PHYSICS 220/230 Lab 6: Oscilloscopes and Multimeters

Introduction: Measurements of voltage, current, resistance, and the frequency and shape of time-varying signals are common to almost any laboratory, regardless of discipline. These laboratory exercises will introduce you to the use of a very common and useful electrical measuring device, the oscilloscope, and will expand the uses of the multimeter. By the end of the lab, you should be familiar enough with the controls and characteristics of both devices that you can use them efficiently in future experiments.

Exericise 1: The Multimeter

The multimeter, which you have already used, is a device that will measure voltage, current, or resistance. Observe the controls for function selection and scale factor selection on the front of your multimeter. Note what ranges it will cover. Also, where the probe wires connect to your instrument, note the panel markings indicating the maximum allowable current and voltage that can be measured.

In making electrical measurements, always remember the following rules: 1) To measure the voltage across a circuit element, the multimeter must be connected across (in parallel with) that circuit element; 2) To measure current through a circuit element, the multimeter must be connected in series with that circuit element; 3) To measure resistance, the resistor should be disconnected from the circuit and placed between the probes of the multimeter.

If ever the voltage, current, or resistance being measured by the multimeter exceeds the maximum value of the range selected, an over-flow indication "1. " will be displayed. You should then select a higher range. In general, you should start on the highest range possible if you have no idea of the value of the variable you are measuring.

Disconnect the power supply before making any modifications to the circuit.

Procedure: 

(a) Measure Resistance

Select the W (OHM) scale and measure the resistance of the resistors marked A and B and record the results.

(b) Measure Voltages in an AC circuit

Connect the 6V secondary of your transformer and your two resistors in series. Plug the primary of the transformer into an AC outlet supplying 120 V rms. With the multimeter function switch on AC voltage, measure the rms voltage across each resistor as well as the total rms voltage across the transformer output. Record these values.

(c) Measure Current in an AC circuit

Measure the rms current in the circuit. To do this, first disconnect the transformer from the AC source, then insert the multimeter in series with the resistors. Now change the function switch to AC current.  Re-connect the 120 VAC source to the transformer primary, and record the current.

 

Analysis/Discussion:  Do the results from parts b and c make sense based on your knowledge of Ohm's Law and Kirchoff's loop rule?

Exercise 2: THE OSCILLOSCOPE 

The easiest and most efficient way to become familiar with an oscilloscope ("scope" for short) is to experiment with its various controls. Do not hesitate to do so---it's almost impossible to damage a scope with the equipment that you have, except in one way. Don't set the intensity excessively bright, especially if the beam (or spot) is staying in a single position. This will avoid the possibility of permanently damaging the screen.

The scope controls differ slightly on different makes and models. However, the following controls are common to almost all general purpose scopes.

GENERAL:

• Power on or off (red lamp indicates on)
• Intensity (avoid excessive brightness)
• Focus
• Scale Light (illuminates graticule)

HORIZONTAL:

• Sweep Speed Selector (usually time/cm)
• Sweep Speed Control (variable or calibrated)
• Sweep Speed Magnifier (xl, x5, xl0)
• Trigger Selector (Internal-CHl or CH2, Line-60Hz, or External)
• Trigger Slope (+ or -)
• Trigger Coupling (AUTO or NORM for general use - ACF for high frequency trigger - TV or TVF for only low frequency trigger)
• Trigger Level
• Position Control
• Input (generally CHl on dual trace scopes)

VERTICAL:(One of each control per input channel)

• Coupling Switch
• AC for variations near mean signal level;
• DC for direct input - adds mean level, too;
• GND for baseline voltage only.
• Relative Gain (usually volts/cm)
• Display Mode (Ch1, Ch2, Alt, Chop, Add on dual trace scopes)
• Gain Control (variable or calibrated)
• Position Control
• Input

MISCELLANEOUS:

• GND (for earth ground terminal connection)
• EXT BLANKING connector (modulates brightness) -located on back

INTERPRETING THE OSCILLOSCOPE DISPLAY

The essential role of an oscilloscope is to display voltages which vary rapidly with time. One source of such voltage functions is a function generator. Make sure that the horizontal and vertical scales are Calibrated.  To do this, make sure all three red knobs are turned and clicked in the full clockwise direction.

