Isolating oscilloscope versus isolating test circuit

Thread Starter

mnevans93

Joined Apr 8, 2016
5
Hello everyone,

New to the forum, and I'm going to start this post by saying I'm a mechanical engineer with no formal electrical training. Try to not laugh too hard if the problem is obvious :oops:

I had a circuit set up to test a unit at work. The basic circuit was as follows:

120 VAC 2 prong power cable ---> Bridge rectifier ~
Bridge rectifier + ---> Multimeter to measure current ---> Large coil ---> Bridge rectifier -

Plugging this into the wall worked fine. The coil received rectified DC voltage and current was about what I expected.

Then, I tried to set up an oscilloscope to look at the voltage waveform the coil was receiving. I was told to use an isolating transformer on the oscilloscope for this purpose to prevent a ground loop. I plugged the 'scope into the isolating transformer, connected the probe to the + coil lead and the ground clip to the - coil lead. As soon as I plugged in the 120 VAC power cable connected to the rectifier, there were sparks and a breaker tripped. There were no visible shorts, as in alligators touching or anything like that.

Later, I tried plugging the rectifier power cable into the isolating transformer and plugging the scope into the non-isolated wall socket, and this time, it worked. What changed?

I read some other posts on similar topics. One in particular caught my eye, which was a post by MrChips in the thread "At what point Isolation transformer needed":

Here is another example of where you could use an isolation transformer.
Most if not all oscilloscopes are grounded and hence voltage measurements are taken with reference to GND.

Now suppose you wanted to measure the voltage across two terminals using the probe and the ground clip. That would not be a good idea because you would be grounding your circuit through the ground clip.

You use an isolation transformer to take the scope off ground, i.e. your scope is now "floating". Now you can use the ground clip as your reference since it is now a floating reference.

Alternatively, you could power your test circuit (power supply and everything connected to it) via the isolation transformer and achieve the same effect.
To my understanding, I was doing the initial option to "float" the scope, but I still tripped a breaker. The second option is the only one that ended up working. Any insight into why this happened would be greatly appreciated. I would like to understand what caused it for future reference. Thank you in advance for your patience with this mechanical engineer! ;)
 

Thread Starter

mnevans93

Joined Apr 8, 2016
5
Sorry for the double post, but it seems that I can't edit my initial post after a certain amount of time.

I wanted to add that I understand this situation is different than the one in the quote because of my use of a bridge rectifier, but I'm not sure what that might do to the circuit to cause this issue one way (with the 'scope isolated) and not the other (with the test circuit isolated).
 

crutschow

Joined Mar 14, 2008
23,776
It sounds like the isolation transformer doesn't isolate the scope's safety ground (connected to earth ground through the 3rd power-plug pin), which is connected to the scopes chassis and input circuit ground. That makes a short-circuit between the mains and the scope ground for a bridge circuit, which you observed.

Alternately, using the transformer for the circuit under test means there is then no path from the mains to the ground, so it works fine and is relatively safe.

Incidentally whoever said you could isolate the scope is flat wrong since that means the circuit still has lethal voltages to earth ground. :eek: Apparently this person is working on a Darwin award.
The isolation transformer should always be connected to the circuit under test.
 

ErnieM

Joined Apr 24, 2011
8,009
Most scopes connect their ground point to the AC power ground, or that 3rd pin on the plug.

The engineering department here keeps a drawer full of those ground pin isolators, or the "cheaters" as anyone with any sense calls them. They break the ground connection when they need to make measurements similar to what you are doing.

At least they insist on using onlye the bright orange adapters so hopefully someone will see them before touching anything.

It is not a recognized practice, but sooner or later we all sucume to the temptation to do it. Just don't leave your now lethal setup for anyone else to stumble over.
 

crutschow

Joined Mar 14, 2008
23,776
Most scopes connect their ground point to the AC power ground, or that 3rd pin on the plug.

The engineering department here keeps a drawer full of those ground pin isolators, or the "cheaters" as anyone with any sense calls them. They break the ground connection when they need to make measurements similar to what you are doing.
-----------------------
But those should only be used to breakup a signal ground loop (which I've done myself at times), but definitely not for working on unisolated line powered circuits (which is an absolute no-no).
 

ian field

Joined Oct 27, 2012
6,539
Hello everyone,

New to the forum, and I'm going to start this post by saying I'm a mechanical engineer with no formal electrical training. Try to not laugh too hard if the problem is obvious :oops:

I had a circuit set up to test a unit at work. The basic circuit was as follows:

120 VAC 2 prong power cable ---> Bridge rectifier ~
Bridge rectifier + ---> Multimeter to measure current ---> Large coil ---> Bridge rectifier -

Plugging this into the wall worked fine. The coil received rectified DC voltage and current was about what I expected.

Then, I tried to set up an oscilloscope to look at the voltage waveform the coil was receiving. I was told to use an isolating transformer on the oscilloscope for this purpose to prevent a ground loop. I plugged the 'scope into the isolating transformer, connected the probe to the + coil lead and the ground clip to the - coil lead. As soon as I plugged in the 120 VAC power cable connected to the rectifier, there were sparks and a breaker tripped. There were no visible shorts, as in alligators touching or anything like that.

