P-type - In P-type doping, or is the dopant. Boron and gallium each have only three outer electrons. When mixed into the silicon lattice, they form "holes" in the lattice where a silicon electron has nothing to bond to. The absence of an electron creates the effect of a positive charge, hence the name P-type. Holes can conduct current. A hole happily accepts an electron from a neighbour, moving the hole over a space. P-type silicon is a good conductor.
A minute amount of either N-type or P-type doping turns a silicon crystal from a good insulator into a viable (but not great) conductor therefore the name "semiconductor."
N-type and P-type silicon are not that amazing by themselves; but when you put them together, you get some very interesting behaviour at the junction.
Because of their properties companies can add various amounts of –type to make the semiconductor the way they want. Thermistors are semiconductors that use the principle of heat conversion into energy to change the resistance value.
Potential Divider:
A voltage divider consists of two resistances R1 and R2 connected in series across a supply voltage Vs. The supply voltage is divided up between the two resistances to give an output voltage Vo which is the voltage across R2. This depends on the size of R2 relative to R1:
- If R2 is much smaller than R1, Vo is small (low, almost 0V)
(because most of the voltage is across R1)
- If R2 is about the same as R1, Vo is about half Vs
(because the voltage is shared about equally between R1 and R2)
- If R2 is much larger than R1, Vo is large (high, almost Vs)
(because most of the voltage is across R2)
If you need a precise value for the output voltage Vo you can use Ohm's law and algebra to work out the formula for Vo shown on the right. The formula and the approximate rules given above assume that only a small current flows from the output. This is true if Vo is connected to a device with a high resistance such as voltmeter. If the output is connected to a transistor Vo cannot become much greater than 0.7V because the transistor's base-emitter junction behaves like a diode.
Why Use Thermistors to Measure Temperature:
- They are cheap, simple and reliable.
- They have a quick response time.
What Does A Thermistor Look Like?
This picture is of a thermistor with plastic sheaths covering the metal wires emerging from the sensor. These will be mentioned later.
Prediction:
I predict that there will be a decrease in resistance as the heat energy (temperature) increases.
Method:
Equipment:
- Thermistor (NTC Disc Thermistor)
- Resistor (1kΩ)
- Cell
- Voltmeter
- Kettle
- Ice
- Wires
- Crocodile clips
- Beaker
- Thermometer
Diagram:
Fig.1
To Calculate the Resistance:
Using the equation R1/R2=V1/V2 first we must first calculate V2.
-
V2= 6-V1
- E.g. 2.75=6-3.25
-
Now we have V2 we can calculate R2 which is the resistance over the thermistor.
-
R1/R2=V1/V2
- Using algebra we can rearrange that to make.
-
R2= (V2xR1)/V1
- E.g. (2.75 x 1000) / 3.25 = 846.15 Ω (2d.p)
Stages:
- Get a kettle boiling.
-
Set up the circuit as shown. In Fig.1
- Put the thermometer in the beaker.
- Put the boiling water in the beaker leaving enough room for the displacement of the ice.
-
Submerge the thermistor in the water close to the edge of the plastic sheath.
-
Take a reading every 5oC, stir the thermometer to ensure average temperature.
- Add ice carefully to speed up the temperature drop (if required).
- If another set of results is required have another quantity of water ready in the kettle.
Pre-Test:
To ensure that there were no anomalies/problems in the main experiment that could be sorted, I took a preliminary set of reading.
Results for Pre-Test:
As the table shows there are a number of anomalies that occurred with my original method. The fault was found to be that the metal rods coming out of the thermistor had been submerged, therefore the water was conducted some of the charge. To improve the results I will in future insure that the metal rods remain dry and out of the water.
This graph shows my pre-test results, there is a decrease in resistance as the temperature increases. There are two anomalies the first at 5oC and the second at 20oC.
Results:
These results were obtained using the improved method, and although some anomalies may have occurred, I have been able to average them using all three results.
This graph shows the average resistance of the thermistor that I studied against the temperature of the water that the thermometer showed. The graph on page 8 is the manufacturer’s specifications. As you can no doubt perceive it is quite difficult to extract precise readings.
Both graphs show the same trend of the resistance decreasing as the temperature increases. The manufacturer’s graphs curve is less steep nearing 10oC than mine. My graph starts with a large dip at the beginning and then levels off whereas the second graph has a lower gradient. A good example of this is at 5oC on my graph is 10 kΩ while the manufacturer’s resistance is 20 kΩ.
This can also be shown in a table.
This shows a difference of 3.05kΩ. This may be accountable to the degree of accuracy used. Although in my experiment I strived to maintain as much accuracy as possible. The manufacturer’s can be expected to have much more accurate measurements as they would have a greater control of measurement and temperature. I only used water influenced by ice and read when the temperature reached a certain level. The manufacturer’s measuring equipment could be expected to measure to a much greater degree of accuracy due to their advanced equipment.
Overall the thermistor showed a large deviation from the specification. Every thermistor would be different no matter what the manufacturers did to maintain similarity. The deviation must come from this fact and also the manufacturer’s improved measuring equipment. It was a shame that the manufacturers didn’t make it easier to take reading from their graph.
Evaluation:
Overall the experiment went well, although there were a few problems with the pre-test but that was what it was there for, and once those problems were rectified the experiment went as smoothly as can be expected.
The wires leading from the thermistor were found to be in the water in the pre-test this means that some of the currant would bypass the circuit as water can conduct electricity. This affected the results so if this test was attempted again I would insulate the wires or try a different way to control the temperature.
The experiment would also have been more accurate if more repeats were taken allowing the average to that much more accurate.
I used the same equipment throughout the experiment so as to minimise the effects of different equipment. I could have used several thermistors and then compared each one to the manufacturer’s specification and averaged each thermistor. The manufacturer’s easily readable examples were limited to three examples, if they had given the resistance more often the graph would have been more accurate and I could have made the trend accuracy greater.
Safety:
Safety was always foremost in this experiment. Steps were taken to check the equipment to make sure there were no sharp edges or fraying wires that could lead to electrocution. Also the boiling water was poured into a Pyrex jug and that was handled with tongs to ensure no burns.
Conclusion:
My prediction based on the background research that the resistance would decrease as temperature increased was proved to be true. So overall it was a successful experiment even with the variation.
This experiment could be useful for products that would need to measure temperature in water e.g. coffee dispensing machines and needed to find a good thermistor for the job.
Reference: