I have acquired an old oscilloscope plus some other equipment for the sake of getting more involved in learning about electronics. While setting up my home test bench I have been reading online and asking those who work with Oscilloscopes about the proper way to set-up the equipment. When it comes to using an Oscilloscope it appears that while many people may "float the ground" by removing the ground prong or other means it seems that is not the approved manner in which to use it. Most of the feedback I've got from various sources is that the device being tested should be powered through an isolation transformer and the scope should have an earth ground. Apparently the reverse will work, but for reasons I haven't figured out that is not the norm.

Any Oscilloscope users here care to share their own preference as to how the set-up their equipment?

The best case is when you can connect the scope normally. Floating the scope is certainly the last thing you want to do. You are potentially putting line voltage on the case of the scope and bad things can start happening very quickly, up to and including death.
Generally speaking, I will look at the wiring diagram and decide where the grounded conductor is and ground the scope there if I absolutely need to but for most things, the small amount of ripple you see if you do not ground the scope to the circuit under test will not affect your measurement.
It is still tied to the ground via the plug.
If you need to null out the voltage drop on the neutral, hook channel 2 to the neutral, set it to invert and add, then probe with channel 1.

If you are looking at something on the load side of a transformer (not autotransformer) you should be able to ground the scope normally. You can ground the scope to the circuit through a light bulb if you are not sure what you are dealing with, voltage wise. If it lights, you are going to have to reconsider what you are doing.
Be sure you have the right probes and ground clips if you are looking at line voltage. It is really not something I like to do unless I have to.

It is best to learn your scope technique with a transformer isolated circuit, preferably at low voltages, like a doorbell transformer or a wall wart.
When I was playing around with dimmers, I found out the scope pictures on the load side of a doorbell transformer was virtually identical to the line side.

Thanks for the information Greg.

I think the bottom line is you don't float scopes.
In 40 years of working with scopes, I have never had the need or the desire.

What kind of scope do you have?
What are you going to be looking at?

Just got home from son's soccer game.

I will get some photos and a good description of the scope posted tomorrow evening.

As for what I will be looking at? Anything I can get my hands on that isn't too sophisticated and can help with the learning curve. I've been saving all kinds of stuff from the scrap metal/recycle bins at work. For example, I brought home an old yet in great condition SCR Variable Speed Drive (old and rare enough not even Google can find it) which I plan on using as a DC power supply and a Variac which I will use as an AC power supply when building circuits on a Protoboard. My only background in electronics was the classes I took in Electrical Apprentice school. BTW the scopes we used in the school lab were powered by an isolated power supply (just confirmed that the other day). That fact is why I've been asking about the proper way to set-up my scope since it's contrary to what you and most everyone else I talk to is telling me.

I would just leave it grounded through the plug and not even bother to try to ground the scope to the circuit under test. On grounded line voltage circuits, there is already a ground reference via the neutral.
Just be sure you have leads rated for the voltage you are probing.

I've had to lift the scope ground in the past when working on TVs with a hot chassis. I did this knowing that I would be in for a jolt if I touched the scope case and Earth at the same time. I would always prefer to plug the TV into the isolation transformer and leave the scope grounded. Anytime your DUT has a polarized cord and feeds directly into a bridge rectifier, you're most likely dealing with a hot chassis and use caution to avoid tingles and arc welding.
Joe
And if I didn't ground the scope to the ckt, the line noise would usually swamp any signal I was trying to scope, PLUS I was forced to AC couple and that's not always a good thing.

Here she is - Philips PM 3230

That looks like it will get the job done.
I doubt you will be looking for delayed sweep or any of those other advanced things for the kind of things you will be looking at.

I don't know what to expect.
Haven't even plugged it in yet. Purchased it for $70 with some accessories. Looks like it needs to be cleaned first.

