lecture (1-6) Circuit Elements 2)

Now let's talk about circuit elements
in a little bit different way.

Let's talk about realistic voltage and

current sources and i-v curves
associated with common elements.

Let's talk about open and

short circuits and switches,
particularly wiring three-way switches.

An ideal voltage and
current source has no resistance.

The voltage source just produces voltage,
the current source just produces current.

But a more realistic non-ideal voltage
source and current source has resistance.

A voltage source has
a resistor in series and

a current source has a resistance in
parallel, called the shunt resistance.

You know that most of your sources
are realistic because they heat up when

you use them.

Your power drill, for example,
gets hot when you use it very long,

and that's because of
its internal resistance.

Here is a graph of the internal
resistance of a battery.

Batteries get hot when we use them too,
certainly my laptop battery does.

So you can see that this battery,
this is an Energizer 1.5 volt AA battery.

As the temperature changes, so
does its internal resistance.

Its internal resistance is just going to
be, oops, I want it to go up, not down,

is going to just be this series resistance
that's right here with a battery.

So let's take a look.

Now, if you were at room temperature,

the internal resistance of that
battery would be about 0.15 ohms.

And if you were in Antarctica, which
is where I did some measurements, and

let's say that you were about
-10 degrees centigrade,

then your resistance
would be about 0.3 ohms.

Let's see how that affects you.

Let's suppose that you just want
to hook up this battery with its

internal resistance to a very small load.

I'm going to use a 1-ohm load for
this example.

I would find the current that's in
the circuit by taking the voltage and

dividing by the sum of the two resistors.

So in this case, if I had an ideal source,
it would be 1.5 volts divided by my

1 ohm load, and so I would have
1.5 amps for the ideal battery.

But now let's say that
I'm at room temperature.

In that case, my internal resistance is
0.15, so if I add that right in here to my

1 ohm, I'm going to have 1.3
amps at room temperature.

It's a little less than I had for
my ideal battery.

Now let's go to my Antarctica battery, and

that's where I have 0.3 ohms
connected on to my 1 ohm load, and

I have 1.15 Amps,
than I really intended to have 1.5.

So what did that do?

This is why our batteries went dead so
fast in the Antarctic cold.

Now let's talk about I-V curves.

Here are the I-V curves for
the simple circuit that I just showed you.

You have a battery
connected onto a resistor.

And what we would be plotting
is voltage on the x axis and

current, or I, on the y axis.

The I-V curve for
a resistor is always a straight line.

The resistor is a linear device.

If I had a 1 kiloohm resistor,
for example,

connected onto a 1 volt battery,
I would have 1 milliamp of current.

If I had a 2 kiloohm resistor,
I'd have half a milliohm of current.

And if I had a really high resistance,

let's say infinity,
I would have no current at all.

On the other hand, if I had a very
tiny resistor, let's say 0 ohms,

I would have no voltage, but
I'd have a very large current.

What does that mean?

Having a resistor of 0 is a short circuit.

That's where my current runs away with me,
I have a huge current, and

my voltage would be 0.

An open circuit has a very high
resistance, resistance of infinity, and

in that case, you have no current,
but you have a very large voltage.

The I-V curves for an LED are non-linear.

The LED is a little light bulb that turns
on when we put a voltage across it.

We're going to be using it in our first or
second lab.

So you can see right here that there
are different colors of LEDs and that

as the voltage increases for each one,
see here's, I'm turning up the voltage,

turning up the voltage, and
the current becomes very high.

This elbow right here,
where the current goes from being almost 0

to becoming almost its final value,
it's called the forward turn on voltage.

And right here for the red one,

you can see the forward turn on
voltage for a red LED is about 1 volt.

The turn on voltage for
a white LED would be a little bit higher.

Now let's go back to our idea of
the open circuit and the short circuit.

If I have an open circuit,
my current is 0.

My circuit may not work, but
at least it's not gonna run away with me.

My short circuit, on the other hand,
has a very small resistance but

extremely high current.

That's what can actually
do you some damage.

A fuse or a circuit breaker is what's
intended to keep the current from running

away with you.

If the current was trying
to go to infinity, but

we had a 10-amp circuit breaker in line
with that, the circuit breaker would

trip when the current got to 10 amps and
it would shut the circuit off.

A circuit breaker is a type of switch.

There's several other types of switches.

The most common that we use when
we do our designs are single pole,

single throw switches.

That's just a switch that you just
turn on, turn off, turn on, turn off.

The other is a single pole double throw,
where you turn on here or

you turn on there, turn on here, turn on
there, and there are many others as well.

I put a link on the website, and I thought
it was pretty interesting for you to see

all the different kinds of switches that
you might be using throughout your career.

One very interesting switch circuit is
the three-way light switch, that's where

you have a voltage source connected on to
two single pull double throw switches.

These are very common along stairways or

hallways, because you want to
able to turn the light on or

off from the top of the stairs as well
as from the bottom of the stairs.

Now right here in the circuit,
you'll notice that these two

switches have to be connected by a double
set of wires going between them,

and then they're connected on to the lamp
or the light bulb along a neutral line.

We often choose to use the light
bulb in the neutral line

instead of the power line.

Because in the event that someone
was taking the lightbulb out, and

let's say that they accidentally
got their finger or

maybe a piece of or
maybe a tool across here.

And somehow they managed to
short themselves to a live line,

sorry to the ground.

If we were connected to the neutral,
nothing should happen, but

if they were connected to a live line,
it should.

So most often, the lamp is put
on the neutral line for safety.

So in conclusion today, we've talked about
voltage and current sources, open and

short circuits, and switches.

I'd like you to take a minute
to try the three-way switch

in your MultiSim simulation.

It's a good chance to get
to practice with MultiSim.

And it's fun to see how
the three-way switch works.

Now, let's talk about where
the picture was in the front.

This is the Sunflowers &
Thunderheads picture from

Blackhawk Trail In Payson Canyon, Utah.

See all these yellow
sunflowers along here?

It's absolutely amazing.

This is normally the week
of the Fourth of July.