Now, let's talk about measuring resistance and voltage.
This device right here is an ohmmeter and a voltmeter.
So here's how I will measure resistance or I can measure DC voltage,
or I can measure RMS, AC voltage.
Today, I am interested in measuring ohms and measuring volts.
So what I have built is a series circuit where I have four resistors and series.
1k, 3k, 4k and 5k.
They'll be connected to a three-volt source which you don't see yet.
In this device right here, this is a protoboard and
I've connected this circuit on my protoboard.
The way a protoboard works is every row is an individual node.
So this is a node, this is a node, this is a node and so on.
When you cross this plastic barrier, here is a new node.
Here is another node.
Here is a third node and so on.
So I have my 1k resistor connected through this node.
This is actually the green node connecting to the the 3k.
The 3k is connected to the 4k with the blue node and
the 4k is connected to the 5k at the yellow code.
The 5k is going to be connected to the negative side of
my voltage through the black node, which is right here.
That is this whole line and the red node or
the positive three-volt source is going to be connected right here.
Now if I want to measure resistance, I am not going to have my source connected,
because that may end up being in series or parallel with my resistance.
So I'm not going to put that there, but
I want to just measure my resistors by themselves.
So I'm going to touch one side of my lead to one side of the resistor.
The other side will lead to the other side of the resistor.
And look, this is 0.987 kiloohms which is just about 1k.
These are five percent resistors, so that's good.
Now, let's measure our 3k resistor.
Touch one side, touch the other side.
There's what my 3k resistor looks like.
Here's the 4k.
One side touch the other side.
There's my 4k and here is my 5k resistor.
Okay, now let's hookup our three-volt source.
The three-volt source is going to be from two batteries in series.
I'm going to connect the red side right there and
the black side right there to my circuit.
All right, now let's measure the voltage across this three-volt source.
And actually, I have a couple little pins here that are just going to help
me be able to hold my things in place.
So here is a little pin that's going to allow me to connect my black loop there,
I made it and here's the other pin that will let me connect my red lead.
So I'm gonna connect those into the black and
red rails on my circuit right here like this.
Okay, so I don't want to measure resistance anymore now.
Now, I want to measure voltage.
So let's turn it over here.
So I am measuring my three-volt source right there.
That's 2.907 volts.
Okay, let's probe my circuit.
The first thing that I am going to do is measure the voltage at each of my nodes.
So right now, I'm measuring the voltage at the red node, see?
This is the part right there connected on to the red node, it's right there.
That's measuring three volts.
Now, I'm going to measure at the green node.
Okay, at the green node, so actually let me bit.
Let me write right here that this was 2.909 volts at the green node number.
I said that was the green nailed right there.
At the grade node, this is two point six eight seven
volts at the blue node there between the 3 and
4k is 2.010 volts and at the junction between the 4 and
5k at the yellow node is 1.138 volts.
Down here at the black node, we're back to zero.
What I just did was that I measured the node voltages.
I measured the red node, the green node,
the blue node, the yellow node and the black node.
Those are node voltages.
The way you measure a node voltage is you plug the black lead of your volt meter
into the ground and
you move the red lead to touch each of the nodes that you are interested in.
Now, let's measure voltage difference.
The way you measure voltage difference is you put your plug on one side of your
resistor right there and
you put your black plug on the other side of the resistor.
This is now going to measure the voltage that's across R1.
From red here to black there, that voltage is 0.223.
We also can say that this is the plus side or the red side and
this is the minus side, or the black side.
Let's do the same thing for this resistor.
Let's measure across R2.
So let's put the black lead on one side and the red lead on
the other side just like that, and that is 0.678 volts.
Put a plus where your red lead is and a minus where your black lead is.
Let's get this one.
Make sure you fill out the cross our 4k now.
That is 0.873 volts measuring from here to here plus on this side minus on that side.
And now, let's measure across the 5k.
Right there, this is 1.140 volts plus a new side minus
on that plus it's always the red lead minus is always the black lead.
So what we measured there were the voltage differences.
Now, let's take a look at what we expect to happen.
So we went from our approximately 3 volts and
we're going to have dropped 0.223 volts down to our next node.
Is that what happened?
Take 2.9 subtract off 0.2.
Yep, we've gotten down to about this voltage,
then we're going to have lost another 0.678 volts across our 2.
In fact, subtract that off from here.
Yep, you get down to the next node.
Now subtract off the voltage difference here,
0.87 and that is bringing you to the 1.138 volts at this node.
Let's also see how those measurements could work this way.
Let's measure the voltage across two of our resistors.
So I'm measuring across R1 and across R2.
What voltage do you expect there?
You would expect to see the voltage drop across R1 and the voltage drop across R2.
So this volt is difference right there is 0.90 and
that's plus were the red is, minus where the black is.
That's another voltage difference.
What if I measured across three of my resistors?
Add up the voltages that you originally had across the individually and
that should give you the voltage that's across from R1 all the way
over here to R3 plus on this side, minus on that side, it's 1.775.
Add those up, so that you can get the right number.
And then finally, when I measure across all of my resistors, sure thing.
I'm back to my 2.9 volts.
Now wait a minute, this says, 2.915 when I started out with 2.909.
Your voltages are not exactly accurate and your volt meters is not exactly accurate.
So you can already see that I wouldn't be trusting these last two digits in this
particular measurement between this volt meter and these voltages.
So let's just focus on the first digit, the 2.9 which means let's get rid of this.
Let's get rid of this.
Let's get rid of this and so on.
Those are your significant digits.
So today, we have measured resistance.
We've measured node voltage and we've measured voltage difference.