Can't find much info on it but I did find this basic description:

"Philips PM3230

Dual trace 10Mhz.
Hybrid valve and transistor components
Produced 1971 in Holland"

Do you have probes?
I looked at the picture and I don't see a calibrate output. Usually scopes have a square wave generator to calibrate the probes and give you a known test source.

That looks like it's only a 100 kHz scope but the Time/Div. knob is obscured. If the DC Bal. adjustment is correct, the trace should stay in the same place when the probe is grounded and you switch through the Volts/Div. That adjustment tends to drift a lot on low end scopes and don't even play with it until the scope has a chance to temperature stabilize.
Joe
http://www.mpja.com/20MHz-Oscilloscope-Probe/productinfo/15085%20TE
Really reasonable 20MHz X1, X10 probes for $8

I do have a couple test probes. I will get some better info on them....

I found 2 manuals online for Philips Oscilloscopes but they are not the same model. The PM 3230 appears to be a bit of an odd ball scope since it looks nothing like the models that came before or after. I did find an owner's manual on the 3230 but it's in Dutch.

The guy who sold it to me used it to fix stereo equipment as recent as 3 years ago.

Can you attach a better picture of the front, showing all the knob and switch names? Most scopes use a 1kHz square wave as the calibration signal. You can bread board up something for that fairly easily.
Joe

Good picture here

http://www.ebay.com/itm/Vintage-Phi...e-Model-PM-3230-/131392647983?rmvSB=true

The calibrator is on the right at the bottom of the trigger section

That is a good pic, g.

As far as a PM 3230 manual is concerned it looks like I might have an easier time learning to speak/read Dutch than finding an English version. If only I could read Dutch:

I contacted another owner of a PM 3230 this weekend who was selling his PM 3230 for parts and I inquired if he had a manual for sale. He said despite having numerous connections through various friends, clubs, and organizations he was unable to locate one.

Here is the details of the scope probes I have.


1st probe is an older model

TEK P6105

Attenuation: 10x within 3%
Input Resistance: 10 Mohms
Input Capacitance: 10.5 pF
Compensation Range: 15 pF to 47 pF
Bandwith: (-3 dB) At least 100 MHz for the 1st and 2nd meter
Max Input Voltage: 500 volts (DC + peak AC), derated with frequency

2nd probe is brand new

CIRCUIT-TEST OP-100A

Attenuation: 10x
Input Resistance: 10 Mohms when used with Oscilloscopes which have 1Mohms input
Input Capacitance: 13 pF
Compensation Range: 10 to 30 pF
Bandwith: DC to 100MHz
Max. Input Voltage 600V CAT I, 300V CAT II (DC + peak AC), derating with frequency

The Tektronic probe is the ones we were using on our newer scopes in 1996. They are good probes. Not sure about the other one. That Ebay picture of the scope I linked is the best I found on the web.
I looked for your manual and all I found was numerous places where you can get a PDF of the Dutch version

That picture is labeled in Hungarian, not Dutch...
/mike

The old joke goes
"A person who speaks 2 languages is bilingual"
"A person who speaks 3 or more languages is a polyglot"
"A person who only speaks one is an American"
(alternately an Englishman)

From what I read all of the original documentation was in Dutch and I found translations into other languages but English seems to be elusive.

Here it is in english



A bigger picture is here
You can print this in "landscape"

http://gfretwell.com/electrical/PM3230.jpg

Here's a nice video on Lissajous patterns. Your external X amplifier is useful for studying phase relationships.
https://www.youtube.com/watch?v=t6nGiBzGLD8
It might remind you of "The Outer Limits".
You would use TV Frame triggering and usually - slope to look at composite video signals. Some scopes can select between even and odd fields. Frame rate is almost 30Hz and the color frame rate is almost 15Hz.
The stability adjustment on my 15MHz Heathkit scope, had to be adjusted to get a full width horizontal trace.
Joe