Later, I tried plugging the rectifier power cable into the isolating transformer and plugging the scope into the non-isolated wall socket, and this time, it worked. What changed?

I read some other posts on similar topics. One in particular caught my eye, which was a post by MrChips in the thread "At what point Isolation transformer needed":



To my understanding, I was doing the initial option to "float" the scope, but I still tripped a breaker. The second option is the only one that ended up working. Any insight into why this happened would be greatly appreciated. I would like to understand what caused it for future reference. Thank you in advance for your patience with this mechanical engineer! ;)
Isolating the scope is a really bad idea - most have all metal case with the same potential as its signal ground.

Some people take off the earth wire in the mains plug - a few only did it once................

Always use the isolating transformer on the equipment under test.
 

Thread Starter

mnevans93

Joined Apr 8, 2016
5
Thank you for all the responses.

I think my main (hah) struggle is a lack of understanding of how exactly ground and ground isolation works, so I'm still not sure I'm understanding you all correctly.

From what I gather after reading your responses, isolating the scope doesn't actually remove the direct path to earth ground it has from the ground clip to the third pin, but it cuts the connection between signal ground and earth ground, so when I attached the oscilloscope probe and ground clip to the DC outputs of the rectifier, the DC voltage could only go to earth ground rather than the signal ground, causing the breaker to trip. I think I understand this part... correct me if I'm wrong.

Now, when you isolate the line powered circuit, you are essentially removing its center tap and the rectified DC voltage goes through the oscilloscope normally. Is this due to the scope's signal ground having a lower impedance than the earth ground, whereas floating the scope removed the signal ground path connection altogether?

And lastly, why does removing the center tap affect how the current wants to flow through the scope, as in, why is it also bad to have neither the scope nor the unit under test? I kind of just took people's word for needing isolation without really understanding why.
 

ian field

Joined Oct 27, 2012
6,539
Thank you for all the responses.

I think my main (hah) struggle is a lack of understanding of how exactly ground and ground isolation works, so I'm still not sure I'm understanding you all correctly.

From what I gather after reading your responses, isolating the scope doesn't actually remove the direct path to earth ground it has from the ground clip to the third pin, but it cuts the connection between signal ground and earth ground, so when I attached the oscilloscope probe and ground clip to the DC outputs of the rectifier, the DC voltage could only go to earth ground rather than the signal ground, causing the breaker to trip. I think I understand this part... correct me if I'm wrong.

.
With any equipment using rectified mains; the lowest negative potential is equal to the negative peaks of the AC waveform. If you connect the scope ground clip to that - it'll go bang.

Isolating the scope is a seriously bad idea, because then its chassis will be at the peak negative potential of the mains AC waveform.

You need to run the equipment under test via an isolating transformer - then you might live long enough to see if this thread gets closed for discussing transformerless power supplies.
 

Thread Starter

mnevans93

Joined Apr 8, 2016
5
With any equipment using rectified mains; the lowest negative potential is equal to the negative peaks of the AC waveform. If you connect the scope ground clip to that - it'll go bang.

Isolating the scope is a seriously bad idea, because then its chassis will be at the peak negative potential of the mains AC waveform.

You need to run the equipment under test via an isolating transformer - then you might live long enough to see if this thread gets closed for discussing transformerless power supplies.
Right, and that's what we are doing now. I'm simply trying to understand what's going on at a circuit level so I can avoid this in the future entirely. I've had a couple incidents with electricity, this one not being the first. Had a run-in with 440 VAC that was particularly terrifying, so I'm trying to do more research to prevent things like this from happening again. I appreciate your insights.
 

crutschow

Joined Mar 14, 2008
23,776
The crux of the danger is that all mains AC is referenced to earth ground so anything connected to the mains has the potential for current to flow from the circuit to anything in contact with ground (such as yourself or the scope common which is connected to earth ground through the 3rd prong safety ground).
An isolation transformer breaks the connection between the mains and earth ground. Its output voltages are only referenced to the other output winding(s), so touching an output winding and earth ground does not have a path and there is no shock hazard to ground.
 

ian field

Joined Oct 27, 2012
6,539
Right, and that's what we are doing now. I'm simply trying to understand what's going on at a circuit level so I can avoid this in the future entirely. I've had a couple incidents with electricity, this one not being the first. Had a run-in with 440 VAC that was particularly terrifying, so I'm trying to do more research to prevent things like this from happening again. I appreciate your insights.
Rules are rules, and not discussing transformerless power supplies is one of them.

Personally I don't understand the logic of closing a thread because someone strays outside a certain parameter - in most cases a TS is going to go ahead and do it anyway, and far more likely to harm themselves without the advice and warnings they'd get on this forum.
 

Thread Starter

mnevans93

Joined Apr 8, 2016
5
Rules are rules, and not discussing transformerless power supplies is one of them.