Procedures:   Connect the function generator to the scope and look at the signal, i.e., connect the Hi output to CHl input and generator GND to scope GND. Adjust the scope's horizontal and vertical scales to observe several repetitive cycles of the waveform.

Analysis/Discussion:

1. Set the function generator for a sine function of 100 Hz.  Then determine the frequency and amplitude of the signal from the scope. (Make sure all controls are in the calibrate position.)  
2. Use the multimeter to measure the voltage output (RMS).
3. Compare the frequency measurement with the generator dial settings.
4. Compare the amplitude measurement with the multimeter reading. 
5. Change the generator settings (frequency and range) and repeat steps 1-4 two more times at frequencies of 1 kHz and 100 kHz.  Are voltage measurements using the multimeter frequency dependent?
6. Sketch one of the waveforms observed on the scope including the scope grid, and label the horizontal and vertical scales.  Briefly explain how you found frequency and amplitude using the oscilloscope.

Exercise 3: RC CIRCUITS

Introduction: In this exercise we will use the oscilloscope to observe time-varying voltages on a capacitor. We will use square waves from the TTL output of the function generator (rather than the HI output as in the drawing) to represent a switch which continuously connects and disconnects a constant voltage source to a capacitor. In this manner, the capacitor will repetitively charge and discharge.  The input coupling should be sent to DC rather than AC.

Procedures: Wire together the following circuit with R = 50 kohms and C = 0.l microfarads. Note that the capacitor C must both charge and discharge through the same resistor R.

Adjust the scope's horizontal and vertical scales to observe one or two periods of the charge and discharge of the capacitor. Now vary the generator frequency so there is enough time in each charging cycle for the capacitor voltage to essentially reach its maximum before the discharge follows.  The charge (and hence the voltage) on the capacitor follows an exponential growth according to:

V = Vo [1 - exp(-t/RC)]

Note that when t = RC, the voltage is about 63% of the initial voltage, i.e.,

                                                                      V = Vo [1 - exp(-1)] = 0.63 Vo

Plots of APPLIED VOLTAGE vs. time and CAPACITOR VOLTAGE vs. time, respectively:

Analysis:

• From the scope, estimate how long it takes the capacitor voltage to reach 0.63 Vo. Compare this to the time constant RC. Do your measurements make sense?  
• Repeat the operation for at least one more different R and C combination. 
• If you wanted to make the capacitor discharge more rapidly, what would you do?
• What happens to the energy stored in the capacitor when it discharges?

Exercise 4: LISSAJOUS FIGURES

Another important use of the horizontal axis of a scope is as a voltage scale analogous to the voltage scale of the vertical axis (i.e., NOT as a time base). In this mode, the scope can be used to make frequency comparisons between two signals.

1. Connect the wave generator and the transformer output to the scope as indicated in the diagram.  Set the amplitudes of both waveforms to be the same as you see them on the oscilloscope display.
2. Turn the time/cm dial to the position marked X-Y or EXT. This removes the time base and connects the X input to the horizontal plates.
3. Adjust the generator frequency slowly around 60 Hz. Study the pattern. When the pattern is a circle and stands still, the generator frequency is matched to the transformer output frequency and is precisely 60 Hz. Draw this pattern in your notebook.  In general, the patterns observed obey the following equation:

VERTICAL FREQUENCY / HORIZONTAL FREQUENCY =

# of peaks along horizontal axis / # of peaks along vertical axis

4.      Return the time/cm dial to its initial position.  Adjust the horizontal scale so that the two input waveforms are displayed with a couple of periods. Does your observation in step 3 make sense based on what you see in this view?

5.      Return to the EXT mode and repeat step 3 at l20, l80, and other multiples of 60 Hz, realizing that you no longer get a circle and making sure the loops are all open and fully visible.

6.      Repeat step 3 at several other frequencies that are a fraction (n/m) times 60 Hz, where n and m are both integers but not equal to each other. Again make sure all loops are visible.