ELEC 243 Lab

Experiment 4.3

I-V Plots

So far our measurements have produced single value, usually temperature. Labview has assisted with this by doing computation, something computers are good at. Another thing computers are good at is repetition, such as automating the task of making (almost) the same measurement over and over. We had an example of this in Part 4 of Experiment 1.1 where we measured and plotted the I-V characteristic of the light bulb.

Our temperature measurement system is based on the measurement of resistance, and our resistance measurement technique is based on the measurement of the resistor's voltage and current. Since Labview is now in control of the applied voltage and can measure the resulting current, it's only one more step to being able to plot I vs. V.

Part 1: An I-V Plot VI

We could continue modifying our temperature/illumination measurement VI to perform this function, but let's take a break from building our own VIs and look at a prefabricated one.


Step 1:

 Load the I-V Plot VI from the ELEC 243 Programs menu. This VI uses the same connections as the resistance measurement VI we have been developing and will display $I_T$ vs $V_T$ . Since the photocell is a resistor, this should be a straight line. Start the VI and verify that this is the case. The slope of the line should be the inverse of the resistance. Is this the case?

Step 2:

 Since the resistance of the photocell changes with illumination, so should the slope of the line on the graph. Shade the photocell with your hand. What happens to the slope?

Step 3:

 Stop the VI and open the block diagram. Since this VI performs many of the same operations as the resistance measurement VI, what you see should be somewhat familiar. Examine the block diagram until you feel that you understand how it works. If you encounter an unfamiliar block, a quick way to find out what it does is to use the context help. A shortcut for turning on context help is to press Ctrl-H in the block diagram.

Question 3:

 What is the purpose of the computation performed by the block which divides by 1000000 and the subtract block to which its output is connected?


Step 4:

 Looking at the I-V relationship of a resistor, even a variable one, is pretty boring. Fortunately, there are more interesting components in your parts kit. Replace the photocell with your red LED, resulting in the following circuit:
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Remember that unlike the resistor and capacitor, the LED is polarized. The short lead denotes the cathode, which is represented by the point of the arrow in the schematic symbol and is connected to ground in the above circuit. The long lead is the anode, represented by the tail of the arrow.

Step 5:

 Start the VI and observe the resulting plot. It should show that the LED is indeed a diode, conducting in one direction and blocking the flow of current in the other. Note that the LED glows dimily during the positive portion of the scan.

Step 6:

 Stop the VI and replace the LED with the photodiode. The photodiode uses same convention as the LED, i.e. the short lead is the cathode.

Step 7:

 Start the VI. You should get a plot similar to that of the LED. (Not surprising, since a photodiode is also a diode.) What are the differences between this plot and that of the LED?

Step 8:

 Turn on the incandescent lamp and shine it on the photodiode from a short distance. Note the difference in the I-V plot. If necessary, press the RESET button to clear the display.

Remark:

 Unlike the CdS photocell, which has a spectral response very close to that of the human eye, the photodiode's response peak is in the infrared. This means that to it the incandescent lamp is much brighter than the flourescent lamp.

Part 2: Measuring I vs V for the Light Bulb

We now have a Labview program which automatically plots I-V curves. We would like to compare the results of this automatic process to those of the same process performed manually. In Experiment 1.1 we manually plotted the I-V characteristics of the light bulb, so we have that manual data available. Unforutnately, we can't use our fully automatic I-V plotter to duplicate this because the D/A output of the DAQ card doesn't produce enough current to drive the lightbulb into its non-linear resistance range. However, we can create a semiautomatic plot by replacing the D/A converter with the power supply and manually sweeping the voltage across the desired range.


Question 4:

 From the DAQ card data sheet and your notes from Lab 1, by what factor do we need to increase the current output of the D/A converter to properly characterize the light bulb?

Step 1:

 Disconnect $V_S$ from dac0 and reconnect it to J1-3. If you have disconnected the 0-6 V power supply from J1-3, reconnect it.

Step 2:

 Remove the photodiode and replace it with the light bulb. Since the light bulb cannot be plugged directly into the breadboard, you will have to exercise your creativity in deciding how to connect it.

Step 3:

 Replace the 10 kΩ resistor ($R_S$ ) with a 10 Ω resistor. On the IV Plot VI front panel, change the Sense Resistor value from 10000 to 10.

Step 4:

 Turn on the power supply and set its output to zero. Start the VI. Manually increase the power supply output to its maximum value (6 volts), then decrease it back to zero.

Question 5:

 Explain the jumps in the I-V plot and the difference between the plots produced by increasing and decreasing voltages.

Step 5:

 Set the power supply output to 6 V. Clear the display. Very slowly decrease the power supply output to zero. Print a copy of the resulting plot.

Question 6:

 Based on your measurements and any necessary additional information, estimate the temperature of the light bulb filament for bulb voltages of 1 V and 4 V.