I lived in the digital world so we never used the X input. You can plot the 2 outputs from a stereo audio amp to get interesting patterns tho.
I knew some guys who rigged an old Heathkit GR25 TV to do it.
I also played with doing it with a laser to project on the wall but lasers were tough in those days. (no diodes)

Never float the scope! If you need to make floating measurements, plug the DUT into an isolation transformer, or use a differential probe on the scope.

http://www.amazon.com/Precision-PR-60-Active-Differential-Probe/dp/B006YYS7L0

When I was younger, I didn't have the $$$ for isolation transformers. I knew that the TV had a hot chassis and my scope had a power transformer that would allow it to reference at the TV chassis ground, if I lifted the scope earth ground. So I removed the three of us from proximity to Earth. Not as dramatic as a guy in a Faraday Suit, hopping off a helicopter, to work on 500KV lines, but the same general idea.
Joe
Really, I removed the scope's and my reference to Earth and the TV chassis shifted us negative WRT Earth.

I don't see the switch on the PM but our Tektronics scopes had the ability to invert and add channel 2. You could put ch2 on the hot common, invert/add and probe with 1. You then just see the differential signals without a ground reference

I don't know if I should ask this question here but it is related.

The other day a friend who is renovating a hospital OR theatre was commenting on the voltage found on the isolated or "floating system". It all made sense. He was getting 60V from L1 to ground and 60V Neutral to ground (or I guess you could call it L2).

After our conversation I was thinking about my Hammond 120:120 isolation transformer that I use to energize devices under test with my scope. I plugged it in to a standard wall outlet that measured 123 V. Then I measured the secondary of the Hammond transformer via the standard female cord end. I found approximately 120 V from L1 to Neautral, 72 V from L1 to ground, and 20 V from Neautral to ground. The last two measurements do not add up to 120 V. Where did the other 28 V go?

I was thinking that the hospital system is 3 phase vs my home being single phase and maybe that's a clue as the voltage readings?

Considering that your earth ground is completely separate from the isolated 120V circuit under test, I would not be surprised at practically any readings one might get, and I would not expect the two readings (L1 – G and L2 – G) to necessarily add up to exactly the L1 – L2 voltage (120V). And having L1 – G and L2 – G appear to split the 120V evenly seems a little suspicious. Could there be some inadvertent coupling going on?

That said – remember that by taking measurements, you are in effect making a connection that did not exist before the measuring operation - albeit a very high impedance connection.

Are you taking your readings on an open circuit? Or is there some load? Would it make any difference? I don’t know. Try it both ways and see.

I recall in Navy shipboard electrical systems, which are intentionally UNgrounded, you could still get a good jolt by touching a leg of the 120V system and any exposed metal around (i.e. – earth ground, so to speak), due I suppose to capacitive coupling.

Working with ungrounded (isolated) circuits can be fascinating – definitely a little different.
Thx.

Appreciate the response.

I found some information online that might help explain why I think the two readings on the secondary should add up to the voltage found between the two isolated conductors:

http://www.aptsources.com/resources/pdf/Floating%20Output.pdf



I might be way off with the above information but I believe that the diagram could represent the secondary of my isolation transformer where the L1 and Neutral (or maybe it should be called "L2" since it is no longer connected to earth ground) are isolated from ground. The ground connects to the shield of the transformer, the chassis bond, and ultimately to the earth ground.

In my test there was wasn't a load on the secondary. I simple took readings with my DMM at the female cord end that is connected to the secondary.

Here's an exact image of the transformer I am using:



As you can see, the male end plugs into a standard 120 V outlet which feeds the primary. The female cord end comes off the secondary.

EDIT: Whoops - that looks like a 240 V male end. Okay, that isn't the exact same transformer. Swap out the 240 V for 120 V and everything else is identical.

More images from the previously posted link:

Just when I couldn't get anymore confused...

This situation has been bugging me all day. Moments ago I went back out into the garage and plugged in my Hammond isolation transformer and took measurements off the secondary with the DMM again. Same results. 120 V from L1 to N, 72 V from L1 to ground, and 20 V from N to ground.

