Beginner Q regarding O'scope grounding

Thread Starter

Johnny7

Joined Feb 5, 2022
20
Excerpted from another thread on this site:

" One thing that you need to be careful of is that you likely have several pieces of equipment, the scope, a power supply or three, a function generator, a computer or network connection, and it is very likely that more than one of these is earth-referred in some way. You need to ensure that these are all compatible. For instance, if your power supply is 12 Vdc and you split it on your board to get +/- 6 V supplies with an artificial "ground" midway, then if that 12 V supply is not floating and you connect your scope probe's ground clip to the circuit "ground" you will be in for a surprise. "

I'm wondering if anyone can clarify (or confirm the truth of) the last statement in this quote. I am as aware as I can be (at this point in my journey) of what can go wrong vis-a-vis improper scope probe grounding, but wouldn't the mid-point of the split supply be 0 volts?

Moreover, if the 12V supply is NOT floating, would it not then be at the same potential as the other earth ground in the circuit?
 

WBahn

Joined Mar 31, 2012
30,071
If you have a 12 VDC supply that is grounded, then the negative terminal of the supply is almost certainly what is at ground potential. When you split it to make a circuit "ground" (really just a common reference point for the circuit), possibly via a voltage divider and opamp, then the circuit "ground" is at 6 V relative to the earth ground. If you now connect your scope probe to your circuit ground, you have created a short circuit that will draw excessive, possibly damaging, current.
 

MrChips

Joined Oct 2, 2009
30,821
Here is what you need to know.

Any instrument that measures voltage (DMM or oscilloscope) requires two connections. A DMM has a red probe and a black probe. An oscilloscope has a probe tip and a probe clip.

On bench oscilloscopes, the probe clip is usually connected to GND.

Before you connect the probe GND clip to any part of a DUT (device under test), make sure that you understand to what node you are connecting. If the DUT is floating, then you can connect the probe GND clip to any point.

If the DUT is not floating, in other words, it is connected to GND, you can either NOT connect the probe GND clip (because the connection to GND is already there) or make sure that you are connecting to a GND node of the DUT (when you want a better probe GND connection).

Edit: If you do connect the probe GND clip to a DUT, you must realize that you have now created a node connected to EARTH GND. Make sure that this is compatible with any and all other instruments connected to the DUT.
 

WBahn

Joined Mar 31, 2012
30,071
But the BIG point that needs to be understood, and is often overlooked (and is a mistake that the TS seems likely to make) is that what you THINK of as the "ground" of the circuit you are making measurements on may not be earth-ground -- some other point of your circuit may be connected to earth ground via some piece of equipment you are using.

Another point that is often overlooked is that, even if your circuit is floating, the moment you connect that probe ground clip to the circuit (on the vast majority of scopes), you have now referred that circuit to earth ground. May people think that the scope probe is like a DMM and that their measurement is the voltage between the probe tip and the ground clip -- and just like they can connect another DMM between two other arbitrary points in the circuit to make an unrelated measurement, they can connect another probe and it's ground clip to any other two points in the circuit.
 

crutschow

Joined Mar 14, 2008
34,459
The thing to always be aware of is that the oscilloscope signal ground is usually connected the the mains earth safety ground through the AC plug.
As long as what your are measuring is not connected to earth ground, then there should not be a problem as you where you connect the oscilloscopes ground clip.
But, of course, if you are using two probes with a dual-channel input, then both ground clips must be connected to the same potential.

And you should always connect all your equipment to a GFCI outlet to minimize the arc flash if you connect something up incorrectly.
 

Thread Starter

Johnny7

Joined Feb 5, 2022
20
If you have a 12 VDC supply that is grounded, then the negative terminal of the supply is almost certainly what is at ground potential. When you split it to make a circuit "ground" (really just a common reference point for the circuit), possibly via a voltage divider and opamp, then the circuit "ground" is at 6 V relative to the earth ground. If you now connect your scope probe to your circuit ground, you have created a short circuit that will draw excessive, possibly damaging, current.
Interestingly enough, the quote in my post comes from an old post by you!

I'm a little confused by your response. You write "If you now connect your scope probe to your circuit ground..." are you referring to the probe or the probe's ground clip?
 

Thread Starter

Johnny7

Joined Feb 5, 2022
20
Firstly, thanks all for your input.

Maybe it's just me, but I don't see where any of these responses answers either of my two specific questions:

• wouldn't the mid-point of the split supply be 0 volts?

• if the 12V supply is NOT floating, would it not then be at the same potential as the other earth ground in the circuit?
 

WBahn

Joined Mar 31, 2012
30,071
Firstly, thanks all for your input.

Maybe it's just me, but I don't see where any of these responses answers either of my two specific questions:

• wouldn't the mid-point of the split supply be 0 volts?

• if the 12V supply is NOT floating, would it not then be at the same potential as the other earth ground in the circuit?
I DID answer your questions -- but you are insisting on making the very mistake I said you were likely to make.

As far as the scope is concerned, there is ONE point in the universe that is at 0 V, and that point is the EARTH ground at the outlet the scope is plugged into.

If you split a 12 V supply, however you do it, then the midpoint of that voltage is NOT at 0 V, it is merely 6 V above the negative side of the supply and 6 V below the positive output of the supply. It is only at 0 V, relative to the scope, if that midpoint is connected to the EARTH ground (which is exactly what is going to happen if you connect the ground clip of the probe to that midpoint).

BUT, if the power supply is not floating, then either the negative or the positive terminal (almost guaranteed to be the negative terminal) is connected to EARTH ground. If the negative terminal of the supply is grounded, then the midpoint of the supply (what you want to think of as your circuit "ground", and which is why "ground" is the wrong name to use for it, it is your circuit "common") is at 6 V.

