series voltage measurement

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.