Ground Oscilloscope to negative on simple circuit?

[davenc]

With a simple breadboard circuit, like an RC network driving a blinking LED, is it appropriate to connect the oscilloscope ground on the probe to the negative power source?

There isn't a ground per-say.

Thanks

[rstofer]

With a simple breadboard circuit, like an RC network driving a blinking LED, is it appropriate to connect the oscilloscope ground on the probe to the negative power source?

There isn't a ground per-say.

Thanks

We don't know anything about how the circuit is powered and that is the crux of the problem with grounding.  Without a schematic, everything is a guess.

In general, guessing, without a schematic, yes, the circuit common or ground is the place to connect.  I do breadboarding all the time and I never have a doubt about where to place the ground lead.  Partly this is because I KNOW that my power supplies don't have an earth ground reference.  If they did have such a reference, this would be the place for the ground lead.  In fact, it would be the only place.

But the key is, I KNOW how my power supply is connected.  In fact, it floats unless I deliberately place an earth ground jumper at the binding posts.  In general, I don't do that.  Wall warts are usually floating, batteries are floating, most everything you will deal with is floating.  Where the problem comes up is with switching mode power supplies.  Those have destroyed a lot of test equipment if I believe what I have read.

[David Hess]

Most circuits use a negative ground and people are just used to that but there are exceptions.  ECL (emitter coupled logic) for instance uses a positive ground and a -5.2 volt supply because its threshold voltage is referenced to its positive supply.

Assuming that everything is setup properly, it does not matter but usually you will want the oscilloscope ground connected to the negative supply or to "common" if a bipolar supply is present.

[davenc]

Thanks for the info.  Permit me to detail my setup then.

I am drawing 9.2 volts from a Tekpower TP3005T Variable Linear DC Power Supply that has a three prong plug into the house.  There is a +, -, and ground post on the output.  I have the + and - powering my breadboard.  The circuit is attached to this message.  I want to measure the waves at various locations in the circuit.  My questions is, where to connect the probe ground?

Thanks again...

[Ian.M]

If in doubt, WHEN DEALING WITH LOW VOLTAGE DC CIRCUITS, connect a small, low current incandescent bulb (e.g. a torch bulb) with an appropriate voltage rating between the scope ground clip and the supply rail you want to connect it to.   If it lights up or blows, don't connect the ground clip to that rail. Test the bulb!

Caution: connecting some other piece of test equipment that introduces a ground to a different point in the circuit may still let the magic smoke out even if up to that moment you had your scope connected and working properly.

[David Hess]

I am drawing 9.2 volts from a Tekpower TP3005T Variable Linear DC Power Supply that has a three prong plug into the house.  There is a +, -, and ground post on the output.  I have the + and - powering my breadboard.  The circuit is attached to this message.  I want to measure the waves at various locations in the circuit.  My questions is, where to connect the probe ground?

The power supply has an external ground post because it supports floating operation allowing you to connect the oscilloscope ground to either the negative or positive output.  For safety, just leave the ground post disconnected; that is what I usually do with my floating power supplies during development.

For your circuit, it is probably most convenient to use a negative ground because that is where the NPN transistors have their emitters connected.  If you mirrored the circuit using PNP transistors with their emitters connected to the positive supply, then a positive ground connection would be more convenient for measurement.

[SirAlucard]

I am drawing 9.2 volts from a Tekpower TP3005T Variable Linear DC Power Supply that has a three prong plug into the house.  There is a +, -, and ground post on the output.  I have the + and - powering my breadboard.  The circuit is attached to this message.  I want to measure the waves at various locations in the circuit.  My questions is, where to connect the probe ground?

The power supply has an external ground post because it supports floating operation allowing you to connect the oscilloscope ground to either the negative or positive output.  For safety, just leave the ground post disconnected; that is what I usually do with my floating power supplies during development.

For your circuit, it is probably most convenient to use a negative ground because that is where the NPN transistors have their emitters connected.  If you mirrored the circuit using PNP transistors with their emitters connected to the positive supply, then a positive ground connection would be more convenient for measurement.

So a power supply like that would be floating? Cause I was looking at one of those myself.

[davenc]

So I will connect the probe ground to the negative where the negative goes into the transistor, if I understand you correctly.  Right?

[danadak]

Oscilloscope grounding and safety -


https://www.tek.com/document/technical-brief/floating-oscilloscope-measurements-and-operator-protection


http://in.tek.com/dl/3AW_19134_2_MR_Letter.pdf



Regards, Dana.

[Richard Crowley]

There are four or five different questions here:

Where is "circuit ground" in my diagram?
Most people would designate the "blue" (-) node as "circuit ground".  Perhaps that is why they made it blue.
If you were measuring DC voltages at various parts of the circuit, that would be the logical reference.

How does  the powering scheme affect whether I can connect a grounded oscilloscope to this circuit?
If you were operating this from an isolated 9V battery, the answer would be simple. Just connect scope ground to circuit ground.
But you are powering your circuit from an external, mains-operated bench supply. Which significantly changes the landscape.