Next, I got out my old Weston Equipment analog voltmeter and took the same measurements again. This time I only got 120 V from L1 to N. The other measurements, L1 to ground and N to ground, failed to move the needle. 0 volts both times! To make sure the meter was responding correctly I unplugged the transformer and took measurements at the wall outlet. Everything checked-out as you'd expect - 120 V from L1 to N, 120 V from L1 to ground, and 0 V from N to ground.

In your friend's case, he's probably looking at a floating system with ground detection on it. The ground detector forces a high impedance ground reference at the center point. In your case, I wonder if they haven't built in a high megohm resistance between their Neutral and case ground. Or some TVS leakage or capacitive coupling between Neutral and Ground. You don't want the two to add up.
Joe

When you are using a digital meter on anything that is "floating" it is going to give you strange results.
You see the same thing if you hook up to a wire that is just floating in a wall.

We had ungrounded isolation transformer driven "convenience outlets" in most of the larger computer equipment. It was deemed to be safer because you would not be shocked if you got across either leg referenced to ground.

I was thinking something similar, that a Filter was responsible for the readings; however it could simply be Capacitive Coupling, as the OP has indicated the Analog Meter's Pointer indicated 0 Volts on the L-G tests, yet the DMM measured E on both L-G tests.

Even though both Meters exhibit a High Input Impedance, the Analog Meter might be 100K Ohm, and the DMM might be 50M Ohm - much higher Z than the Analog Meter.

--Scott (EE)

My friend is working in an OR theatre where there is an isolation transformer for each theatre and yes, there is ground current detection monitors in each theatre, as well. He claims his tests add up to 120 V when metering the separate conductors from each to ground. He might be incorrect.

In the article I posted above the images show each conductor metered separately which adds to 120 V. I realize that I might be confused and that the article has nothing to do with either his or my situation.

For my transformer you suggest that you don't want the voltages to add up to the supply voltage. Why?

Your friend is correct. If he disconnects his ground detector(s), he should get different results, ideally zero volts.
You don't want them to add up because there is not supposed to be any connection to ground. I don't really like the terms, "phantom" or "ghost" when discussing voltages. We did have a ghost at WCLQ-TV in Cleveland, but that's a completely different story. My original saying is, "All the world's a voltage divider." Your DMM probably has an input impedance of around 10 Megohm. A VOM could be around 50Kohm/volt. For kicks and giggles, connect your VOM and DMM, both on AC volts, in series across your transformer. The voltages that you observe on both, should reflect the ratio of their input impedances. You might not see much on your Weston, while observing almost full voltage on your DMM.
So if you hook your 10 Mohm DMM up to one leg and ground, and read about 60 volts, you are probably dealing with about a 10 Mohm combination of resistive leakage, and/or capacitive, and/or inductive coupling.
Joe

I think I should clarify a couple of things. By TVS, I mean Transient Voltage Suppression. Floating supplies can be a wonderful thing, but not if they float too far away from Earth reference. Various forms of coupling can steer them to potentially deadly differences WRT ground. For this reason, it's not unusual to use TVSs, to clamp at least the unreferenced floating Neutral to ground. Let's say you use a 1.5KE51CA, between Isolated Neutral and Ground. It would start conducting around 51 VAC, and clamp your supply. But below that point, there is a leakage current that must be considered.
I just went to the Digi-Key site and pulled the datasheet for the Littelfuse parts. The PDF was too large to attach but it shows that the 51 volt part might have a leakage current of up to a 1 uA. That's not enough to cause a problem, but plenty to confuse an electrician with a Fluke.
Joe

Okay, just to make sure I understand and that I am providing the right information I dug up a diagram of how an OR theatre should be wired according to CSA's Z32 code book regarding health care facilities. I have done some work in an OR theatre as far as taking the cover off the panel and inspecting the guts so some of this looks familiar:



If I understand you correctly, a DMM would measure a percentage of supply voltage from L1 to grnd and N to grnd because of the ground detection device (Line Isolation Monitor?) but if it were out of the circuit then there would be zero voltage measured?

