PHYSICS 220/230
Lab 7: Oscilloscopes and Multimeters

Measurements of voltage, current, resistance, frequency, and the shape of a time-varying signal 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.

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. To measure the voltage across a circuit element, the multimeter must be connected across (in parallel with) that circuit element. To measure current through a circuit element, the multimeter must be connected in series with that circuit element. 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.

(a) Measure Resistance

Select the 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 ACV, 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. Change the function switch to ACA, and select a range which you estimate (DO THIS!) to be larger than the current expected. Connect the 120 VAC source to the transformer again, and record the current.

(d) Analyze AC circuit measurements

Do the results from parts b and c make sense based on your knowledge of Ohms Law and circuits?

 

THE OSCILLOSCOPE CONTROLS

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

Make sure that the horizontal and vertical scales are Calibrated.

The essential function of the cathode-ray oscilloscope is to display voltages which vary rapidly with time. One source of such voltage functions is a signal or function generator. 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 sweep speed and vertical gain to observe several repetitive cycles.

• Determine the frequency and amplitude from the scope. Make sure all controls are in the calibrate position.
• Use the multimeter to measure the voltage output (amplitude).
• Compare the frequency measurement with the generator dial settings.
• Compare the amplitude measurement with the multimeter reading. Are amplitude measurements using the multimeter frequency dependent?
• Change the generator settings (frequency and range) and repeat steps a to d. Do this a total of three times for widely different frequencies.

RC CIRCUITS

In this exercise, you will use square waves from the function generator to represent a switch which continuously connects and disconnects a constant voltage source to a capacitor. Wire together the following circuit. Note that the capacitor C must both charge and discharge through the same resistor R.

Activity: RC Circuits

With R = 50 kohms and C = 0.l microfarads, adjust the scope sweep frequency and vertical gain to observe one or two periods of the charge and discharge of the capacitor. Vary the generator frequency so there is enough time to each charging process for the capacitor voltage to essentially reach its maximum before the discharge follows. Since 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

From the scope, estimate how long it takes the capacitor voltage to reach 0.63 Vo. Compare this to the time constant RC.

Repeat the operation for at least one more different R and C combination.

Activity: LISSAJOUS FIGURES

An important use of the horizontal amplifier of a scope is as an amplifier (i.e., NOT as a time sweep). In this mode, the scope can be used to make frequency comparisons between two signals.

• Connect the wave generator and the transformer output to the scope as indicated in the diagram.
• 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.
• 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. In general, the patterns observed obey the following equation:

• Adjust the horizontal scale so that the two input waveforms are displayed with a couple of periods. Does your observation make sense?
• Repeat the same operations at l20, l80, and other multiples of 60 Hz, making sure the loops are all open and fully visible.
• Repeat at several other frequencies which are (n/m) times 60 Hz, where n and m are both integers. Again make sure all loops are visible.