. The earth wire was cut off inside the plug
(UK plug).

Guess what? Finding out about it *was* a shocking experience.

Rick.

13. ### Roger JohanssonGuest

If the ground connection gives us an absolute reference, absolutely free
from noise and absolutely stable within microVolts, there is no reason to
float the 'scope.

If that is not the case the noise and voltage changes from the ground
connection will be added to all measurements.

Maybe the former owner did not trust his ground connection to be
perfectly stable and noise free.

14. ### Fred AbseGuest

More likely he needed to measure above ground and either didn't know about

Fault finding on the input side of switchmode PSUs tends to encourage that
sort of misbehavior.

15. ### Fred AbseGuest

That's one advantage of molded plugs. Tampering is obvious.

What I was commenting on was on the lines of "surely nobody actually
Plus the revelation of the sort of home it had come from.

Tek actually used to make a device for floating 'scopes up to 20 or so
volts. Anything over, it would trip to ground. I've seen one.

16. ### Rich GriseGuest

Actually, I have "floated the scope", but in very controlled conditions,
and we (it was a whole class in tech school) followed all of the safety
rules.

It's doable, but not recommended, for all of the obvious reasons. Then
again, I've seen some pretty badly pitted 'scope ground clips! i.e., if
the scope is properly grounded, what you ground the clip to has to be
very, very close to zero potential from earth.

Cheers!
Rich

17. ### Clarence_AGuest

I have always used an isolation transformer for the entire test
station to avoid such problems, but even this precaution requires
care and use of save techniques.