Lab #3 Help Document
DC power supply
DMM
Oscilloscope
Function Generator
Bread Board
4 Resistors (2 of each value):
Resistance Value in ohms (Ω)
Color Bands
10M Ω
Brown-black-blue
100K Ω
Brown-black-yellow
1. Measure the actual value of the two 10 MΩ resistors and the two 100 kΩ resistors.
This part is pretty basic. Configure the DMM to measure resistance and measure the resistance across
each of the two 10 M ohm resistors and each of the two 100 k ohm resistors. If you are unsure of how
to do this, refer to lab #1 help document section “How to use the DMM”.
2. Set the DC supply to +10 Volts, and apply to the two 10 MΩ resistors connected in series. Calculate
the voltage you would expect to measure across each resistor, respectively. After calculating the
expected voltage measurements, measure each, determine the % difference from the expected
values, and explain the difference.
Again, pretty basic. Set the two 10M ohm resistors up in series using a breadboard. Set the DC power
supply to +10V. Configure the DMM to measure DC voltage. Set up the leads of the DC power supply
and DMM to make a voltage measurement. (If you are unsure of how to configure the DMM for a DC
voltage measurement or how to set up the leads of the DMM and DC power supply to measure voltage
drops, refer to the lab #1 help document sections “How to make a voltage measurement” and “How to
measure voltage drops”). Now measure the voltage across each resistor, which basically means
measure the voltage drop across each resistor (this would be done in the same way that you measured
the voltage drops across each resistor for lab 1). You will notice that the two voltage drops do not add
up to the input voltage. This is due the input impedance of the DMM. Here’s what happens:
Why the off reading?
The DMM has an internal impedance of 10Mohm. Impedance is not quite the same as resistance, but in
this case where we are making a voltage measurement, it acts like resistance. The DMM is connected in
parallel with the resistor that you are measuring when making a voltage drop measurement, so the
10Mohm impedance is in parallel with the 10Mohm resistor. The impedance acts like resistance in this
case, so you basically have 2-10Mohm resistors in parallel. The resulting resistance of the set will be
only 5Mohm (remember that resistors in parallel have lower total resistance than that of the smallest
resistor. In this case, it’s half). This 5Mohm equivalent resistance is in series with the other 10Mohm
resistor. In a series circuit with multiple resistors in series, the voltage drop across each one is
proportional to resistance (larger resistance translates to larger voltage drop). If you run through the
voltage drop equation using this scenario, you discover that the expected voltage under these
conditions is:
For the resistor being measured: Vout = 10*(5Mohm/15Mohm) = 3.333 V
For the resistor not being measured: Vout = 10*(10Mohm/15Mohm) = 6.666V
This will agree with your observations, however if you were unaware of this internal impedance and its
resulting effect on the circuit, you would expect a different voltage drop across each resistance than
what you will observe.
Calculate the % difference between the measured voltage drops and the expected voltage drops using
the % difference equation:
% difference = [(expected – measured)/ expected]*100
3. Draw the schematic of how you connected everything.
If you included a picture or diagram as a description for your DMM/DC power supply lead setup with the
two resistors, that will meet this requirement. If not, add a picture or diagram that shows how you
measured the circuit by showing where the leads were connected.
4. Set the function generator to 10 kHz, 10 Vp-p, sine wave, using the DMM to set the frequency and
the amplitude. Be sure to turn the offset (symmetry) of the function generator to OFF.
From here, you will pick up the use of two new pieces of equipment. One is the Function Generator and
the other is the Digital Oscilloscope. This part of the lab will have you configure the function generator
to produce a certain voltage and frequency which will later be measured using the oscilloscope.
To set up the function generator, first push the 10K button under the Range (Hz) section. Tune it to 10k
Hz by setting the knob on the bottom left to read 1.0. Next, press the sine wave button under function.
It will have a sine wave over it. Make sure the DC OFFSET knob is pushed in not pulled out. Connect the
function generator leads to the left output labeled MAIN.
Now configure the DMM to measure AC voltage. The only difference between the AC voltage and DC
voltage configuration is that the AC voltage button is pushed instead of the DC voltage button. Lead
configuration remains the same for both.
Now connect the red leads of your DMM and Function generator together. Then connect the black
leads of both together.
To adjust the voltage, turn the knob on the Function generator that is labeled AMPLITUDE.
NOTE: The DMM will read the output AC voltage as RMS voltage. To convert between a peak-to-peak
voltage of 10V and the RMS voltage you will read on the DMM divide the peak-to-peak voltage by 2 and
then multiply that by 0.707.
RMS = (p-p/2)*0.707
Where p-p means peak-to-peak voltage
You should get 3.535 V RMS. Adjust the amplitude knob on the function generator until you read that
on the DMM.
Leave the function generator on and set the way it is currently set . Disconnect the DMM from the
function generator and turn off the DMM.
