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Hello, my name's Dr.

George.

This lesson is called, "Measuring the resistance of a diode," and it's part of the unit, circuit components.

You may be wondering how there can be a whole lesson on the resistance of a component.

But a diode is more complicated than a resistor, and there's an investigation for you to do.

The outcome for the lesson is, I can interpret an I-V graph of a diode in order to describe its properties.

Here are the keywords for the lesson.

I'll explain them as we go along, and if you need to remind yourself of the meanings anytime, come back to this slide.

The lesson has two parts, they're called, investigating the resistance of a diode, and, properties of a diode.

So, what are diodes? They're electrical components used in electrical circuits and in lights.

Here are some diodes in this circuit.

And here are some light-emitting diodes, also known as LEDs.

And these are often used in household lighting.

In electrical devices, diodes, the ones that aren't light-emitting, look like this.

And in circuit diagrams, the symbol is this.

So, there's a triangle with a pointed end meeting the line, and a circle around that.

If it's a light-emitting diode, the symbol is slightly altered.

There are two arrows parallel to each other, representing light being emitted.

And you may see other symbols being used for diodes in different places.

These two are also used.

So, which of the following are correct symbols for a diode? And when I ask a question, I'll wait for you for five seconds, but you may need longer, in which case, press pause while you're thinking, and press play when you have your answer ready.

And the correct answers are A, which is the one we'll be using.

But also B and C.

Not D, which represents something else.

Diodes are made from semiconductors such as silicon or germanium.

And semiconductors are materials that have some metallic properties and some non-metallic properties.

And this material enables a diode to control how current flows.

One end of the diode is positive, and the other end is negative.

And you'll need to be aware of that when you do the investigation later.

And this is how you identify the positive end and the negative end on a real diode, and also on the diode symbol.

Current in a diode can only flow from positive to negative.

So, if you connect the positive terminal of your power supply to the right-hand end of this diode, and the negative to the left-hand side, current won't flow through that diode.

You're going to use a circuit like this one to investigate the p.

d.

across, and current through, a diode.

So, you'll simply connect a diode and a variable resistor in series with a DC supply.

That's a direct current supply, the current flows in only one direction.

You'll use a volt metre to measure the p.

d.

across the diode.

And a milliammeter to measure the current through it.

The reason for using a milliammeter rather than a normal ammeter is because the currents through the diode are going to be very small.

Remember, milliamp means a thousandth of an amp.

So, which of the amateurs below is the most appropriate for measuring the current through a diode? And of course, it's A.

The mA on this ammeter shows that it measures milliamps.

The other ammeters can't detect milliamps at all.

The middle one only goes to two decimal places of an amp, that's a hundredth of an amp.

And the one on the right only goes to tenths of an amp.

We use a variable resistor because it enables us to vary the p.

d.

across the diode.

Components in series share the p.

d.

of the power supply, and they share that p.

d.

in the same ratio as their resistance is.

So, if we change that resistance ratio, we change the p.

d.

across each component.

An appropriate range of p.

d.

for a diode is minus one volt to plus one volt.

So, that's one volt in one direction, and one volt in the other direction.

And to get these negative values of p.

d.

, you can reverse the leads to the power supply.

And turning the power supply off, of course gives a p.

d.

of exactly zero volts, and you can include that as one of your data points.

An appropriate set of results will let you plot a graph on which it's clear where the best fit line goes.

We don't know for sure yet if that best fit line is going to be straight or if it's going to be curved.

And you'll need enough points to be able to see that.

It's quick to take readings in this investigation, you simply adjust the variable resistor, you get a new p.

d.

, you measure that p.

d.

and the current.

And so, we can take a lot of readings, which will help us see how to draw the best fit line.

You don't need to take repeat readings in that case.

You also don't need to spend time trying to get exact round values of the p.

d.

, like 0.

10 volts.

You should just try to get the readings at roughly equal intervals, but it doesn't have to be exact.

Which of the following sets of results will allow a valid conclusion to be made? And a valid conclusion is one that answers the question that the investigation is asking.

We are trying to find out the relationship between p.

d.

and current for a diode.

And the best one is C, because it has a larger number of data points.

A has round numbers of the p.

d.

, but that's not important.

And both A and B have fewer results.

You can then use the p.

d.

and the current each time to calculate the resistance of the diode using the equation, resistance is p.

d.

divided by current.

For this equation to give you an answer that's in ohms, you need to use standard units for p.

d.

, vaults, and current, amps, when measuring current in milliamps.

So, include a column in your table for converting those milliamps to amps.

You just divide by a thousand.

If the p.

d.

is zero, the resistance will also be zero.

