# Experiment about diode related to temperature and potential difference

• songoku
Producing graph usually would be a lot better to analyse the behaviour of the device and also to know how the resistance changes when the current and voltage change.Yes, producing an IV graph at each temperature would be a good idea.f

#### songoku

Homework Statement
A student was asked to investigate how changing the temperature of a diode will change the potential difference at which the diode starts to conduct.
(a) Draw a labeled diagram showing how she could carry out this investigation using school laboratory apparatus.
(b) Identify one safety issue with this investigation and how it may be dealt with
Relevant Equations
Not sure
(a)
I know some of the apparatus needed for the experiment, such as DC power supply, ammeter, voltmeter, maybe rheostat. But I don't know how to change the temperature of diode. What is the correct and safe way to change the temperature of diode?

Thanks

Start by drawing a circuit diagram and refer to it to describe how you would measure the potential difference at which the diode conducts when it is at room temperature. Then worry about changing the diode's temperature.

songoku
Start by drawing a circuit diagram and refer to it to describe how you would measure the potential difference at which the diode conducts when it is at room temperature. Then worry about changing the diode's temperature.

I change the value of rheostat until there is reading in ammeter so the reading of voltmeter will be the potential difference at which the diode conducts. Is this correct?

But I still don't know how to change the temperature of the diode.

Thanks

A hair dryer? An ice cube?

songoku
View attachment 324926
I change the value of rheostat until there is reading in ammeter so the reading of voltmeter will be the potential difference at which the diode conducts. Is this correct?
It's not wrong but could be improved if aiming for a high grade. For example, what value (maximum resistance) of rheostat would you use? What supply voltage? Is any protection for the diode (and ammeter) needed to prevent against damage from excessive current? What is the size of the ammeter reading you are looking for? 1pA? 1mA?, 1A? etc.Would a single reading be enough or are there any advantages in producing an IV graph at each temperature?

But I still don't know how to change the temperature of the diode.
If you were asked to heat a small object to , say, 40ºC (you are given as thermometer and any other simple equipment needed) how could you do it? Imagination and a bit of common-sense are needed!

songoku
A hair dryer? An ice cube?
Wow it is actually that simple. It just never crosses my mind because I think I focus too much on "school laboratory apparatus".

But wouldn't it be hard to measure the temperature of the diode when I am taking the reading of voltmeter and ammeter if I heat it using hair dryer?

It's not wrong but could be improved if aiming for a high grade. For example, what value (maximum resistance) of rheostat would you use? What supply voltage? Is any protection for the diode (and ammeter) needed to prevent against damage from excessive current? What is the size of the ammeter reading you are looking for? 1pA? 1mA?, 1A? etc.Would a single reading be enough or are there any advantages in producing an IV graph at each temperature?
I am really doing the experiment and never did this before. I googled the safe limit of current through the diode is 10 mA and forward biased voltage for diode is around 0.7 V so I think I will use around 9 V for the power supply and 2 kΩ for rheostat.

For protection, isn't rheostat already the protection to adjust current, preventing overheating?

Producing graph usually would be a lot better to analyse the behaviour of the device and also to know how the resistance changes when the current and voltage change.

If you were asked to heat a small object to , say, 40ºC (you are given as thermometer and any other simple equipment needed) how could you do it? Imagination and a bit of common-sense are needed!
My initial idea was to use beaker with water and heat the water. Put the diode above the water surface (using something to hold the diode, maybe string) and measure the temperature of water using thermometer and take it as the temperature of diode.

I am not sure this will work. I think the temperature of diode will increase but the measurement of the temperature of the diode won't be accurate because the temperature of the diode will not be the same as temperature of water (although I am not sure how far off it will be)

Thanks

If you search for 'finding the characteristics of a diode' for example, you should be able to find general of experimental guidance.

I am really doing the experiment and never did this before. I googled the safe limit of current through the diode is 10 mA
That seems rather low. E.g. a typical LED requires about 10mA in normal operation. It depends on the type and size of diode you use. (Some large diodes can handle many amps.)

For protection, isn't rheostat already the protection to adjust current, preventing overheating?
Suppose you have no additional protective series resistor. Say your rheostat's resistance varies from 0 to 2kΩ. If you accidentally set it to zero you will have the full supply voltage across your diode and burn it out.

Producing graph usually would be a lot better to analyse the behaviour of the device and also to know how the resistance changes when the current and voltage change.
The problem you have to deal with is that the cutoff point is not clear. Look at this graph: https://knovhov.com/wp-content/uploads/2021/01/curve.jpg. The 'usual' value of 0.7V (for silicon) is not a 'sudden' switch-on value; it is simply the approximate beginning of the steep part of the IV curve. You need to decide exactly how you will use your results to check for temperature-dependance.

My initial idea was to use beaker with water and heat the water. Put the diode above the water surface (using something to hold the diode, maybe string) and measure the temperature of water using thermometer and take it as the temperature of diode.

I am not sure this will work. I think the temperature of diode will increase but the measurement of the temperature of the diode won't be accurate because the temperature of the diode will not be the same as temperature of water (although I am not sure how far off it will be)
If you wrap the diode and leads in 'clingfilm' (or similar) to protect direct contact with the water, you can immerse it in the water. Since the clingfilm is thin and the diode is small, it will settle to the same temperature as the water fairly quickly. With hot water from, a kettle and some ice you can cover a reasonable temperature range.

songoku
Suppose you have no additional protective series resistor. Say your rheostat's resistance varies from 0 to 2kΩ. If you accidentally set it to zero you will have the full supply voltage across your diode and burn it out.
I see. There is still the need to use a fixed resistor as a safety measure.

The problem you have to deal with is that the cutoff point is not clear. Look at this graph: https://knovhov.com/wp-content/uploads/2021/01/curve.jpg. The 'usual' value of 0.7V (for silicon) is not a 'sudden' switch-on value; it is simply the approximate beginning of the steep part of the IV curve. You need to decide exactly how you will use your results to check for temperature-dependance.
After heating the diode, I am thinking about just connecting it to the circuit and decreasing the value of the rheostat until I get reading in ammeter. Then I make table consisting of the value of voltage at which there is reading in ammeter and temperature, and draw the graph of voltage against temperature

Thanks

I see. There is still the need to use a fixed resistor as a safety measure.
As well as the diode, you are also protecting the ammeter (or its fuse if it has one) from damage.

After heating the diode, I am thinking about just connecting it to the circuit
I would keep the diode permanently connected (with power off) and let settle to each temperature before taking readings.

and decreasing the value of the rheostat until I get reading in ammeter. Then I make table consisting of the value of voltage at which there is reading in ammeter and temperature, and draw the graph of voltage against temperature
That's one way. You will need an ammeter of suitable sensitivity. What do you think is the smallest value of current that you need to measure?

songoku
That's one way. You will need an ammeter of suitable sensitivity. What do you think is the smallest value of current that you need to measure?
I am thinking it would be in order of mA

I am thinking it would be in order of mA
Yes, that will work ok.

songoku
Thank you very much for the help and explanation Steve4Physics

berkeman and Steve4Physics