In this video on ground reference points it shows how you can get a negative voltage across a resistor by changing the position of ground. I don't understand this. I always thought that ground was by definition the negative terminal of the power source, 0 volts. I bread-boarded the circuit shown in the video and in the screen shot photo I've attached but I have no idea how to establish ground between R3 and R4. Please help, thanks.
https://www.allaboutcircuits.com/video-lectures/ground-reference-points/

- Jack

 

You could set it up according to the proper use of the symbol which is Earth Ground and this would set up a reference point.
Max.

 

I always thought that ground was by definition the negative terminal of the power source, 0 volts.

Nope, no such definition.
Ground is an arbitrary voltage reference point that is often the negative terminal of the power source but it is not mandatory to be that.
Some older cars used to have the batteries positive terminal as ground, with all the accessories running on a negative voltage.
And some of the old ECL logic circuits were operated with a positive ground and a negative voltage.
Today most circuits do run with the ground being negative but that's just a common convention.

 

Likewise a secondary of a transformer where you want to make one zero reference, neutral or 'Ground'. Which one qualifies?
Ans: Either one can be assigned.
Max.

,

 

OK thanks guys, but saying "Ground is an arbitrary voltage reference point" doesn't help me understand it in practical terms. How do I wire up my bread-board so that "ground" is between R3 and R4 and I get a negative voltage reading when measuring from point D to ground as he says in the video. The only way this will make sense to me is if I see it with my own eyes, I'm not very good with abstracts. Thanks in advance.

 

For this purpose you just need to connect the meter negative terminal to the 'ground reference' and connect meter positive to point 'D'. Here the 'ground reference' is the reference point for voltage measurement.

 

OK, I think I figured it out. In the video he says "we PLACE ground here between R3 and R4" which led me to believe that he changed the circuit physically. But now I'm realizing that he just changed the location of the symbol on the schematic, meaning "ground" in this context is completely arbitrary, as has been already stated (and which I stubbornly could not digest). But that raises another question for me then. What is the practical benefit of this (other than having some fun confusing noobs like me)? Why not just call 0V ground and keep it simple? It seems to me if ground can be anything it loses any meaning.

 

I prefer the word Common, and this can be made at any point in a circuit as a reference point, which that is all it is.'
I don't know the source of your example, but I suspect it is intended to indicate any reference point can be determined as common or GND if you prefer.

Max.

 

Why not just call 0V ground and keep it simple? It seems to me if ground can be anything it loses any meaning.

It is simple.
But what do you mean by 0V? There's no such thing as an isolated "0V".

The meaning (definition) of ground is that it is the point selected as the 0V reference point, no more, no less.
And ground can indeed be selected as any point.

You have to get rid of the idea that there is something particularly different about "ground".
It's just a common reference point for the other voltages in the circuit.

 

Why not just call 0V ground and keep it simple? It seems to me if ground can be anything it loses any meaning.

Where you call ground is what gives it meaning. Every circuit can have a place called "ground" and it means a very specific thing to THAT circuit. If you don't like it, you can draw the circuit with a different ground point and that ground point will mean something very specific to THAT circuit.

 

It is simple.
But what do you mean by 0V? There's no such thing as an isolated "0V".

The meaning (definition) of ground is that it is the point selected as the 0V reference point, no more, no less.
And ground can indeed be selected as any point.

You have to get rid of the idea that there is something particularly different about "ground".
It's just a common reference point for the other voltages in the circuit.

I guess it goes back to the idea I formulated when I first took a basic electronics course regarding what voltage is. My formulation is that voltage represented a certain amount of "work" that can be done as determined by the amount of current and resistance. The voltage is dropped across the resistance in the circuit so that when you consider the negative terminal of the power source you can consider the voltage at that point to be 0V, in other words, there is no more "work" to be done, the voltage has been dropped elsewhere in the circuit. Maybe I had a bad teacher.

 

Everybody makes short cuts. When our understanding matures to include all the variations, our beginner short cuts usually don't work. They used to be short cuts because they simplified what we understood at that time. Now, they are just crutches.

 

Likewise a secondary of a transformer where you want to make one zero reference, neutral or 'Ground'. Which one qualifies?
Ans: Either one can be assigned.
Max.

,

I wonder if Dr. Bruce also had a cartoon about someone being "Charged With Assault & Battery"?

When I first heard the term back in my childhood days, I thought you could make a battery from table salt and a lemon and connect it to your fingers with a jumper cable. I thought it was similar to getting a static charge from walking on a carpet and creating a spark when you touched a door knob.

