ELEC 243 Lab

Experiment 1.2

Measuring Heat and Light

Equipment

Components

Part 1: Thermistor

In Part 4 of Experiment 1.1 we found that the resistance of the light bulb filament increased with increasing temperature. Materials (such as metals) which exhibit this behavior are sead to have a positive temperature coefficient. Materials (such as semiconductors) whose resistance decreases with temperature have a negative temperature coefficient (NTC). The thermistor is a piece of NTC material whose resistance is described by the equation $R=R_0 e^{B(\frac{1}{T}-\frac{1}{T_0})}$ , where $R$ is the resistance at the measured temperature $T$ and $R_0$ is the resistance at the reference temperature $T_0$ . Note that the temperatures must be absolute temperatures (in K).


Step 1:

 In this experiment we will need to make resistance measurements in situations where using the DMM probes or banana plug patch cords will not be satisfactory. Fortunately, there is very convenient way to do this. Get a BNC clip lead cable from the equipment room and attach it to the BNC-banana adapter from your tool kit, thereby creating a banana plug clip lead cable.
Plug the banana plug end of this assembly into the V and COM terminals of your DMM and set the selector switch to Ω.

Step 2:

 Connect the clip leads to the terminals of the thermistor.


Step 3:

 Wait until the resistance reading on the DMM has stabilized, and record the value. This represents the ambient room temperature.

Step 4:

 Devise a means of placing the thermistor in intimate contact with your body (or your lab partner's body). When the resistance reading has stabilized, record the value.

Question 5:

 Based on the nominal values for resistance and B value, what are the ambient temperature and your (or your lab partner's) body temperature?

Part 2: CdS Photocell

Also known as a photoconductive cell or photoresistor, this device changes its resistance as a function of incident illumination. One useful characteristic of cadmium sulphide is that its spectral response closely matches that of the human eye. That means that a CdS photocell can be used in some photographic and photometric applications without additional filtering.


Step 1:

 Remove the thermistor from the clip leads and replace it with the CdS photocell.

Step 2:

 Note the change of resistance as the illumination of the photocell changes. Record a few interesting values (e.g. ambient, illuminated by incandescent lamp, covered with hand, etc.).

Question 6:

 Based on the nominal characteristics of the photocell, what is the ambient illumination level in the lab? What is the illumination level at a distance of one foot from the incandescant lamp?

Step 3:

 Measure the dark resistance of the photocell. How long does it take for the measured value to stabilize?

Part 3: The Photodiode

The thermistor and photocell are passive transducers: in order to convert the changes in resistance into an electrical signal an external source of electrical power is required. There is another class of sensors, called self-generating, which convert the applied input directly to an electrical signal. One such transducer is the photodiode which converts incident optical power into electrical power.


Step 1:

 Remove the CdS photocell from the clip leads and replace it with the photodiode.

Step 2:

 Set the DMM selector to DC Volts.

Step 3:

 Observe the changes in voltage produced by the photodiode as the amount of light reaching it is changed. Record the voltages for the same conditions you did for the CdS photocell in Part 2.

Step 4:

 Examination of the data sheet for the photodiode shows that it is the current rather than the voltage which is proportional to the incident illumination. Set the DMM to DC Current and connect the clip leads to the COM and 300 mA terminals. (Since the spacing between these two terminals is not the same as the spacing between the plugs on the banana plug adapter, you will have to insert one plug into one of the terminals and use a banana plug patch cord to connect the other.)

Step 5:

 Observe the changes in the current produced by the photodiode as the amount of light reaching it is changed. Record the current values for the same conditions you did previously in Part 2 and Step 3.

Question 7:

 Based on the nominal characteristics of the photodiode, what is the ambient illumination level in the lab? What is the illumination level at a distance of one foot from the incandescant lamp? How do these values compare with those measured with the CdS photocell?