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Investigating a Thermistor.

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Introduction

INVESTIGATING A THERMISTOR After deciding to investigate the properties of a thermistor, I chose to be more specific, and to look at repeatability, accuracy, and sensitivity. I also thought of many different ideas as to what the experiment could be used for: for fridge...??? controlled???. The proceeded by looking at different circuits suitable for exploring sensitivity. I looked into the 'whetstone bridge' circuit: The wheatstone bridge circuit enables more accurate readings, However, I decided on another circuit that acted as a potential divider only using one fixed resistor, which seemed equally suitable for detecting small changes of volts. The reason for this being that I thought it would be interesting to see the effect(s) of changing the fixed resistor (R): After recognizing the importance of (R), I decided on the equipment I was going to use, and then tried using some algebra to tackle the problem of finding the value of R that would give me the biggest change DVo. Equipment: 1 power generator set at 2volts 1 thermistor (RS 0.47Kohms 232-4538) an assortment of fixed resistors some leads some crocodile clips a digital voltmeter Here are my jottings: Vo/Vs=R/R+RTh Vo=Vs(R/R+RTh) (Vo=Vo t2 - Vo t1 (Vo=VsR[1/ R+RTh t2 - 1/ R+RTh t1] At this point I thought it appropriate to take a numerical approach, and use Excel to plot a graph of the equation as I was having difficulty simplifying or manipulating it further. ...read more.

Middle

Bellow are a few diagrams to illustrate this: Experiment: After gaining my background knowledge, and after doing my planning, I felt I was ready to start my experiments. I decided to produce more accurate results, that I would take two sets of results, which would also help to test for repeatability. I started by testing the 470ohm resistor, and the results are as follows: Vo(volts) set 1 Vo(volts) set 2 Average Vo Temp. (degrees Celsius) 1.86 1.86 1.86 86 1.85 1.85 1.85 85 1.83 1.85 1.84 84 1.84 1.84 1.84 83 1.83 1.85 1.84 82 1.83 1.83 1.83 81 1.82 1.82 1.82 80 1.82 1.82 1.82 79 1.8 1.82 1.81 78 1.8 1.8 1.8 77 1.8 1.8 1.8 76 1.79 1.79 1.79 75 1.79 1.74(void) 1.79 74 1.77 1.79 1.78 73 1.77 1.77 1.77 72 1.77 1.77 1.77 71 1.75 1.77 1.76 70 1.75 1.75 1.75 69 1.73 1.75 1.74 68 1.74 1.74 1.74 67 1.73 1.73 1.73 66 1.71 1.73 1.72 65 1.7 1.72 1.71 64 1.71 1.71 1.71 63 1.7 1.7 1.7 62 1.7 1.68 1.69 61 1.68 1.68 1.68 60 1 1.66 1.66 59 1.65 1.65 1.65 58 1.65 1.63 1.64 57 1.63 1.63 1.63 56 1.62 1.62 1.62 55 1.59 1.61 1.6 54 1.59 1.59 1.59 53 1.58 1.58 1.58 52 1.57 1.57 1.57 51 1.56 1.56 1.56 50 1.54 1.54 1.54 49 1.53 1.53 1.53 48 1.51 1.51 1.51 47 1.5 1.5 1.5 46 1.5 1.48 1.49 45 1.47 1.47 1.47 44 1.46 1.46 1.46 43 1.45 1.45 ...read more.

Conclusion

To try and justify my results, I went back to my equation (Vo=VsR[1/ R+RTh t2 - 1/ R+RTh t1] However I manipulated it to give me the specific value of RTh, not a range of values: (Vo=VsR[1/ R+RTh] I proceeded to plot these graphs on Excel: I was then faced with the problem of converting 'resistance of Rth' into temperature, and at first thought that the best way to do this was to was by finding the equation of the line of the calibration graph provided by RS. However, I faced with the problem of logarithmic scales, and due to the large jump in numbers, thought that the equation I produced would not be accurate enough. However, from the 5 ohm graph is clear that DVo increases rapidly with low values of resistance, which still contradicts the calibration graph. The graph of the 100ohm resistor is harder to tell which for which values gives a sharp increase in DVo, as the curve is quite steady. Therefore I am none the wiser as to why my results given do not indictate that 100ohms is more sensitive to low temperatures, and 5ohms is more sensitive to high temperatures. Unfortunately I ran out of time with this project, however I would have liked to have taken more readings, and a second set of results for the experiments above. However, I do think that they were sufficient to conclude that the thermistor has good repeatability, and also that there was little difference between using the 560ohm and 27k ohm fixed resistors. Also, from the fixed resistors I evaluated, 100ohms appears to be the most sensitive. ...read more.

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