 

Way back when, "ground was Earth". No such thing as plastic or double-insulated.

Some instruments have their common side connected to Earth ground.
Some instruments are allowed to float (say 30 V) above Earth ground.

Now we have to be a little more particular.

Without getting into a lot of specifics (simplifying a lot)

1. Earth ground is the Earth and is the 3rd prong of a 3 prong 120 outlet. This can be a shield and nothing else.
2. Chassis ground is indeed the chassis (tube era primarily)

So, we can have (simplifying):
a) Digital ground (this is a dirty ground)
b) Analog ground (This is a clean reference point)
c) High current ground
d) Earth ground

What we try to do is to separate the kinds of grounds AND tie them together at one place. Earth.

In some cases the isn't enough. I've seen instruments where (a,b and c) can be electrically "MOVED" to say an external D/A card located in a computer.

it gets really nasty sometimes.
One easy way to solve some of the problems is to make the D/A converter current based and put a resistor at the instrument to change to a voltage. In this case, it's referenced to the ends of the resistor.

When reading a voltage from the instrument, employ pseudo-differential where the common point on the instrument is measured with respect to the computer's reference and the output of the instrument is also measured relative to the computer's reference. These voltages are now subtracted. The method doesn't introduce loops.

A "more serious" example is when there are a bunch of instruments with say a single 12 v and single 5V supply. The output is relative to each mini instrument and the input needs to be relative to that same mini-instrument.

The power supplies +5 and +12 are connected together, but if you measure with respect to that point, you get the wrong answer. You have to measure with respect to each mini-instrument. The setpoint was provided by an effectively isolated potentiometer. To replace the 0-5V signal generated locally by the potentiometer gets to be really troublesome because the current drawn by each card changes the +5/+12 common point.

 

I wonder if Dr. Bruce also had a cartoon about someone being "Charged With Assault & Battery"?

Some of his lecture video's are quite amusing.
Max.

 

they look like the Tek cartoons: http://vintagetek.org/tektronix-schematic-cartoons/

 

Nope, no such definition.
Ground is an arbitrary voltage reference point that is often the negative terminal of the power source but it is not mandatory to be that.
Some older cars used to have the batteries positive terminal as ground, with all the accessories running on a negative voltage.
And some of the old ECL logic circuits were operated with a positive ground and a negative voltage.
Today most circuits do run with the ground being negative but that's just a common convention.

So we established that in the video example I started with the circuit didn't change, he just choose another point in the circuit to call ground (I'm still wondering what benefit this provides by the way). In your car example it sounds like the older cars you refer to actually were wired different than they are these days, it wasn't just a matter of moving the symbol on the schematic. So I am still confused.

 

In reality, there is no 12V point in a circuit. Similarly, there is no 0V point in a circuit.
Voltage is measured as a voltage difference between point A and point B.

It is like a distance measurement.
No one says "it is 200 miles at New York City". That doesn't make any sense.
You can say "it is 200 miles from New York City to Washington D.C."

Hence you can pick any location and call it your zero reference point. Everything else is measured with respect to your reference point.

 

he just choose another point in the circuit to call ground (I'm still wondering what benefit this provides by the way).

No particular benefit.
He just wanted you to get around the idea that ground is some magical fixed point.

In your car example it sounds like the older cars you refer to actually were wired different than they are these days, it wasn't just a matter of moving the symbol on the schematic. So I am still confused.

No they were wired the same.
It's just that any polarity sensitive devices, like a generator, starter motor, or radio for example, had to be designed to work with a negative voltage with the positive (chassis) being ground.
In those days they didn't have that many polarity sensitive devices on a car as now.

 

No particular benefit.
He just wanted you to get around the idea that ground is some magical fixed point.

When one services tube things, resistance measurements were a common technique. The tube, except for the filament should be a series of OPEN connections thus resistance measurements were meaningful.

The next part of the procedure is to define a set of conditions that the voltage measurements were taken and define the impedance of the meter and take the measurements relative to the most common point what "happened" to be ground and even earth ground.

When calibrating certain tube testers, it was important to use an analog meter with say 50,000 ohms/volt input Z. So, yes the resistance presented to the circuits varied with the measured voltage. A common meter with a 10 M input Z actually UPSETS the desired value. High input Z meters were not commonplace.

In an audio amp, bias is measured as a voltage across a resistor. The B-E voltage is important to know.