BTW, the measurements I took on my isolation transformer were done with a Fluke 117 with 5 Mohm R and the Weston has 11.535 Kohm R at 150 V setting.

"If I understand you correctly, a DMM would measure a percentage of supply voltage from L1 to grnd and N to grnd because of the ground detection device (Line Isolation Monitor?) but if it were out of the circuit then there would be zero voltage measured?"

That's right, your measured voltages should be close to zero. Also, with the 2-pole breaker for the ground detector off, I'm guessing that you would measure a resistance through the detector that is substantially lower than your meter input impedance.
Joe

Alright, I follow that.

Here's a diagram of my Hammond isolation transformer from the website:



http://www.hammondmfg.com/171.htm

In my opinion it is similar to the diagram of the isolated system above minus the ground detection device. The above system has a grounded shield and my transformer, according to the website information, has an electrostatic shield (Faraday shield?) that is grounded.

My DMM measurements on the secondary show voltage where ideally their shouldn't be. The Weston shows no voltage where, according to what you're saying, there shouldn't be voltage.

So here's the million dollar question - is it possible to suggest that the DMM is simply the wrong meter for measuring voltage on the secondary of an isolated system/isolation transformer. I think more than one of you may have already suggested that earlier with me being slow to catch on.

I'm just trying to nudge you along towards a deeper understanding of things. They mostly fall under the "nice to know", rather than "need to know" heading. I would never suggest that you give up a hi-z DMM for a VOM or a Wiggy. The reason is that you can always make a hi-z lower, while making a lo-z higher, results in much lower readings. You can always toss a power resistor with leads into your meter pouch if you want to load down your measurement. A 50K, 2W resistor across your DMM will yield readings more like a Simpson or Weston, and the difference in the readings gives clues to the source impedance that you're looking into. Just always consider power dissipation in cases like that. A 2W resistor would be fine for a 240 volt system, but not 480V.
Joe

Here we go again Joe. I'm trying to wrap my head around this so hopefully I'm not wearing your patience thin...

The DMM has a high input impedance and therefore should have minimum impact (meter loading?) on the measurements I was taking on the secondary of the Hammond Isolation transformer - right? Therefore the result was the voltage readings of 72 V and 20 V.

The analog meter with it's lower input resistance would have an impact on the measurements and act as a load - right? Thus I got the 0 volts reading.

After reading the information below am I on the right track? Without a substantial load on the secondary of the transformer the voltage readings I am getting with the DMM are a result of, and I quote, "under lightly loaded conditions, the reactive drop may not provide sufficient voltage reduction, or attenuation"???

Forgot to mention that the above quoted information was put out by Square D.

I think Joe is saying (in part) that to try to measure the “difference in potential” between two electrically random points that have no electrical connection to one another (until you connect a meter) is sorta nonsensical in a way, meaning that any reading you get means next to nothing. And taking two different (& rather random) voltage readings between two sets on electrically unrelated points – I would not expect the sum of the two readings to add up to anything significant.

I think the higher the meter impedance, the wilder the readings you may expect due to a reduced impact on the circuit under test. The lower the meter impedance, the closer to 0V you will likely read. Remember you can connect (i.e. SHORT) any ONE point of an ungrounded (or isolated) system you wish directly to earth ground without ill-effect. That’s why Navy shipboard electrical systems are intentionally UN-grounded. Tactical reliability.

However, caution is needed, as Joe has mentioned, due to various forms of coupling, the actual effective EMF to ground of a given point in an isolated system can be dangerously high, if left to float completely independently (no anchor). If that point is touchable, it can be dangerous.

