Extech Instruments MT310 Instrukcja Użytkownika Pobierz pdf (Strona 5)

P31220 Lab

Experiment 4: Measuring Resistance as a Function of Voltage

Now, things get a little more interesting. Carbon resistors, light bulbs, and semiconductors

behave very differently. While V=IR is always true for fixed values of V, I, and R, the graph of

V vs. I is not necessarily linear. This is because R cannot be assumed to be a constant. We will

plot V vs. I for several different devices so that you can observe this.

Basic setup: Modify your potentiometer circuit by adding an ammeter, as shown in Fig. 5. The

resistor marked “X” is whatever device we are measuring – either a resistor, light bulb, or diode.

We will measure the current through X and the voltage across X at the same time. For starters,

use the 27Ω resistor as “X”.

Fig. 5: Potentiometer Circuit for Measuring Resistor X

What to do:

1. Measure and graph V vs. I for the 27 Ω resistor. Measure and graph the V vs. I curve for

both positive and negative voltages. The easiest way to get the negative voltages is to switch

the wires from the power supply. You can expect this first graph to be linear. Remove the

“connect the dots” and do a linear fit to your data. The slope of this line should be the

resistance of X. This is the classic “Ohm’s Law” measurement.

IMPORTANT: Complete each graph before changing to the next element. You may

want to fill in some more data points after you see the graph.

2. Next, remove the 27Ω resistor and replace it with the light bulb. Make a graph of V vs. I,

using both polarities and taking care to obtain more data points in the regions of the graph

that are changing more rapidly. Notice how the resistance is changing and observe whether

reversing polarity makes a difference. Forget the linear fit. Tell this graph to connect the dots

“to guide the eye”.

3. Finally, replace the light bulb with the diode. Measure and graph both the positive and

negative voltages as before, even if it looks like nothing is happening. Take more data points

where things are happening fast. Tell this graph to connect the dots also.

25Ω potentiometer

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Extech Instruments MT310 Instrukcja Użytkownika Strona 5