EXPERIMENT 4: DC-I
CURRENT-VOLTAGE CHARACTERISTIC OF OHMIC AND NON-OHMIC DEVICES
Object: To determine the current-voltage characteristic curve of a resistor and silicon diode,
and to determine the resistance of the resistor.
Prior to lab: A particular measurement of current and potential difference can be used to
determine a characteristic resistance for an electrical element operating under specific
conditions. For example a current of 0.250 A through a resistor when a potential
difference of 0.815 V is applied yields (according to Ohm’s Law) a specific resistance for
those operating conditions.
Predict the resistance and its measurement uncertainty for the specific current and
potential difference stated in the above example. Assume the measurement uncertainty
is that given for the DMM’s used in lab.
For the DMM DC voltage reading: ΔV = 0.5% V + 1 digit (right most displayed decimal
position).
For the DMM DC current reading: ΔI = 2% I + 1 digit.
Discussion:
1. Definition of terms:
DC or direct current is a current that is constant in magnitude and direction.
Voltage is used as another term for electric potential difference.
I-V characteristic curve or current-voltage characteristic curve is a graph of the relation
between the current through a device and the potential difference across the terminals
of the device.
Resistance is the result of moving charges (current) colliding with atomic cores in a
conductor when a voltage is applied. It can be calculated from Ohm’s Law as R =
V
.
I
A device is considered ohmic if the ratio of voltage to current is constant.
Polarity is a way of showing the relative magnitude of voltage or the direction of current. In
terms of voltage, the higher value of voltage is relatively more positive and should be
connected to the plus terminal of a voltmeter. Conventional current is taken as the
direction of motion of positive charges. The current flows from higher electric potential
to lower electric potential.
Bias refers to the polarity of the connections to a circuit element (resistor, diode, etc.).
Determine how bias may be used to describe the behavior of the circuit elements used
in this experiment.
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Figure 1. The wire wound resistor used in the
experiment. The value of this resistor is 10 Ω ±10%. The
maximum current capacity of the resistor is 0.45
amp.
Figure 2. The diode used in the experiment. The
maximum current capacity of the diode is
1.0 amp.
Power Supply with Switch
A
Resistor
V
Figure 4. A wiring
schematic for the
apparatus shown at the
left.
Figure 3. A representative arrangement of the apparatus. The diode will
replace the resistor for the second part of the experiment.
PROCEDURE:
1. Wire the circuit as shown. Connect the resistor in the position labeled sample. DO NOT
PLUG IN POWER SUPPLY OR SWITCH IT ON. Do ask your instructor to check your
circuit for correctness of wiring. Your instructor will adjust the output of the power
supply to a maximum of 1.95 volts and set the current limit for 190 mA. DO NOT
CHANGE THE CURRENT SETTINGS!
2. Measure the voltage and current pairs of the resistor for 10 different values. Reverse the
connections to the resistor and make another 5 measurements. Label this second set
of readings as negative or reverse bias. DO NOT EXCEED 190 mA ON THE
AMMETER!
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3. Remove the resistor, and connect the diode. Repeat the above instructions. HINT: In the
forward bias orientation of the diode, the current changes very rapidly, choose
very small changes in the voltage. DO NOT EXCEED 190 mA ON THE AMMETER!
ANALYSIS:
1. Create a table for your measured values of I and V for the resistor, and another table for
the diode.
2. For the resistor only, calculate the resistance for each I-V pair you measured. Using the
definitions in the write up on error, determine the mean (average) resistance and
standard deviation for your measurements. Use the spreadsheet to quickly do all these
calculations. (Ask your instructor for assistance, if necessary!)
3. For the resistor only, graph the V vs I characteristic curve and determine the best
average value of R by using a trend line determination with I as the independent variable
(x axis), and V as the dependent variable (y axis). (Does the trend line have a y-intercept
equal to zero?). Include the results of the trend line as a graphed line on your V-I plot. Also
be sure the equation of the trend line is shown on the graph.
4. For the resistor only, determine the maximum instrumental error (uncertainty) for your
set of measurements. Report this uncertainty with the value of R determined from the
trend line. NOTE: The calibration and reading error for the DMM on DC amps is 2% of
the reading + 1 digit.
5. For the diode, graph the I vs. V characteristic curve. Plot the V on the x-axis.
REPORT:
Your report should include a discussion on the nature of the characteristic I-V curves for
the diode and resistor
Compare your value of R from the trend line with the mean value of the set of calculated
values of R and with the manufacturer's claimed value. Be sure to include the relevant
uncertainties with each value of R you report. What do you conclude about the nature
of your measurements. Report any sources of error in the experiment.
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