I'm not sure about any of this but I do know that connecting a meter across the secondary of an isolation transformer or isolated system will indicate supply voltage. Measuring voltage from either conductor to ground is where it seems to get wiggy.

BTW, what you stated about a floating/isolated system and it's built in safety aspect as far touching one conductor and ground is something I tested with my Hammond isolation transformer. I connected one conductor directly to ground and flipped the power on to see if it would function as it should. As advertised, nothing happened and no measurable current found with my Fluke clamp-on meter (with the conductor wrapped around it several times).

Navy shipboard AC electrical systems are ungrounded for that very reason. You can inadvertently ground any one point with no mishap whatsoever. But one point only.

Those systems have ground detection circuits built in, to alert the crew of an inadvertent ground, so it can be cleared before some other point becomes inadvertently grounded.

However - there is some coupling between the floating electrical system and ship's ground that occurs because of the ground detection circuitry, and additional coupling that occurs due to capacitance. One can receive a severe shock, fatal even, by coming in contact with an ungrounded system "Hot" and the ships hull (which is all around, by the way).

But that hazard aside, while the electrical system floats, you can still ground any single point you like in the electrical system and nothing will happen.

I have lots of patience for people who give a darn and are willing to take the time to figure things out!
That said, your last attachment is leading you down the wrong path. That material is letting you know that harmonics, noise, and transients, that wouldn't typically have much amplitude in a normal load impedance, can create problems with a high load impedance. Again, the ratio of impedances comes into play. Your source 50 or 60 Hz power should have an extremely low source impedance, while the harmonics and noise should be a hi-z source. But if your load z is high, you might see close to the same amplitude on the secondary.

You're dealing with more like what we consider "parasitics" in electronics. You showed a description of a shielded isolation transformer. Think of a twisted, shielded cable Vs a non-shielded twisted-pair cable. The pF/foot is much higher on the shielded cable so although you have more noise immunity, you suffer more high frequency attenuation. Your transformer probably has a greater amount of capacitive coupling between the secondary Neutral and the grounded case. You should draw a resistor, a cap, and an inductor, between each leg and ground. Then draw a voltmeter symbol to represent your DMM, and another for your Weston. Then write down the input resistance and capacitance for each. All of the Rs and Cs and Ls make complex voltage dividers that cause readings like the ones that you're getting.

A ground detector is there to detect grounds, not to protect from getting shocked. It actually creates a path that may or may not limit currents to a safe level.
Joe

I forgot to suggest a couple of things for your observations:
1.)Check your readings again with your isolation TX unplugged. It helps you see how much your setup is acting like an antenna. If the readings are non-zero(They will be.), try moving the leads around on the table or placing your hands along the wire insulation in various places.
2.) Switch your meter to read frequency if it has that feature. See if you're reading line frequency or a harmonic of same.
Joe

Yes, I have noticed the possibility of interference. For example, I will later post a couple images of the wave form shown on my oscilloscope when a nearby LED magnifying lamp is turned on and off. The wave form shape remains the same but the "lines get fuzzy".

Well better late than never...

Scope showing square wave with nearby LED lamp off:

And with LED lamp on:

The most preferred scope is to connect normally. Floating the scope is the last thing you would want as an alternative.
You are potentially putting line voltage on the case of the scope and bad things can start happening very quickly, this could impose a risk of life too.

We suggest you look at the wiring diagram and decide where the grounded conductor is and ground the scope at that point. The small amount of ripple you see will not affect your measurement. It is tied to the ground via an external plug.

If you are looking at something on the load side of a transformer you should be able to ground the scope normally. You can ground the scope to the circuit through a light bulb, voltage wise. If it sparks, you are going to have to reconsider your plan of action.

Be sure you have the right ground clips if you are looking at line voltage.
It is best to learn your scope technique with a transformer isolated circuit, preferably at low voltages, like a doorbell transformer or a wall wart.

Electrical companies can provide the necessary advise based on the requirement and concern with the floating ground.