Personally I don't understand the logic of closing a thread because someone strays outside a certain parameter - in most cases a TS is going to go ahead and do it anyway, and far more likely to harm themselves without the advice and warnings they'd get on this forum.
I would assume it has to do with liability and legal issues, because if the TS hurts themselves or someone else, and it somehow comes back to the forum that they were given advice here, even if it was misinterpreted, legal issues could ensue. I didn't mean to break the rule to begin with and I apologize. I also don't mean to be clueless when it comes to electricity and wiring, but... yeah.

Thank you for bearing with me regardless!
 

ian field

Joined Oct 27, 2012
6,539
I would assume it has to do with liability and legal issues, because if the TS hurts themselves or someone else, and it somehow comes back to the forum that they were given advice here, even if it was misinterpreted, legal issues could ensue. I didn't mean to break the rule to begin with and I apologize. I also don't mean to be clueless when it comes to electricity and wiring, but... yeah.

Thank you for bearing with me regardless!
You could also argue that the forum denied them advice that might've saved their life................
 

IBCNU

Joined Jul 19, 2018
6
I read and read and never heard anyone say the neutral and ground are connected at the beaker panel and Isolation transformers not only lift the ground but also isolate the neutral and line
 

ErnieM

Joined Apr 24, 2011
8,009
I read and read and never heard anyone say the neutral and ground are connected at the beaker panel and Isolation transformers not only lift the ground but also isolate the neutral and line
I say unto you neutral and ground shall and must be connected at the breaker panel and shall not no never be connected past the panel.

There. Now you can read it for yourself. It is part of standard wiring per "the code."

I know not what you mean by "lift."

Isolation transformers can break any common connections wherever they may be.
 
Simple demonstration circuit of the output of a line connected power supply. (not something you would want to build)
Notice that the DC rides on top of an AC voltage. Hence the danger that beginners never see.
upload_2019-6-24_21-1-0.png
 

ebeowulf17

Joined Aug 12, 2014
2,960
From what I gather after reading your responses, isolating the scope doesn't actually remove the direct path to earth ground it has from the ground clip to the third pin, but it cuts the connection between signal ground and earth ground, so when I attached the oscilloscope probe and ground clip to the DC outputs of the rectifier, the DC voltage could only go to earth ground rather than the signal ground, causing the breaker to trip. I think I understand this part... correct me if I'm wrong.
Not quite how I'd describe it. See if this makes sense:

A normal isolation transformer *should* not have any connection between the "ground" pins on input and output. In other words, it *should* break the ground path completely.

It sounds like, in your case, the isolation transformer did not break the ground path on the scope as expected. Although perhaps another possibility is that the transformer was set properly but something else completed the ground path. For example, maybe the scope chassis is metal and was resting on a grounded metal table or countertop, which provided a ground path. In any case, it sounds like your scope had its chassis grounded when you expected it not to be.

As for why isolating the rectifier circuit worked, even though the transformer may not be isolating the way we want it to, there are several factors here:

1) As @IBCNU stated above, in North American mains power, the neutral is tied to earth ground at the breaker panel, so electrically they're basically the same (there are important safety reasons to treat them differently and follow all applicable electrical codes, but for the purposes of the following analysis, they're the same.)

2) Your rectifier circuit used a two-prong plug, meaning it utilizes 120V Line and Neutral, but no third connection for Earth Ground.

3) Because of the Earth Ground-Neutral connection in item 1, your rectifier circuit is still ground referenced, even when it's only connected to Line and Neutral.

4) Because the rectifier circuit is ground-referenced through the neutral on the input side, you can see in @CharlesWMcDonald's simulation above that the rectified output "rides" the AC waveform. In other words, the difference between the two output terminals is rectified DC, but both output terminals still carry a high AC voltage relative to Earth Ground.

5) Assume for the sake of analysis that *your* isolation transformer is providing transformer isolation for Line and Neutral, but is providing a direct (not isolated) connection for Earth Ground.

6) For all practical purposes in terms of safety, the signal ground, the Earth Ground, and any exposed metal frame on your scope are all equally grounded.

When you add all these facts together, this is what you get:

In the first scenario, when you tried to isolate your scope, it was still grounded. When you connected your signal ground to the rectifier output, you actually connected AC mains (on the rectifier output) directly to Earth Ground (on the scope "signal" ground clip.)

In the second scenario, isolating the rectifier circuit, the transformer isolation worked, because you only have a two prong plug. The rectifier circuit was then "floating" with no DC connection to Earth Ground. When you connected the scope probes to it, that grounded the negative side of the output to Earth Ground, providing a stable reference against which to measure the positive output.

Sorry I wrote quite a lot there. Hopefully this makes sense. Let me know if you have any questions.
 
Last edited:

IBCNU

Joined Jul 19, 2018
6
I say unto you neutral and ground shall and must be connected at the breaker panel and shall not no never be connected past the panel.

There. No w you can read it for yourself. It is part of standard wiring per "the code."

I know not what you mean by "lift."

Isolation transformers can break any common connections wherever they may be.
Hello ErnieM, I have only studying electronics for a very short time and know nothing of code, as for your reply lift was probably the wrong choice
of words and forgive me but your reply was just a little anal for me. I say unto you Lighten up.
 
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