Is my bench supply isolated from ground?
Your TP3005T appears to have a SEPARATE green banana jack labeled "GND".
From that, we might assume that the main + and - outputs are isolated from mains ground/PE.
However it would good to know that for sure. Especially since neither the marketing blurb, nor the "user manual" even mention "ground" or "isolated".
I would confirm it by internal inspection, reference to a published circuit diagram, or by measurement. Then you would always know.

How does ground from the power source affect how you connect an oscilloscope?
If your power source is referenced to ground, then the ONLY safe way of connecting an oscilloscope (assumed to be mains-ground referenced) is to connect scope ground to the same circuit node (typically identified as "circuit ground") as the grounded power supply node is connected to.
Of course, there are standalone, isolated, battery-powered oscilloscopes (or equivalent). They would be more similar to the meter rule below....

How does ground from the power source affect how you connect a meter?
Most meters (even "bench" meters powered from mains) are fully floating and isolated.
And certainly hand-held, battery-operated meters (DMMs etc.) are inherently isolated.
So, in a low-voltage circuit (generally anything < 50V), you should be safe probing ANY node in your circuit.

[Shock]

So I will connect the probe ground to the negative where the negative goes into the transistor, if I understand you correctly.  Right?

The most convenient place close to the measurement on the negative rail is ok unless it's a precision measurement.

When you know you are not introducing other earths/grounds and the circuit is fully floating you can put it anywhere. Important to note after you have an attached oscilloscope ground (or from any other equipment) your circuit is no longer floating.

Your bench supply has a ground output and is linear so you would assume floating... BUT they can come with a jumper clip that connects negative to earth, this makes it earth referenced. So first check to see it's removed.

To make an reasonable assessment if the power supplies floating you can do the following.

Unplug the bench supply from the wall and your circuit. Test the continuity and resistance (multimeter) between the mains plug earth pin and negative on the supplies outputs. Do the same now to the positive output. There should be no connection. Test continuity and resistance (multimeter) from the earth pin to the earth output socket. It should be a good and low resistance connection.
 
Plug in the supply and dial up a say 12V output on (without a load), take a voltage measurement (multimeter) on the supplies outputs, first positive and earth, then negative and earth. There should be no voltage.

As Ian mentioned we can perform a bulb/load test to see if current passes between two points. So put an incandescent bulb (and dial up the correct voltage for it) on your supplies outputs between positive and earth, then negative and earth. If it lights you have current. You can also substitute a resistor for this as well then measure voltage across the resistor, the idea in both cases is to load the circuit down and see if current flows. Use ohms law to find a suitable resistor value and power rating.

Don't ever load random parts of unknown circuits down (this is why we are careful where we put the ground), only power rails and again use ohms law to use suitable load. Drawing current (too much current) through sensitive components is bad.

[Brumby]

@ davenc

Don't let the above responses scare you off.  They are all good points that you should understand at some point.

The reason they have been presented is that your question shows that you are very new to using a scope and that there are some important things you need to know to protect yourself, the scope and the circuit you are working on.

As such, it is advisable that you get to understand them sooner than later.

The fact that you have brought the question here indicates you are aware that there is "something" important about where you put an earth and we want to make sure (as much as we can) that you not only know what to do ... but, more importantly, WHY.

Read the above - and when it all makes sense, then you are much better prepared for scope usage - BUT - don't think that's all you need to know.  There are still a few more things to learn, especially with mains connected equipment.

Here is the video we roll out for people such as yourself - and if it seems a bit too much for you to take in right now, come back and look at it later.  It is worth the effort.

[rstofer]

Short answer:  Your power supply is floating as long as there is no link to the GROUND binding post.  Connect the scope ground to the transistor emitters.  It would be highly unusual to have the link installed and it certainly wouldn't be for a circuit like this.

Probe away!

[davenc]

I've enjoyed and learned a bunch from him already.  His video digging into the RIGOL 2000 scopes helped me decide to buy one. 

[davenc]

That video explained it very well.  Now I get it.  I checked the scope and yes, both probes and the house ground are connected.  I verified my power supply's negative isn't, and that I have been at-risk powering my test circuits off my MacBook USB.  The USB ground goes right to house ground.  Got it.  Now using a wall wart to USB.  I am a bit disappointed my scope isn't separated from house ground.  It seems that would be an important design feature.  I'm using the RIGOL MSO2202A.

I appreciate everyones times and expertise on this forum.  I am very glad I found it.  You folks rock!

[David Hess]

I am a bit disappointed my scope isn't separated from house ground.  It seems that would be an important design feature.

They make special oscilloscopes with inputs which are isolated from the AC power ground and from each other.  They are very handy for measurements of off-line circuits and at high frequencies where a high voltage differential probe lacks common mode rejection.  Bench multimeters have similarly isolated inputs.  Some signal generators have isolated outputs.

Note that the isolated "ground" which is more properly called "common" still has considerable capacitance to ground, 100 to 200 picofarads is typical, and usually higher leakage compared to the signal.  So the ground or common of an isolated input needs to be considered low impedance.  If high impedance is required for both the signal and ground, then a differential input is used which actually has *three* connections, two signals and a ground, whether the ground is explicitly available like on your power supply or not.