5. Measure the amplitude and frequency of the 10 kHz sine wave using the oscilloscope. Make an
entry in you lab book for all the units the oscilloscope gives for these measurements.
To access the oscilloscope, go to start, All programs, scroll down to the folder that says “Syscomp Digital
Oscilloscope” and click it once, then click the third option entitled “Syscomp Digital Oscilloscope”. This
will open the digital oscilloscope. There may be an alert that says, “Unable to connect to device,
Examine connection settings?” This means that for some reason, the oscilloscope isn’t connected. First,
make sure that the oscilloscope is connected to one of the computer’s front USB ports. Then close out
of and restart the program. If you still get that same connection alert, there are two ways to address it:
First way, click yes on the alert that says “Unable to connect to device, Examine connection settings?”
This will open a small window that looks like this:
Click Autodetect.
This will open another window that looks like this:
Before clicking okay, disconnect and then reconnect the oscilloscope to the computer by unplugging it
and then re-plugging it into the USB port. Now click okay.
It should automatically assign a port. Click save and exit below autodetect when the process is done.
If that doesn’t work, try this:
Go to the top toolbar and click Hardware → Port Settings. This will open up the following window again:
Now, one at a time, try each of the COM ports. Select one, click “Save and Exit” and wait for the
dialogue box. If the box that pops up says, “Unable to connect to device”, try a different port. When
you get a box that says, “Unknown device in Port X,” you’ve found the right port. Close the oscilloscope
program and reopen it. The oscilloscope should now be connected and the program should have a
green box that says “Connected” at the top. If instead it still gives an alert or has a red box that says
“Disconnected” at the top, either try the above steps again or try a different oscilloscope. If you did the
above steps, the oscilloscope should work now.
Once the Oscilloscope is set up, all you do is connect the red lead from the function generator to the
larger black lead of the oscilloscope (has a hook in its tip), then connect the black lead from the function
generator to the smaller black lead coming off of the larger black lead on the oscilloscope. This smaller
black lead is ground for the oscilloscope just like the black lead for the function generator, DMM and
power supply is ground. Keep the function generator set up like it is because you will still need to use
those settings in part 7 of the lab.
NOTE: The measurements window will have two columns labeled A and B. The column with the
measurements you are taking will be the same as the port your leads are connected to. In other words,
if your oscilloscope leads are connected to port A on the oscilloscope, measurements you are taking will
be in column A. The same is true for port B and column B.
6. Calculate the voltage you would expect to measure across each of the 100 k ohm resistors and
across both resistors, with the 10 kHz, 10 Vp-p signal applied, for the oscilloscope.
Treat 10 Vp-p as just 10V and use it as Vin in the voltage divider rule to calculate the expected voltage
across each resistor. You would expect to measure the input voltage across both resistors.
7. Apply the above output of the function generator to the 100 k ohm resistors (remove the DC
voltage). Using the oscilloscope, observe the voltage across both resistors, and the voltage across the
resistor closest to ground. Do not try to measure the voltage across the other resistor. Compare the
measured values to the expected values.
First, set up a two resistor series circuit on the breadboard with the two 100kohm resistors. Then,
connect the function generator to the circuit in the same manner you would connect the DC power
supply (red lead to one side of circuit and black lead to other). The reason for this is because the
function generator is acting as the power supply in this situation only it’s AC instead of DC. The
oscilloscope will act as the DMM. To connect it to the circuit, place the smaller black lead (ground) on
the same side of the circuit as the Function Generator’s black lead. Place the larger black lead on the
same side as the Function Generator’s red lead. This will allow you to measure the voltage across the
entire circuit.
Now look at the measurements window for the oscilloscope. The value for the voltage will be the
second one from the bottom entitled Peak-Peak. (I used port A to make these measurements, so I take
the Peak-Peak measurement from column A).
Record this value as the measured voltage across both resistors. Now move the larger black lead from
the oscilloscope so that it is between the two resistors. This will allow it to measure the voltage across
just the resistor closest to ground.
Record the Peak-Peak voltage as your voltage across the resistor closest to ground. If you want to
measure the voltage drop across the other resistor, you will have to switch the position of the two
resistors or switch all of the leads for both the function generator and the oscilloscope around such that
that resistor becomes closest to ground. Here’s why. If you were to take the large black lead of the
oscilloscope, place it on the side of the circuit with the red lead from the function generator, and then
take the smaller black lead from the oscilloscope and place it between the two resistors (this is what you
would do if you were using the DMM) you would measure a voltage drop of 10V across just that one
resistor. The reason for this is because the oscilloscope ground is connected directly to the building
ground, so the electricity never goes through the second resistor. A DMM ground does not connect to
the building ground. Its ground is relative, so you can move it around the circuit to measure voltage
drops because the current will still go through the second resistor.
NOTE: If when you measure the voltage drop across the resistor closest to ground you get something
lower than expected, don’t worry, you probably did it right. The oscilloscopes have a history of not
giving the expected result for this measurement.