UNIVERSITY OF MASSACHUSETTS DARTMOUTH
COLLEGE OF ENGINEERING
EGR 101 INTRODUCTION TO ENGINEERING THROUGH APPLIED SCIENCE I
VOLT-AMPERE CHARACTERISTICS
BACKGROUND
The behavior of two-terminal devices can be described by a volt-ampere characteristic, a
graphical representation of the relationship between the current flowing through the device as
a function of the voltage across the device’s terminals. The circuit setup shown below in
Figure 1 can be used to obtain the volt-ampere (V-I) characteristic of a two-terminal device.
Current
+
0.000
A
Z=A+jB
DC 1e-009Ohm
Vsupply
12 V
Device
under test
+
0.000
Voltage
V DC 1MOhm
-
Figure 1. Circuit setup for V-I Characteristic
A point-by-point plot of the device’s V-I characteristic could be obtained by varying the
amplitude and polarity of the DC voltage supply while recording the ammeter and voltmeter
readings on coordinate axes as shown in Figure 2.
i
v
Figure 2. Coordinate system for V-I Characteristic
1
For a 1 kΩ resistor, R, the circuit setup and resulting V-I Characteristic are displayed in below in
Figure 3.
Current
+
i
A
0.012
DC 1e-009Ohm
R
1kOhm
Vsource
12 V
+
Voltage
12.000 V DC 1MOhm
-
v
Figure 3. Circuit setup and V-I Characteristic for a resistor, R
From Ohm’s Law,
i =v/R.
If we compare this to the equation for a straight line plotted on the i-v axes,
i = mv + b,
where m = the slope of the line and b = the i-intercept, we see that the resistance R is the
reciprocal of the slope, or
R = 1/m
and the i-intercept is zero.
The second device for which we will obtain a V-I Characteristic is the diode, more specifically, a
Light-Emitting-Diode, or LED. A diode is a two-terminal device which will allow a current to
pass through it in one direction only. The symbols for two types of diodes are shown in Figures
4a and 4b shown below.
LED
Diode
1N4007GP
Figure 4a. Diode symbol
Figure 4b. LED symbol
A diode will allow the current to flow through it when it is connected to an external circuit that
will establish conventional current in the same direction as the “arrow” in the diode symbol
(forward bias). The diode will behave as a low resistance (the forward resistance), and the
voltage across the diode will depend upon the material from which the diode is fabricated
(approximately 0.7V for Silicon, 1.2V for GaAs) and the amount of current. If the external circuit
tries to establish a conventional current opposite to the direction of the “arrow” in the diode
2
symbol (reverse bias), the diode will “block” the flow of current through it, thus acting as a very
high resistance (the reverse resistance).
If the diode is an LED, it will glow under forward bias and remain unlit under reverse bias.
LABORATORY PROCEDURE / RESULTS
1. Construct the circuit of Figure 1 using your DC powered breadboard, digital multimeter, and
multi-range analog milliammeter.
2. Using a 1,000 Ω resistor for the device under test, vary the amplitude and polarity of the DC
power supply to obtain enough data points to plot the V-I Characteristic. Using EXCEL, create a
table of the measured values of voltage and current and plot the results.
Be sure that you measure only the voltage across the resistor. (Resistor voltage from -10
to +10 Volts is good.)
3. Repeat step 2 using an LED with a 220 Ω series resistor (to prevent burn-out) in place of the
device under test. Measure the voltage across the LED only, using power supply voltages
between -3 and +5 volts. (The LED voltage will begin to “limit” at about 2 volts.). Use EXCEL
to plot the V-I Characteristic. Compare it with that of the resistor, noting any differences.
4. Determine the resistance of the 1,000 Ω resistor from the V-I Characteristic obtained in
step 2. Document the calculation in your report.
5. Determine the forward and reverse resistances of the LED from the V-I Characteristic
obtained in
step 3. Show your method of calculation and the results. Why are the resistances different?
6. Using the V-I Characteristics which you have sketched, determine
a. the voltage across the resistor at IR = 3.4mA
b. the current through the resistor at VR = -4.5V
c. the voltage across the LED at I = 5mA.
7. Be sure to include the complete circuit diagram of each setup used to obtain the V-I
Characteristics in your report.
RESULTS TO BE HANDED – IN
1. The EXCEL plot of the resistor Volt – Ampere Characteristic.
2. The calculation of the resistance from the Volt- Ampere Characteristic.
3. The EXCEL plot of the LED Volt – Ampere Characteristic.
4. The calculation of the forward and reverse resistances of the LED from the Volt – Ampere
Characteristic.
3