FVCC Physics Laboratory 2009
Testing Ohm’s Law by Plotting Voltage as a function of Current
Objective:
Plot the data for voltage vs. current for three different electronic components and
determine which of these components is ohmic.
Background:
Ohm’s law says the voltage is proportional to current with the proportionality constant
being the resistance. Equation one is Ohm’s law mathematically.
𝑉 = 𝑅𝐼
(1)
Here it is assumed that V represents delta V. Said another way, if an electronic component responds
linearly to changes in current (or voltage), than it is said to be ‘ohmic’. To see if an electronic
component is ohmic, we need only to plot voltage against current and verify that the plot is a straight
line. The slope of the line is simply the resistance. If the component being tested is not a line, it is said
to be non-ohmic. The importance of knowing whether a given electronic component is ohmic is evident
if we consider how to solve for current and voltage drops in a circuit using the Kirchhoff laws. Consider
the simple circuit of figure 1.
Figure 1. Basic circuit with 2 electronic components
If the 2 components (C1 and C2) in the figure are ohmic, then Loop law for circuits gives:
∆𝑉 = 0 = 𝑉𝐵𝑎𝑡𝑡𝑒𝑟𝑦 − 𝐼𝑅1 − 𝐼𝑅2
(2)
If one or more of the electronic components is not ohmic, you must replace the voltage drops across
that component with the formula that really describes what happens. For example, if component 1 is
ohmic and component 2 responds in a quadratic fashion, then the loop law will give something like
equation 3.
∆𝑉 = 0 = 𝑉𝐵𝑎𝑡𝑡𝑒𝑟𝑦 − 𝐼𝑅1 − (𝐼𝐴2 − 𝐼 2 𝐵2 )
(3)
Here A and B represent constants describing the behavior of the second component. The loop law is
still valid, the circuit can still be analyzed, but the equations are clearly more complicated. So we try to
restrict the beginning physics courses to ohmic materials. In this lab, we will check 3 electronic
FVCC Physics Laboratory 2009
components and determine how they respond in current as the voltage is increased. In this way we can
determine what is, and what is not ohmic.
Procedure: The lab is composed of 3 different tests.
First we will measure an actual resistor, second
we will measure an incandescent light bulb and third we will measure a Zener diode. A basic picture of
the equipment used in the lab is shown in Figure 2.
Figure 2. FVCC equipment used in the lab.
Part I. Measuring a resistor.
Figure 3. A circuit diagram showing how the resistor, C1, is to be measured.
FVCC Physics Laboratory 2009
1) Turn off all the power to the multi-meters and the voltage supply! Also, turn the variable
voltage knob fully CCW, its lowest level. Switch the variable power supply to the 0-8V range.
Save my fuses.
2) Choose a resistor and plug it into the protoboard. (White push pin board) You can estimate the
resistance value by reading the color coded band on the resistor. The resistor color code is
given in Figure 4. Choose a resistor in the 10 to 1000 Ohm range.
3) Connect the circuit as shown in Figure 3.
4) Turn on the meters and the DC power supply.
5) Slowly increase the supply voltage, stopping frequently to collect current and voltage data. You
should collect data in the voltage range of 0V to 6V. Note that if you get too much current, the
resistor will get hot. Be careful and be quick. Take 20-30 data points in the voltage range given.
6) Plot the V vs. I data and use a linear fit to estimate the resistance as measured.
7) Use an Ohm meter to measure the actual resistance.
8) Compare the 3 values of resistance. 1) Resistance estimated from the color code. 2) Resistance
measured with an Ohm meter. 3) Resistance measured with the slope of your line.
Figure 4. Decoding resistor color coded bands. Note that there are two types of resistors, the 4-band
code shown at the top of the figure and the 5-band code shown at the bottom.
FVCC Physics Laboratory 2009
Part II. Measuring a light bulb.
This procedure repeats the steps taken in Part I except that the resistor is replaced with a light bulb. The
circuit diagram is given in Figure 3 where C1 is a light bulb rather than the resistor of part I. The steps
of part I are repeated here with some slight changes due to the component change.
1) Turn off all the power to the multimeters and the voltage supply! Also, turn the variable voltage
knob fully CCW, its lowest level. Switch the variable power supply to the 0-8V range. Save my
fuses.
2) Connect the circuit as shown in Figure 3 with the incandescent bulb in place for C1.
3) Turn on the meters and the DC power supply.
4) Slowly increase the supply voltage, stopping frequently to collect current and voltage data. You
should collect data in the voltage range of 0V to 6V. Note that if you get too much current, the
bulb will get very bright and burn out. Take 20-30 data points in the voltage range given.
5) Plot the V vs. I data and use a polynomial fit to estimate the response as measured.
6) Also use the best fit linear line which is the approximation of the bulb’s resistance.
7) In your report, estimate how much difference there is in power dissipated by the bulb when the
linear assumption is used as when the parabolic fit is used. Use worse case voltage from the
range of volts measured. (i.e. the worst value in the 0-6V range)
Part III. Measuring a Zener Diode
A diode is Ohmic. In fact, it is not even close. In the case of a Zener diode, it will forward conduct at a
lower voltage and reverse conduct at a breakdown voltage. We will measure this with a power resistor
in place to keep the net current in the circuit for getting too high. The circuit diagram is given in Figure
5. The step-by-step procedure is again repeated for this third part of the lab with modifications made
for the Zener diode.
1) Turn off all the power to the multi-meters and the voltage supply! Also, turn the variable
voltage knob fully CCW, its lowest level. Switch the variable power supply to the 0-16V range.
Save my fuses.
2) Connect the circuit as shown in Figure 5A with the 100 power resistor in place for R. (this will
limit the current to a measureable value) Note the direction of the band on the component
which will identify the cathode. This is the forward bias so voltage and current are positive in
your data.
3) Turn on the meters and the DC power supply.
4) Slowly increase the supply voltage, stopping frequently to collect current and voltage data. You
should collect data in the voltage range of 0V to 16V as observed on the dial indicator of the
power supply, not the value on the multimeter. Your voltage data is still coming from the
multi-meter since this is the voltage drop across the Zener diode. But you monitor the power
FVCC Physics Laboratory 2009
5)
6)
7)
8)
supply since this is the total voltage across the power resistor and the Zener diode. Take 20
data points in the voltage range given.
Turn the voltage off (including turning the control dial fully CCW) and reconfigure the circuit into
the reverse bias condition of Figure 5B. This can be done simply by unplugging the Zener diode
and turning it around. Note that you are now going through the diode backwards so the voltage
and current values will all be negative even though the readings on the instrument look positive.
Slowly increase the supply voltage, stopping frequently to collect current and voltage data. You
should collect data in the voltage range of 0V to 16V as observed on the dial indicator of the
power supply. Take 20 data points in the voltage range given.
Plot the data.
Explain when the Zener diode becomes conductive using your plot in the explanation. Also, how
would you account for a Zener diode when setting up an algebraic equation as derived in the
loop law?
Figure 5. A) Measuring the Zener diode in the forward bias configuration. B) Measuring the Zener Diode
in the reverse bias configuration.