Imagine driving a stake into the ground. That's your EARTH ground. Your scope probe's ground clip is connected to this stake (this is literally the case -- there is an unbroken connection going from your scope probe ground clip to a stake driven intot he ground somewhere relatively close by (in most countries).

Now imagine you have a circuit powered by two 9 V batteries connected in series so that you have +9 V and -9 V supplies for your circuit. The midpoint of your supplies is your 0 V common reference for your circuit. But these voltage are ONLY relative to each other -- they have no defined value relative to the rest of the universe.

Your batteries (and circuit) are floating, so when you make a measurement and you connect your ground clip to your circuit's common, you are connecting the midpoint of your batteries to EARTH ground. NOW your circuit is no longer floating, but rather it is grounded THROUGH the scope ground clip.

Now take a step back and imagine that your batteries have a wire connecting the negative terminal of the "bottom" battery to that stake. Now, before your scope ever enters the picture, your circuit is no longer floating and your supply voltages, relative to that stake, are at 0 V and +18 V, with your circuit common at +9 V relative to that stake.

When you then connect your scope's ground clip to that midpoint, you are shorting out the bottom battery because both it's positive and negative terminals are connected to that stake.
 

Thread Starter

Johnny7

Joined Feb 5, 2022
20
The mid point of a split supply is a 'zero' reference point, floating GND in most cases usually refers to a non-connection to Earth GND.
I suspect that many of these beginner q's are so elementary in relation to the knowledge base of many here, that it becomes hard to identify exactly what the beginner is 'missing'.

Thanks for at least addressing my question.

More specifically -- in the quoted text, with which I began this post, the author is discussing a split supply (+6V/-6V) and a situation in which the PS is NOT floating.

In this scenario, when he refers to a "surprise", what exactly is he claiming might happen?
Is is that the potential at the 'zero' reference point may be above the earth potential of the 'scope, thereby potentially harming the instrument?
 

MaxHeadRoom

Joined Jul 18, 2013
28,696
Read #5 by @crutschow
As long as what your are measuring is not connected to earth ground, then there should not be a problem as you where you connect the oscilloscopes ground clip.
Often the danger occurs where you are working on a circuit that has no galvanic isolation from the Earth grounded mains supply.
But using a mains power oscilloscope, the common (GND) connector is usually at earth GND potential.
So this can often be the cause of a fuse blowing and/or destroying the GND clip of the 'scope when it is connected to a above-ground potential! :(
There is generally no adverse effects when using a portable battery 'scope.

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MrChips

Joined Oct 2, 2009
30,821
You can have a system with a single supply, a split supply, or any number of supplies with different voltages and polarity. These can all be independent of EARTH GND.

The "surprise" comes from not knowing where and how your system is connected to EARTH GND.
Before connecting the probe GND clip, make sure you know where is EARTH GND on the DUT.
 

Thread Starter

Johnny7

Joined Feb 5, 2022
20
You can have a system with a single supply, a split supply, or any number of supplies with different voltages and polarity. These can all be independent of EARTH GND.

The "surprise" comes from not knowing where and how your system is connected to EARTH GND.
Before connecting the probe GND clip, make sure you know where is EARTH GND on the DUT.
Yes -- as I stated, this is NOT new information for me. The whole point of my question revolves around the fact that I'm trying to apply the quoted text to the issue of 'scope grounding.

Any wisdom you can offer on exactly how to identify a proper earth ground point?
 

WBahn

Joined Mar 31, 2012
30,071
I suspect that many of these beginner q's are so elementary in relation to the knowledge base of many here, that it becomes hard to identify exactly what the beginner is 'missing'.

Thanks for at least addressing my question.

More specifically -- in the quoted text, with which I began this post, the author is discussing a split supply (+6V/-6V) and a situation in which the PS is NOT floating.

In this scenario, when he refers to a "surprise", what exactly is he claiming might happen?
Is is that the potential at the 'zero' reference point may be above the earth potential of the 'scope, thereby potentially harming the instrument?
No, the quoted text is talking about a 12 V supply, not a split supply. It is common practice to take a single-supply and artificially create a split supply by establishing a mid-point reference in the circuit.

What is going to happen, as I have said in my prior posts, is you are going to short-circuit one the supply rails. The effects of this are not well-defined, since it depends on what gives first. It depends on how the supply was split and how much current the artificial ground can support. In a typical low-powered circuit, it is unlikely to damage the scope, because something else will likely limit the current to a non-damaging level. It might simply result in an opamp saturating at it's max current, which may or may not damage it. It might let the magic smoke out of it. But if it is a high-power circuit using stiff transistors to enforce the midpoint reference, you could get sparks, smoke, fire, and a destroyed scope.
 

Thread Starter

Johnny7

Joined Feb 5, 2022
20
No, the quoted text is talking about a 12 V supply, not a split supply. It is common practice to take a single-supply and artificially create a split supply by establishing a mid-point reference in the circuit.

What is going to happen, as I have said in my prior posts, is you are going to short-circuit one the supply rails. The effects of this are not well-defined, since it depends on what gives first. It depends on how the supply was split and how much current the artificial ground can support. In a typical low-powered circuit, it is unlikely to damage the scope, because something else will likely limit the current to a non-damaging level. It might simply result in an opamp saturating at it's max current, which may or may not damage it. It might let the magic smoke out of it. But if it is a high-power circuit using stiff transistors to enforce the midpoint reference, you could get sparks, smoke, fire, and a destroyed scope.
Thanks for the clarification.
 
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