And if just the current is zero as in this third row, the resistance will be infinite.

You can write the infinity symbol there.

<v ->Now, which of the following is the same as 200 milliamps?</v> Well, a milliamp is a thousandth of an amp, just as a millimetre is a thousandth of a metre.

And 200,000th of an amp is 0.

200 or 0.

2 amps.

And now for the investigation, you're going to investigate the p.

d.

across and the current through a diode for values of p.

d.

between minus 1.

0 volts and plus 1.

0 volts.

Set up this table before you start taking measurements.

Include a column for resistance, and calculate the resistance for each p.

d.

and current pair of values.

And then plot a graph of current in milliamps against p.

d.

in vaults, and draw a best fit line.

Pause the video while you do all that, and come back when you finish your graph.

Here's a set of example results.

When the current is zero, the resistance is infinite.

And the resistance falls quickly between 0.

5 and 0.

7 volts.

If for any reason you didn't manage to get a set of results, you could use these to plot a graph.

And here is the graph.

And also written here is a summary of what the graph shows.

Current remains at zero, below 0.

5 amps, and then increases rapidly as the p.

d.

has increased.

I hope you saw something similar in your results.

And we'll look in detail at this graph and what it shows us in the next part of the lesson, properties of a diode.

So, here's the graph again.

And we can see that for a diode, zero current can flow when the p.

d.

is negative.

And zero current flows when the p.

d.

is positive, but below around 0.

5 volts.

Take a look at graph and you can see that.

Above 0.

7 volts, the resistance falls quickly.

It must do because the current goes up quickly.

And 0.

7 volts is the threshold p.

d.

for the diode.

Which of the following statements about a diode are correct? Press pause when you read these and choose your answers.

Well, it's true that current can flow in just one direction of the p.

d.

The p.

d.

has to be the right way round for a diode to let current through.

And it's also true that current only flows when p.

d.

is above the threshold p.

d.

The I-V graph of a diode is called its characteristic.

When the p.

d.

is negative, current can't flow, and we say the diode is reverse-biased.

So, that describes the way round it is.

When the p.

d.

is positive, current can flow, and we say the diode is forward-biased.

And current can only flow above a threshold p.

d.

of about 0.

7 volts.

Let's recap.

Which of the following statements is correct if a diode is connected with a negative p.

d.

across it? And it's reverse-biased in that situation, and forward-biased the other way around.

The resistance of a diode then depends on the p.

d.

across it.

When the diode is reverse-biased, the resistance is extremely high.

And when the diode is forward-biased, then the resistance is very low, as long as the p.

d.

is above 0.

7 volts.

So, when is the resistance of a diode very low? Press pause while you read these.

And of course, it's when it's forward-biased and the p.

d.

is above 0.

7 volts.

And take a look at this results graph.

Can you explain what mistakes have been made with the results shown here, and suggest what steps should be taken to correct them? Press pause while you think about your answers, and press play when you're ready to check them.

Here's an example answer.

There is an anomalous point at a p.

d.

of about minus 0.

8 vaults.

Let's take a look.

You see that one on the left-hand side? This reading should be repeated to check it.

If it's not correct, the correct value should be used instead.

And this is why it's a good idea to plot your graph before you put your equipment away.

Because if you then realise there's an anomalous point, you can go back and retake that measurement.

Also, there are only two readings in the negative p.

d.

part.

Let's take a look.

Okay, there's only minus 0.

4 and minus 0.

8 volts.

More readings should be taken to make sure the conclusion can be trusted.

And finally, there's a large gap in p.

d.

values from 0.

4 volts to 0.

9 volts.

You can see that here, there's nothing in between those values.

That would make it hard to see exactly how the best fit line should go.

More reading should be taken so we can be more certain of where to draw the line of best fit.

So, well done if you spotted those points.

If you're doing an investigation to try to find something out, it's important that you take suitable measurements.

Otherwise you can't draw a valid conclusion.

And now we're at the end of this lesson, so here's a summary.

Diodes are used to control the flow of current in electric circuits.

In forward bias, when p.

d.

is above a threshold voltage, they have a very low resistance.

In reverse bias, when p.

d.

is reversed, they have a very high resistance.

Light-emitting diodes, LEDs, are a type of diode that give out light when a current flows through them.

And you may well be using bulbs in your home that have light-emitting diodes inside them.

And the home electricity supply is alternating current, which is current that keeps changing its direction.

But there's a device also inside the bulb that turns that into direct current so that the LEDs light all the time.

I hope you enjoyed this lesson, and I hope your investigation went well.

And perhaps I'll see you again in a future lesson.

Bye for now.